1

CCNP-SWITCH (V60) Building Cisco Multilayer Switched Networks (BCMSN) v30

l Module 4 Implementing Inter-VALN routingsect Origin Cisco Academic Presssect Update 이훈재(李焄宰) HoonJae Lee 동서대학교

sect e-mail hjleedongseoackrsect Homepage httpkowondongseoackr~hjleel httpcryptodongseoackr

Module 4 Implementing Inter-VLAN routing

- cisco flash v50 -

SWITCH v60 MCMSN v50 (Cisco flash v50)module 4

Dongseo UniversityHoonJae Lee

2

Module 4 Implementing Inter-VLAN routing (flash v50)

41 Describing Routing Between VLANs42 Enabling Routing Between VLANS43 Deploying CEF-Based Multilayer Switching44 Inter-VLAN Routing Lab Exercises

4

Inter-VLAN Routing

l A VLAN is a logical group of ports usually belonging to a single IP subnet to control the size of the broadcast domain

l Even though devices in different VLANs may be ldquophysicallyrdquo connected as shown in the previous slides these devices cannot communicate without the services of a default gateway a router

l Because VLANs isolate traffic to a defined broadcast domain and subnet network devices in different VLANs cannot communicate with each other without the use of a router

l This is known as Inter-VLAN Routing

3

5

Inter-VLAN Routingl The following devices are

capable of providing inter-VLAN routing1 Any Layer 3

multilayer Catalyst switch

2 Any external routerwith an interface that supports trunking(ldquorouter-on-a-stickrdquo)

3 Any external router or group of routers with a separate interface in each VLAN

Or trunk port

Inter-VLAN Routing with External Router

bull a single trunk link between the switch and the router that can carry the traffic of multiple VLANs and which in turn can be routed by the router

6

4

l The advantages are as follows Oslash Implementation is simple Oslash Layer 3 services are not required on the switch Oslash The router provides communications between VLANs

l The disadvantages are as followsOslash The router is a single point of failure Oslash The single traffic path between the switch and the router may

become congested Oslash Latency is higher than on a Layer 3 switch

Inter-VLAN Routing with External Router

7

8

Router On a Stick

l Router on a stick is very simple to implement because routers are usually available in every network

l Most enterprise networks use multilayer switches to achieve high packet-processing rates using hardware switching

l Multilayer (layer 3) switches usually have packet-switching throughputs in the millions of packets per second (pps) whereas traditional general-purpose routers provide packet switching in the range of 100000 pps to just over 1 million ppsagrave 110 speed down

5

9

Connecting VLANs with Multilayer Switches

Layer 2 Interfacesl Access portmdash Carries

traffic for a single VLANl Trunk portmdash Carries

traffic for multiple VLANs using Inter-Switch Link (ISL)encapsulation or 8021Q tagging

Describing Inter-VLAN Routing Using External Router Configuration Commands

10

6

Inter-VLAN Routing on External Router 8021Q Trunk Link

switch(config)interface FastEthernet 00switch(config-if)switchport trunk encapsulation dot1qswitch(config-if)switchport mode trunk

11

Inter-VLAN Routing on External Router ISL Trunk Link

switch(config)interface FastEthernet 00switch(config-if)switchport trunk encapsulation islswitch(config-if)switchport mode trunk

12

7

Verifying Inter-VLAN Routing

13

l MLS(multilayer switch) = Switch + Router

into one device

l A multilayer switch combines the functionality of a

switch and a router into one device therefore

enabling the device to switch traffic when the

source and destination are in the same VLAN and

to route traffic when the source and destination are

in different VLANs (that is different subnets)

Oslash The same VLAN agrave switching

Oslash Different VLANs agrave routing

l ASIC wire speed

l Routing table access control list (ACL) agrave

store in CAM TCAM

Explaining Multilayer Switching

14

8

Layer 2 Switch Forwarding process ndash In MLS

15

Logical Flow for a Multilayer Switch

16

9

Frame Rewrite

17

l The source MAC address changes from the sender MAC address to the router MAC address l The destination MAC address changes from the router MAC to the next-hop MAC address l The TTL is decremented by one and as a result the IP header checksum is recalculated l The frame checksum is recalculated

l Routing switching ACL and QoS tables are stored in a high-speed table memory so that forwarding decisions and restrictions can be made in high-speed hardware

l Cisco Catalyst switches create and use two primary table architectures Oslash CAM (content addressable memory)

two results 0 (true) or 1 (false) MAC address tables

Oslash TCAM (ternary content addressable memory ) ndash Ternary Logic three results 0 (donrsquot care) 1 (true) 2 (false)IP tables routing ACL QoS

Switching Table Architectures - Details

CAM TCAM

18

10

CAM Application

Key

VLAN ID

Key

19

l The information a switch uses to perform a lookup in a CAM table is called a key

l For example a Layer 2 lookup would use a destination MAC address and a VLAN ID as a key

l In specific high-end switch platforms the TCAMis a portion of memory designed for rapid hardware-based table lookups of Layer 3 and Layer 4 information In the TCAM a single lookup provides all Layer 2 and Layer 3 forwarding information for frames including CAM and ACL information

l How the values are stored in the TCAM access-list 101 permit ip host 10111 anyaccess-list 101 deny ip 10110 000255 any

l Longest match region Each longest match region consists of groups of Layer 3 address entries (ldquobucketsrdquo) organized in decreasing order by mask length All entries within a bucket share the same mask value and key size The buckets can change their size dynamically by borrowing address entries from neighboring buckets Although the size of the whole protocol region is fixed you can reconfigure it The reconfigured size of the protocol region takes effect only after the next system reboot

l First-Match region The first-match region consists of ACL entries Lookup stops after the first match of the entry

TCAM

20

11

Module 4 Implementing Inter-VLAN routing (flash v50)

41 Describing Routing Between VLANs42 Enabling Routing Between VLANS43 Deploying CEF-Based Multilayer Switching44 Inter-VLAN Routing Lab Exercises

22

Layer 3 Interfaces

The Catalyst multilayer switches support three different types of Layer 3 interfaces

l Routed portmdash A pure Layer 3 interface similar to a routed port on a Cisco IOS router

l Switch virtual interface (SVI) mdash A virtual VLAN interface for inter-VLAN routing In other words SVIs are the virtual routed VLAN interfaces

l Bridge virtual interface (BVI) mdash A Layer 3 virtual bridging interface (Not discussed)

12

23

MLS Layer 3 Interface Routed Port

MLS Layer 3 Interface Routed Portl A routed switch port is a physical switch port on a multilayer switch that is

capable of Layer 3 packet processing A routed port is not associated with a particular VLAN as contrasted with an access port or SVI The switch port functionality is removed from the interface A routed port behaves like a regular router interface except that it does not support VLAN subinterfaces Routed switch ports can be configured using most commands applied to a physical router interface including the assignment of an IP address and the configuration of Layer 3 routing protocols

l A routed switch port is a standalone port that is not associated with a VLAN whereas an SVI is a virtual interface that is associated with a VLAN SVIs generally provide Layer 3 services for devices connected to the ports of the switch where the SVI is configured Routed switch ports can provide a Layer 3 path into the switch for a number of devices on a specific subnet all of which are accessible from a single physical switch port

l The number of routed ports and SVIs that can be configured on a switch is not limited by software However the interrelationship between these interfaces and other features configured on the switch may overload the CPU because of hardware limitations

24

13

l A routed port has the following characteristics and functionsOslash Physical switch port with Layer 3 capability Oslash Not associated with any VLAN Oslash Serves as the default gateway for devices out that switch port Oslash Layer 2 port functionality must be removed before it can be

configured

MLS Layer 3 Interface Routed Port

25

Configuration of Routed Ports on a Multilayer Switch

26

14

27

MLS Layer 3 Interface SVI

l Switch virtual interfaces (SVI) are Layer 3 interfaces that are configured on multilayer Layer 3 Catalyst switches that are used for inter-VLAN routing

l An SVI is a virtual VLAN interface that is associated with the VLAN-ID to enable routing capability on that VLAN

l Note These are virtual interfaces

DLSwitch(config)interface vlan 1DLSwitch(config-if)ip address 1721611 2552552550DLSwitch(config)interface vlan 10DLSwitch(config-if)ip address 17216101 2552552550DLSwitch(config)interface vlan 20DLSwitch(config-if)ip address 17216201 2552552550DLSwitch(config)interface vlan 30DLSwitch(config-if)ip address 17216301 2552552550

28

MLS Layer 3 Interface SVI

l To configure communication between VLANs you must configure each SVI with an IP address and subnet mask in the chosen address range for that subnet

l The IP address associated with the VLAN interface is the default gateway of the workstation

DLSwitch(config)interface vlan 1DLSwitch(config-if)ip address 1721611 2552552550DLSwitch(config)interface vlan 10DLSwitch(config-if)ip address 17216101 2552552550DLSwitch(config)interface vlan 20DLSwitch(config-if)ip address 17216201 2552552550DLSwitch(config)interface vlan 30DLSwitch(config-if)ip address 17216301 2552552550

15

Layer 3 SVI

l To provide a default gateway for a VLAN so that traffic can be routed between VLANs

l To provide fallback bridging if it is required for non-routable protocols

l To provide Layer 3 IP connectivity to the switch

l To support routing protocol and bridging configurations

29

Configuring Inter-VLAN Routing on a Multilayer Switch

30

16

Module 4 Implementing Inter-VLAN routing (flash v50)

41 Describing Routing Between VLANs42 Enabling Routing Between VLANS43 Deploying CEF-Based Multilayer Switching44 Inter-VLAN Routing Lab Exercises

32

MLS and CEF

l One of the bottlenecks in high-speed networking is the decision-making process within the router

l Two of the methods used by Cisco devices to speed up this process are

1 Multilayer Switching (MLS) 2 Cisco Express Forwarding (CEF)

17

33

Internal route processors

Route Processors includeOslash Route Switch Module (RSM) ndash 4000 5000 6000 7000Oslash Route Switch Feature Card (RSFC) - 5000Oslash Multilayer Switch Module (MSM) - 6000Oslash Multilayer Switch Feature Card (MSFC) - 6000

Other terms usedOslash Layer-3 Card or Layer-3 ldquoBladerdquoOslash MultiLayer Switch Route Processor (MLS-RP)

uumlThe router in the network (handles the first packet in every flow)

34

Introduction to MLS

l MLS is a technology used by a small number of older Catalyst switches to provide wire-speed routing

l MLS is sometimes known as Route once switch manyOslashThe first packet of a flow is routed by the router in

software and the remaining packets are forwarded in hardware by the switch

18

35

Introduction to CEF

l CEF is the technology used by newer Cisco devices to provide wire-speed routing

l Unlike MLS which requires the route processor to route the first packet of a flow CEF enables packet switching to circumvent the route processor altogether OslashThis is accomplished by the communication process

between the route processor and the switch processor to create the shortcut info before the first packet arrives

OslashldquoRoute never switch alwaysrdquo

36

Multilayer Switching

l Multilayer switching refers to the ability of a Catalyst switch to support switching and routing of packets in hardware with optional support for Layers 4 through 7 switching in hardware as well

l Hardware switching A route processor (Layer 3 engine) must download software-based routing switching access lists QoS and other information to the hardware for packet processing

Traditional MLS CEF-Based MLS

19

37

Traditional and CEF-based MLS

l To accomplish multilayer switching (packet processing in hardware) Cisco Catalyst switches use eithersect Traditional multilayer switching (traditional MLS) sect Cisco Express Forwarding (CEF)-based MLS architecture

l Traditional MLS is a legacy feature whereas all leading-edge Catalyst switches support CEF-based multilayer switching (CEF-based MLS)

38

Multilayer Switching

l The term multilayer switching (MLS) is a general networking term that refers to hardware-based PDU header rewriting and forwarding based on information specific to one or more OSI layersOslash When used in the context of this class MLS refers to Cisco MLS

20

39

Traditional MLS

l MLS enables specialized application-specific integrated circuits (ASICs) to perform Layer 2 rewrite operations of routed packets

l Layer 2 rewrites include rewriting the source and destination MAC addressesand writing a recalculated cyclic redundancy check (CRC)

l Because the source and destination MAC addresses change during Layer 3 rewrites the switch must recalculate the CRC for these new MAC addresses

40

Traditional MLS

l For Catalyst switches that support traditional MLS the switch learns Layer 2 rewrite information from an MLS router via an MLS protocol sect Also known as netflow-based switching

l With traditional MLS the Layer 3 engine (route processor) and switching ASICs work together to build Layer 3 entries on the switch

l Each entry contains a source a source and destination or full flow information including Layer 4 protocol information

21

41

Traditional MLS

l With traditional MLS the switch forwards the first packet in any flow to the Layer 3 engine for processing using software switching

l After the routing of the first packet in the flow the Layer 3 engine programs the hardware-switching components for routing for subsequent packets

dot1q Tag(inside Eth Hdr)

Ethernet Header IP Header IP Data

VLAN 1

D-MAC= 00-00-0C-11-11-11

S-MAC= 00-AA-00-11-11-11

S-IP = 101110

D-IP = 101220

MLS-SE

MLS-RPMLS-RP The Destination MAC Address is one of the routerrsquos interfacesThere is not an existing flow so I will flag this as a candidate packet

Candidate Packet Info

Layer 3 InfoS-IP 101110 D-IP 101220

Layer 2 InfoS-MAC 00-AA-00-11-11-11 D-MAC 00-00-0C-11-11-11

dot1q Tag(inside Eth Hdr)

Ethernet Header IP Header IP Data

VLAN 1

D-MAC= 00-00-0C-11-11-11

S-MAC= 00-AA-00-11-11-11

S-IP = 101110

D-IP = 101220

Traditional MLS

42

22

MLS-SE

MLS-RP

dot1q Tag(inside Eth Hdr)

Ethernet Header IP Header IP Data

VLAN 2 D-MAC= 00-AA-00-22-22-22

S-MAC= 00-00-0C-22-22-22

S-IP = 101110

D-IP = 101220

Traditional MLS

43

MLS-SE

MLS-RPCandidate Packet Info

Layer 3 InfoS-IP 101110 D-IP 101220

Layer 2 InfoS-MAC 00-AA-00-11-11-11 D-MAC 00-00-0C-11-11-11

Dst IP Src IP Port Dst Port

Src Port

Dst MAC

Src MAC

VLAN Interface

101220 101110 TCP 23 1238 00-AA-00-22-22-22

00-00-0C-22-22-22

2 31

Future Packets

MLS Cache

Found match in MLS Cache rewrite Ethernet

Header and send directly to Host B forget the

router

Traditional MLS

44

23

45

CEF-based MLS

46

CEF

l CEF-based MLS forwarding model is used to download the control plane information such as the access lists to the data plane on the supervisor port or line card for hardware switching of packetsOslash Control plane represents the Layer 3 engine (route processor) Oslash Data plane represents the hardware components such as ASICs

used by the switch for hardware switching l CEF is a topology-based forwarding model in which all routing

information is prepopulated into a forwarding information base (FIB)

l As a result of the prepopulation of routing information Catalyst switches can quickly look up routing information such as IP adjacencies and next-hop IP and MAC addresses

24

47

CEF

The two main components of CEF are FIB and Adjacency Tablebull Forwarding information base (FIB)

Oslash Used make IP destination prefix-based switching decisions Oslash Similar to a routing table or information base Oslash It maintains a mirror image of the forwarding information contained in the

IP routing table Oslash When routing or topology changes occur in the network the IP routing

table is updated and those changes are reflected in the FIB Oslash The FIB maintains next-hop address information based on the

information in the IP routing table Oslash In the context of CEF-based MLS both the Layer 3 engine and the

hardware-switching components maintain an FIB

Routing Table

48

CEF

l Adjacency tablesOslash Network nodes in the network are said to be adjacent if they can

reach each other with a single hop across a link layer (OSPF EIGRP)

Oslash A router normally maintainssect Routing table containing Layer 3 network and next-hop

informationsect ARP table containing Layer 3 to Layer 2 address mapping sect These tables are kept independently

25

49

CEF

l Adjacency tablesOslash Recall that the FIB keeps the Layer 3 next-hop address for each

entry Oslash To streamline packet forwarding even more the FIB has

corresponding Layer 2 information for every next-hop entry Oslash This portion of the FIB is called the adjacency table consisting of

the MAC addresses of nodes that can be reached in a single Layer 2 hop

Layer 2 MAC Addresses Next Hop Information

50

CEF

l Adjacency tables (summary more detail coming)Oslash The adjacency table information is built from the ARP table Oslash As a next-hop address receives a valid ARP entry the adjacency

table is updatedOslash If an ARP entry does not exist the FIB entry is marked as ldquoCEF

glean(이삭 줍기)rdquo Oslash This means that the Layer 3 forwarding engine cant forward the

packet in hardware due to the missing Layer 2 next-hop address Oslash The packet is sent to the Layer 3 engine so that it can generate an

ARP request and receive an ARP reply Oslash This is known as the ldquoCEF gleanrdquo state where the Layer 3 engine

must glean the next-hop destinations MAC address

No ARP entry L3 forwarding engine canrsquot

forward packet in hardware

must send to L3 Engine Irsquoll generate the ARP

Request and get an ARP Reply

26

51

CEF

l Adjacency tablesOslash During the time that a FIB entry is in the CEF glean state waiting for

the ARP resolution subsequent packets to that host are immediately dropped so that the input queues do not fill and the Layer 3 engine does not become too busy worrying about the need for duplicate ARP requests

Oslash This is called ARP throttling(목 조르기) or throttling adjacency Oslash If an ARP reply is not received in two seconds the throttling is

released so that another ARP request can be triggered Oslash Otherwise after an ARP reply is received the throttling is released

the FIB entry can be completed and packets can be forwarded completely in hardware

Explaining Layer 3 Switch Processing

52

27

Explaining Layer 3 Switch Processingl Layer 3 switching refers to a class of high performance routers

optimized for the campus LAN or intranet providing both wire-speed Ethernet routing and switching services

l A Layer 3 switch router performs the following three major functionsOslash Packet switching Oslash Route processing Oslash Intelligent network services

l Compared to other routers Layer 3 switch routers process more packets faster by using ASIC hardware instead of microprocessor-based engines Layer 3 switch routers also improve network performance with two software functions route processing and intelligent network services

l Layer 3 switching software employs a distributed architecture in which the control path and data path are relatively independent The control path code such as routing protocols runs on the route processor whereas most of the data packets are forwarded by the Ethernet interface module and the switching fabric 53

Explaining Layer 3 Switch Processingl Each interface module includes a microcoded processor that

handles all packet forwarding The control layer functions between the routing protocol and the with the firmware datapath microcode following primary dutiesOslash Manages the internal data and control circuits for the packet-

forwarding and control functions Oslash Extracts the other routing and packet forwarding-related control

information from the Layer 2 and Layer 3 bridging and routing protocols and the configuration data and then conveys the information to the interface module to control the datapath

Oslash Collects the datapath information such as traffic statistics from the interface module to the route processor

Oslash Handles certain data packets sent from the Ethernet interface modules to the route processor

54

28

Explaining Layer 3 Switch Processing

55

Explaining Layer 3 Switch Processingl Layer 3 switching can occur at two different locations on the switch

Oslash Centralized Switching decisions are made on the route processor by a central forwarding table typically controlled by an ASIC

Oslash Distributed Switching decisions are made on a port or line-cardlevel Cached tables are distributed and synchronized to various hardware components so that processing can be distributed throughout the switch chassis

l Layer 3 switching uses one of these two methods depending on the platform Oslash Route caching Also known as flow-based or demand-based

switching a Layer 3 route cache is built in hardware since the switch sees traffic flow into the switch

Oslash Topology-based Information from the routing table is used to populate the route cache regardless of traffic flow The populated route cache is called the forwarding information base (FIB) CEF builds the FIB 56

29

Explaining CEF-based Multilayer Switches

bull CEF Operation modesbull Central CEFbull Distributed CEF

bull CEF-based Cisco MultiLayer SwitchesCatalyst 2970 Catalyst 3550Catalyst 3560 Catalyst 3750 Catalyst 4500Catalyst 4948 Catalyst 6500 two card modules of CP and DP

57

Explaining CEF-based Multilayer Switches

l Cisco Layer 3 devices can use a variety of methods to switch packets from one port to another The most basic method of switching packets between interfaces is called process switching Process switching moves packets between interfaces on a scheduled basis based on information in the routing table and the Address Resolution Protocol (ARP) cache As packets arrive they are put in a queue to wait for further processing When the scheduler runs the outbound interface is determined and the packet is switched Waiting for the scheduler introduces latency

l To speed the switching process strategies exist to switch packets on demand as they arrive and to cache the information necessary to make packet-forwarding decisions

l CEF uses these strategies to expediently switch data packets to their destination It caches information generated by the Layer 3 routing engine CEF caches routing information in one table (the FIB) and caches Layer 2 next-hop addresses for all FIB entries in an adjacency table Because CEF maintains multiple tables for forwarding information parallel paths can exist and enable CEF to load balance per packet

58

30

Explaining CEF-based Multilayer Switches

l CEF operates in one of two modesOslash Central CEF The FIB and adjacency tables reside on the route

processor and the route processor performs the express forwarding Use this mode when line cards are not available for CEF switching or when features are not compatible with distributed CEF

Oslash Distributed CEF (dCEF) Supported only on Cisco Catalyst 6500 switches Line cards maintain identical copies of the FIB and adjacency tables The line cards can perform the express forwarding by themselves relieving the main processor of being involved in the switching operation Distributed CEF uses an interprocesscommunications (IPC) mechanism to ensure that the FIBs and adjacency tables are synchronized on the route processor and line cards

59

Identifying the Multilayer Switch Packet Forwarding Process

60

31

Identifying the Multilayer Switch Packet Forwarding Process

61

l CEF separates the control plane hardware from the data plane hardware and switching ASICs separate the control plane and data plane thereby achieving higher data throughput Oslash The control plane is responsible for building the FIB and adjacency

tables in software Oslash The data plane is responsible for forwarding IP unicast traffic using

hardware l When traffic cannot be processed in hardware the traffic must receive

processing in software by the Layer 3 engine thereby not receiving the benefit of expedited hardware-based forwarding A number of different packet types may force the Layer 3 engine to process them Some examples of IP exception packets are the following Oslash IP packets that use IP header options (Packets that use TCP header options are

switched in hardware because they do not affect the forwarding decision) Oslash Packets that have an expiring IP Time to Live (TTL) counter Oslash Packets that are forwarded to a tunnel interface Oslash Packets that arrive with non-supported encapsulation types Oslash Packets that are routed to an interface with non-supported encapsulation types Oslash Packets that exceed the maximum transmission unit (MTU) of an output interface

and must be fragmented

62

Identifying the Multilayer Switch Packet Forwarding Process

32

Identifying the Multilayer Switch Packet Forwarding Process

l CEF-based tables are initially populated and used as follows Oslash The FIB is derived from the IP routing table and is arranged for maximum lookup throughput Oslash The adjacency table is derived from the ARP table and it contains Layer 2 rewrite (MAC) information for the

next hop Oslash CEF IP destination prefixes are stored in the TCAM table from the most specific to the least specific entry Oslash When the CEF TCAM table is full a wildcard entry redirects frames to the Layer 3 engine Oslash When the adjacency table is full a CEF TCAM table entry points to the Layer 3 engine to redirect the

adjacency Oslash The FIB lookup is based on the Layer 3 destination address prefix (longest match)

l The FIB table is updated when the following occursOslash An ARP entry for the destination next hop changes ages out or is removed Oslash The routing table entry for a prefix changes Oslash The routing table entry for the next hop changes

l These are the basic steps for initially populating the adjacency tableOslash Step 1 The Layer 3 engine queries the switch for a physical MAC address Oslash Step 2 The switch selects a MAC address from the chassis MAC range and assigns it to the Layer 3 engine

This MAC address is assigned by the Layer 3 engine as a burned-in address for all VLANs and is used by the switch to initiate Layer 3 packet lookups

Oslash Step 3 The switch installs wildcard CEF entries which point to drop adjacencies (for handling CEF table lookup misses)

Oslash Step 4 The Layer 3 engine informs the switch of its interfaces participating in MLS (MAC address and associated VLAN) The switch creates the (MAC VLAN) Layer 2 CAM entry for the Layer 3 engine

Oslash Step 5 The Layer 3 engine informs the switch about features for interfaces participating in MLS Oslash Step 6 The Layer 3 engine informs the switch about all CEF entries related to its interfaces and connected

networks The switch populates the CEF entries and points them to Layer 3 engine redirect adjacencies

63

Identifying the Multilayer Switch Packet Forwarding Process

64

33

Identifying the Multilayer Switch Packet Forwarding Processl These are the steps that would occur when you use CEF to forward frames between

host A and host B on different VLANs Oslash Step 1 Host A sends a packet to host B The switch recognizes the frame as a

Layer 3 packet because the destination MAC (MAC-M) matches the Layer 3 engine MAC

Oslash Step 2 The switch performs a CEF lookup based on the destination IP address (IP-B) The packet hits the CEF entry for the connected (VLAN20) network and is redirected to the Layer 3 engine using a glean adjacency

Oslash Step 3 The Layer 3 engine installs an ARP throttling adjacency in the switch for the host B IP address

Oslash Step 4 The Layer 3 engine sends ARP requests for host B on VLAN20 Oslash Step 5 Host B sends an ARP response to the Layer 3 engine Oslash Step 6 The Layer 3 engine installs the resolved adjacency in the switch

(removing the ARP throttling adjacency) Oslash Step 7 The switch forwards the packet to host B Oslash Step 8 The switch receives a subsequent packet for host B (IP-B) Oslash Step 9 The switch performs a Layer 3 lookup and finds a CEF entry for host B

The entry points to the adjacency with rewrite information for host B Oslash Step 10 The switch rewrites packets per the adjacency information and forwards

the packet to host B on VLAN2065

Populating FIB and Adjacency Table

66

34

Identifying the Multilayer Switch Packet Forwarding Process

67

Identifying the Multilayer Switch Packet Forwarding Process

68

l An example of ARP throttling which consists of these stepsOslash Step 1 Host A sends a packet to host B Oslash Step 2 The switch forwards the packet to the Layer 3 engine

based on the ldquogleanrdquo entry in the FIB A glean adjacency entry indicates that a particular next hop should be directly connected but there is no MAC header rewrite information available

Oslash Step 3 The Layer 3 engine sends an ARP request for host B and installs the drop adjacency for host B At this point subsequent frames destined for host B from host A are dropped (ARP throttling)

Oslash Step 4 Host B responds to the ARP request The Layer 3 engine installs an adjacency for host B and removes the drop adjacency

35

69

ARP Throttling

l When a router is directly connected to a multiaccess segment(Ethernet) the router maintains an additional prefix for the subnet

l This subnet prefix points to a glean adjacencyl When a router receives a packets that needs to be forwarded to a

specific host the adjacency database is gleaned for a specific prefix l If the prefix does not exist the subnet prefix is consultedl The glean adjacency indicates that any address with this range

should be forwarded to the Layer 3 engine ARP processing

70

ARP Throttling

1 Host A sends a packet to Host Bl CEF lookup shows glean adjacency (ARP entry does not exist so

no entry in adjacency table) l No rewrite information exists2 Packet passed to Layer 3 Engine for processing

36

71

ARP Throttling

3 Obtaining rewrite informationl L3 Engine sends an ARP Request for Host B and waits for ARP Replyl Throttling Adjacency While in glean state subsequent packets to that

host are dropped so that input queues do not fill and so the Layer 3 engine isnrsquot busy with duplicate ARP Requests (Note Ciscorsquos routers drop the first packet when there is no ARP entry while sending the ARP Request)

ARP Request

Drop packets until ARP Reply received (Throttling Adjacency)

Throttling Adjacency is removed when no ARP Reply is received in 2 seconds This allows for another packet to initiate a new ARP Request

Throttling Adjacencyrelieves the Layer 3 Engine of excessive ARP processing or ARP-based DoSattacks

72

ARP Throttling

4 Host B sends ARP Reply

ARP Reply

Drop packets until ARP Reply received (Throttling Adjacency)

37

73

ARP Throttling

5 The Layer 3 Engine installs Adjacency for Host B and removes the throttling (drop) adjacency

Next Packet Rewrite (Coming)

Drop packets until ARP Reply received (Throttling Adjacency)

1020102

Describing CEF Configuration Commands

74

38

Verifying CEF

75

l Does the Ideal switching (CEF DCEF) used l CEF table is perfected or accuracy

Common CEF Problems

76

39

Verify Layer 3 Switching

Switchshow interface type modport | port-channel number | begin L3

Switchshow interface fastethernet 33 | begin L3L3 in Switched ucast 0 pkt 0 bytes - mcast 12 pkt 778 bytes mcastL3 out Switched ucast 0 pkt 0 bytes - mcast 0 pkt 0 bytes

4046399 packets input 349370039 bytes 0 no bufferReceived 3795255 broadcasts 2 runts 0 giants 0 throttles

Switch

77

Displaying Hardware Layer 3 Switching Statistics

Switchshow interfaces type modport | port-channel number include switched

Switchshow interfaces gigabitethernet 95 | include switchedL2 Switched ucast 8199 pkt 1362060 bytes - mcast 6980 pkt 371952 bytesL3 in Switched ucast 3045 pkt 742761 bytes - mcast 0 pkt 0 bytes mcastL3 out Switched ucast 2975 pkt 693411 bytes - mcast 0 pkt 0 bytes

78

40

Adjacency Information

Switchshow adjacency [type modport | port-channel number | detail | internal | summary]

Switchshow adjacency gigabitethernet 95 detailProtocol Interface AddressIP GigabitEthernet95 1722053206(11)

504 packets 6110 bytes00605C865B82000164F83FA50800ARP 034931

79

Debugging CEF Operations

Switchdebug ip cef drops | access-list | receive | events | prefix-ipc | table

bull Displays debug information for CEF

Switchdebug ip cef ipc | interface-ipc

bull Displays debug information related to IPC in CEF

Switchping ip

bull Performs an extended ping

80

41

l Things to check(1) Check Layer 3 operations(2) Verify the FIB and adjacency table

l Step 1 Verify CEFshow ip cef summary show ip cef vlan 10

l Step 2 Verify the running configurationl Step 3 Verify the routing

Switchshow ip route | include 1921681500l Step 4 Verify an ARP entry on the route processor

At Switch check ARP atable for 1921681993

l Step5 Verify the CEF FIB table entry for the routeSwitch show ip cef 1921681500

l Step 6 Verify an adjacency table entry for the destinationSwitchshow adjacency detail | begin 1921681993

l Step 7 Verify CEF from the supervisor engine for modular switch platforms

Troubleshooting Layer 3 CEF-Based MLS

81

Module 4 Implementing Inter-VLAN routing (flash v50)

41 Describing Routing Between VLANs42 Enabling Routing Between VLANS43 Deploying CEF-Based Multilayer Switching44 Inter-VLAN Routing Lab Exercises

42

Module 5 Implementing Inter-VLAN routing

l Multilayer Switched Networksl CCNP 3 version 5l Rick Graziani

84

Internetwork Communications

l Can two hosts on different subnets communicate without a routerl What would happen if a host tried to ping another host

No they cannot communicatel Would it send an ARP Request Why or why not

The host would not send an ARP Request because there is no default-gateway

Cgtping 1721630100

43

85

Trunking with Default Gateway

l What difference would it make if these hosts were on different VLANsThe Broadcasts would not be forwarded out all ports by the switch

l Why does the host send the ARP Request to the router and not the destination host After all theyrsquore on the same switch

The host doesnrsquot know where the destination host is just that itrsquos not on itsrsquo network

Cgtping 1721630100

86

Internetwork Communications

l Then Destination MAC Address is that of the same device as the Destination IP Address

l Check ARP cache for entry of Destination IP Address and its MAC AddressIf no entry ARP Request Destination IP Address asking for MAC Address

bull Then Destination MAC Address will be that of the Default Gatewaybull Check ARP cache for entry of Default Gatewayrsquos IP Address and its MAC

Addressndash If no entry ARP Request Default Gatewayrsquos IP Address asking for MAC

Address

44

87

Inter-VLAN Routing

l VLAN is a logical group of ports usually belonging to a single IP subnet to control the size of the broadcast domain

l Even though devices in different VLANs may be ldquophysicallyrdquo connected these devices cannot communicate without the services of a default gateway a router

This is known as Inter-VLAN Routing

88

Inter-VLAN Routingl The following devices are

capable of providing inter-VLAN routing

Any external router or group of routers with a separate interface in each VLANAny external router with an interface that supports trunking(router on a stick)Any Layer 3 multilayer Catalyst switch

Or trunk port

45

89

External Router separate interface in each VLAN

l Download PT-Topology-MLS-1 l Configure the router to route between VLANsl Is a routing protocol necessary Why or why not

No because all of our networks are directly connected

90

Router-on-a-Stick

bull Download PT-Topology-MLS-2pktbull Single trunk link carries traffic for multiple VLANs to and from router

172161010024 172162010024

46

91

Configure Router On A Stick 8021Q Trunk Link

interface GigabitEthernet11switchport mode trunk

interface GigabitEthernet50no shutdown Does not show in configinterface GigabitEthernet501description VLAN 1encapsulation dot1Q 1 nativeip address 1721611 2552552550interface GigabitEthernet5010description VLAN 10encapsulation dot1Q 10ip address 17216101 2552552550interface GigabitEthernet5020description VLAN 20encapsulation dot1Q 20ip address 17216201 2552552550interface GigabitEthernet5030description VLAN 30encapsulation dot1Q 30ip address 17216301 2552552550interface GigabitEthernet5040description VLAN 40encapsulation dot1Q 40ip address 17216401 2552552550

172161010024

172162010024

bull Router on a stick is very simple to implement because routers are usually available in every network

92

Multilayer Switches

Layer 2 Interfacesl Access portmdash Carries

traffic for a single VLANl Which are the access

portsl Trunk portmdash Carries

traffic for multiple VLANs using Inter-Switch Link (ISL) encapsulation or 8021Q tagging

l Which are the trunk ports

47

93

Connecting VLANs with Multilayer Switches

l Cisco IOS Switchport commandThe switchport command configures an interface as a Layer 2 interface

Note The no switchport command configures an interface as a Layer 3 interface

SwitchA(config)interface fa 01SwitchA(config-if-range)switchport mode accessSwitchA(config-if-range)switchport access vlan 10

SwitchA(config)interface gigabitethernet 12SwitchA(config-if-range)switchport trunk encapsulation dot1qSwitchA(config-if-range)switchport mode trunk

Layer 2 Interfaces

94

Layer 3 Interfaces

The Catalyst multilayer switches support three different types of Layer 3 interfaces

l Routed portmdash A pure Layer 3 interface similar to a routed port on a Cisco IOS router

l Switch virtual interface (SVI)mdash A virtual VLAN interface for inter-VLAN routing In other words SVIs are the virtual routed VLAN interfaces

l Bridge virtual interface (BVI)mdash A Layer 3 virtual bridging interface (Not discussed)

48

95

MLS Layer 3 Interface Routed Port

l Download PT-Topology-MLS-3pktl A routed port is a physical port that acts similarly to a port on a

traditional router with Layer 3 addresses configured Not associated with a particular VLAN Like a regular router interface except that it does not support subinterfaces

96

Core1(config) interface GigabitEthernet01Core1(config-if) no switchportCore1(config-if) ip address 19216815 255255255252

Core1(config) interface GigabitEthernet02Core1(config-if) no switchportCore1(config-if) ip address 19216811 255255255252

MLS Layer 3 Interface Routed Port

1921681430

1921681030 1921681830

bull Configure the other Core and Distribution switches

49

97

Core2(config) interface GigabitEthernet01Core2(config-if) no switchportCore2(config-if) ip address 19216816 255255255252

Core2(config) interface GigabitEthernet02Core2(config-if) no switchportCore2(config-if) ip address 19216819 255255255252

MLS Layer 3 Interface Routed Port

1921681430

1921681030 1921681830

98

DLS1(config) interface GigabitEthernet02DLS1(config-if) no switchportDLS1(config-if) ip address 19216812 255255255252

MLS Layer 3 Interface Routed Port

1921681430

1921681030 1921681830

DLS2(config) interface GigabitEthernet02DLS2(config-if) no switchportDLS2(config-if) ip address 192168110 255255255252

50

99

MLS Layer 3 Interface SVI

l Switch virtual interfaces (SVI)Layer 3 interfaces that are configured on multilayer Layer 3 switches Used for inter-VLAN routing A virtual VLAN interface

Associated with the VLAN-IDEnable routing capability on that VLAN

l Note These are virtual interfaces

SVI

100

MLS Layer 3 Interface SVI

l To configure communication between VLANs you must configure each SVI with an IP address and subnet mask in the chosen address range for that subnet

l The IP address associated with the VLAN interface is the default gateway of the workstation

DLS1(config) interface vlan 1DLS1(config-if) ip address 1721611 2552552550DLS1(config-if) no shutdownDLS1(config) interface vlan 10DLS1(config-if) ip address 17216101 2552552550DLS1(config-if) no shutdownDLS1(config) interface vlan 20DLS1(config-if) ip address 17216201 2552552550DLS1(config-if) no shutdown

51

101

MLS Layer 3 Interface SVI

bull The switch routes frames between VLANs directly on the switch via hardware switching without requiring an external router ndash An SVI is mostly implemented to interconnect the VLANs on the

Building Distribution submodules or the Building Access submodules in the multilayer switched network

DLS1(config) interface vlan 1DLS1(config-if) ip address 1721611 2552552550DLS1(config-if) no shutdownDLS1(config) interface vlan 10DLS1(config-if) ip address 17216101 2552552550DLS1(config-if) no shutdownDLS1(config) interface vlan 20DLS1(config-if) ip address 17216201 2552552550DLS1(config-if) no shutdown

102

MLS Layer 3 Interface BVI

l httpwwwciscocomenUStechtk389tk689technologies_tech_note09186a0080094663shtml

l BVIPDFl A bridge virtual interface (BVI) is a Layer 3 virtual interface that acts

like a normal SVI to route packets across bridged or routed domains Bridging Layer 2 packets across Layer 3 interfaces is a legacy method of moving frames in a network To configure a BVI to route use the integrated routing and bridging (IRB) feature which makes it possible to route a given protocol between routed interfaces and bridge groups within the same device Specifically routable traffic is routed to other routed interfaces and bridge groups while local or unroutable traffic is bridged among the bridged interfaces in the same bridge group As a result bridging creates a single instance of spanning tree in multiple VLANs or routed subnets This type of configuration complicates spanning tree and the behavior of other protocols which in turn makes troubleshooting difficult

l In todays network however bridging across routed domains is highly discouraged

52

103

IP Broadcast Forwarding

l DHCP use IP subnet broadcasts to the 255255255255 address l Routers do not route these packets by default l Routers and Layer 3 switches can be configured to forward these

DHCP and other UDP broadcast packets to a unicast directed broadcast address

104

DHCP Relay Agent

l Layer 3 devices do not pass broadcastsl What issue does this cause for DHCP Servers

Each subnet requires a DHCP serverl To enable the DHCP relay agent feature configure the ip helper-address

command with the DHCP server IP address(es) on the client VLAN interfaces

MLS(config)interface vlan 1MLS(configif)description DHCP Server VLANMLS(config-if)ip address 10111 2552552550MLS(config-if)no ip directed-broadcast

MLS(config)interface vlan 2MLS(config-ig)description DHCP clientsMLS(config-if)ip address 10211 2552552550MLS(config-if)no shutdownMLS(config-if)no ip directed-broadcastMLS(config-if)ip helper-address 1011254

53

105

DHCP Relay AgentThe ip helper-address command not only forwards DHCP UDP packets but also forwards TFTP DNS Time NetBIOS name server and BOOTP packets by default

By default the ip helper-address command forwards the eight UDPs services

106

DHCP Relay Agent

l ip helper-address - make sure the ip directed-broadcast is notconfigured on any outbound interfaces that the UDP broadcast packets need to traverse

l The no ip directed-broadcast command configures the router or switch to prevent the translation of a directed broadcast to a physical broadcast (MAC FF)

l This is a default behavior since Cisco IOS Release 120 implemented as a security measure

MLS(config)interface vlan 1MLS(configif)description DHCP Server VLANMLS(config-if)ip address 10111 2552552550MLS(config-if)no ip directed-broadcast

MLS(config)interface vlan 2MLS(config-ig)description DHCP clientsMLS(config-if)ip address 10121 2552552550MLS(config-if)no shutdownMLS(config-if)no ip directed-broadcastMLS(config-if)ip helper-address 1011254

See Improving Security on Routers httpwwwciscocomwarppublic70721html

54

107

UDP Broadcast Forwarding

l To specify additional UDP broadcasts for forwarding by the router when configuring the ip helper-address interface command use the following global commandip forward protocol udp udp_ports

l Use the no option to remove default or configured applications

Router(config)interface vlan 1Router(config-if)ip address 1010011 2552552550Router(config-if)ip helper-address 102001254

Router(config)ip forward-protocol udp mobile-ipRouter(config)no ip forward-protocol udp netbios-ns

Traditional and CEF Based Multilayer Switching

55

109

Multilayer Switching

l Multilayer switching - ability of a Catalyst switch to support switching and routing of packets in hardware

Optional support for Layers 4 through 7 switching in hardware as well

l Hardware switching A route processor (Layer 3 engine) must download software-based routing switching access lists QoS and other information to the hardware for packet processing

Traditional MLS CEF-Based MLS

110

Traditional and CEF-based MLS

l Cisco Catalyst switches use eitherTraditional multilayer switching (traditional MLS)

A legacy featureCisco Express Forwarding (CEF)-based MLS architecture

All leading-edge Catalyst switches support CEF-based multilayer switching

Traditional MLS CEF-Based MLS

56

111

Traditional MLS

l Specialized application-specific integrated circuits (ASICs) perform Layer 2 rewrite operations of routed packets

Source MAC addressDestination MAC addressCyclic redundancy check (CRC)

Because the source and destination MAC addresses change during Layer 3 rewrites the switch must recalculate the CRC for these new MAC addresses

112

Traditional MLS

l Switch learns Layer 2 rewrite information from an MLS router via an MLS protocol

netflow-based switching l With traditional MLS the Layer 3 engine (route processor) and

switching ASICs work together to build Layer 3 entries on the switch l Each entry can be populated in one of three ways

Source IP address only Source and destination IP addressesFull Flow Information with Layer 4 protocol information

57

113

Traditional MLS

l The switch forwards the first packet in any flow to the Layer 3 engine for processing using software switching

l After the routing of the first packet in the flow the Layer 3 engine programs the hardware-switching components for routing for subsequent packets

dot1q Tag(inside Eth Hdr)

Ethernet Header IP Header IP Data

VLAN 1

D-MAC= 00-00-0C-11-11-11

S-MAC= 00-AA-00-11-11-11

S-IP = 101110

D-IP = 101220

S-MAC= 00-AA-00-11-11-11

114

l When workstation A sends a packet to workstation B workstation A sends the packet to its default gateway

The default gateway is the RSM l The switch (MLS-SE) recognizes this packet as an MLS candidate packet

because the destination MAC address matches the MAC address of the MLS router (MLS-RP)

l As a result the switch creates a candidate entry for this flow

MLS-SE

MLS-RPMLS-RPThe Destination MAC Address is one of the routerrsquos interfacesThere is not an existing flow so I will flag this as a candidate packet

Candidate Packet Info

Layer 3 InfoS-IP 101110 D-IP 101220

Layer 2 InfoS-MAC 00-AA-00-11-11-11 D-MAC 00-00-0C-11-11-11

dot1q Tag(inside Eth Hdr)

Ethernet Header IP Header IP Data

VLAN 1

D-MAC= 00-00-0C-11-11-11

S-MAC= 00-AA-00-11-11-11

S-IP = 101110

D-IP = 101220

58

115

l Next the router accepts the packets from workstation A rewrites the Layer 2 MAC addresses and CRC and forwards the packet to workstation B

l The switch refers to the routed packet from the RSM as the enabler packet

MLS-SE

MLS-RP

dot1q Tag(inside Eth Hdr)

Ethernet Header IP Header IP Data

VLAN 2 D-MAC= 00-AA-00-22-22-22

S-MAC= 00-00-0C-22-22-22

S-IP = 101110

D-IP = 101220

116

l MLS-SE recognizes various matches including CAM details not includedl Basically the MLS-SE recognizes that the packet going out of VLAN 2

was the same one that came in on VLAN 1l The switch upon seeing both the candidate and enabler packets creates an

MLS entry in hardware (MLS Cache) such that the switch rewrites and forwards all future packets matching this flow

MLS-SE

MLS-RP

dot1q Tag(inside Eth Hdr)

Ethernet Header IP Header IP Data

VLAN 2

D-MAC= 00-AA-00-22-22-22

S-MAC= 00-00-0C-22-22-22

S-IP = 101110

D-IP = 101220

Candidate Packet Info

Layer 3 InfoS-IP 101110 D-IP 101220

Layer 2 InfoS-MAC 00-AA-00-11-11-11 D-MAC 00-00-0C-11-11-11

59

117

l As future packets from the ldquoflowrdquo arrive the MLS-SE uses the destination IP address to look up the entry in the MLS cache

l Finding a match rewrite engine modifies the necessary header information and forwards the frame (the packet is not forwarded to the router)

l The rewrite operation modifies all the same fields initially modified by the router for the first packet including the source MAC and destination MAC addresses

MLS-SE

MLS-RPCandidate Packet Info

Layer 3 InfoS-IP 101110 D-IP 101220

Layer 2 InfoS-MAC 00-AA-00-11-11-11 D-MAC 00-00-0C-11-11-11

Dst IP Src IP Port Dst Port

Src Port

Dst MAC

Src MAC

VLAN Interface

101220 101110 TCP 23 1238 00-AA-00-22-22-22

00-00-0C-22-22-22

2 31

Future Packets

MLS Cache

Found match in MLS Cache rewrite Ethernet

Header and send directly to Host B forget the

router

118

CEF-based MLS

60

119

CEF

l CEF-based MLS forwarding model is used to download the control plane information such as the access lists to the data plane on the supervisor port or line card for hardware switching of packets

Control plane represents the Layer 3 engine (route processor) Data plane represents the hardware components such as ASICs used by the switch for hardware switching

l CEF is a topology-based forwarding model in which all routing information is prepopulated into a forwarding information base (FIB)

l Result is switches can quickly look up routing information such as IP adjacencies and next-hop IP and MAC addresses

120

CEF

The two main components of CEF are FIB Adjacency Table

bull Forwarding information baseMake IP destination switching decisions Similar to a routing tableMirror image of the forwarding information contained in the IP routing table When routing or topology changes occur in the network the IP routing table is updated and those changes are reflected in the FIB Maintains next-hop address information based on the information in the IP routing table Both the Layer 3 engine and the hardware-switching components maintain a FIB

Routing Table

61

121

CEF

l Adjacency tablesNetwork nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer (OSPF EIGRP)A router normally maintains

Routing table containing Layer 3 network and next-hop informationARP table containing Layer 3 to Layer 2 address mapping These tables are kept independently

122

CEF

l Adjacency tablesThe FIB keeps the Layer 3 next-hop address for each entry To streamline packet forwarding even more the FIB has corresponding Layer 2 information for every next-hop entry This portion of the FIB is called the adjacency table consisting of the MAC addresses of nodes that can be reached in a single Layer 2 hop

Layer 2 MAC Addresses Next Hop Information

Next hop

62

123

CEF

l Adjacency tables (summary more detail coming)Built from the ARP table As a next-hop address receives a valid ARP entry the adjacency table is updated If an ARP entry does not exist the FIB entry is marked as ldquoCEF gleanrdquo This means that the Layer 3 forwarding engine cant forward the packet in hardware due to the missing Layer 2 next-hop address The packet is sent to the Layer 3 engine so that it can generate an ARP request and receive an ARP reply This is known as the ldquoCEF gleanrdquo state where the Layer 3 engine must glean the next-hop destinations MAC address

No ARP entry L3 forwarding engine canrsquot

forward packet in hardware

must send to L3 Engine

Irsquoll generate the ARP Request and get an

ARP Reply

124

CEF

l Adjacency tablesWhat happens to subsequent packets while FIB entry is in glean state (L3 engine is sending ARP Request)

These packets are droppedSo input queues do not fillSo Layer 3 engine does not become too busy worrying about the need for duplicate ARP requests

This is called ARP throttling or throttling adjacency If an ARP reply is not received in two seconds the throttling is released so that another ARP request can be triggered

After ARP reply is receivedThrottling is releasedFIB entry can be completed Subsequent packets can be forwarded in hardware

63

125

ARP Throttling

1 Host A sends a packet to Host Bl CEF lookup shows glean adjacency (ARP entry does not exist so

no entry in adjacency table) l No rewrite information exists2 Packet passed to Layer 3 Engine for processing

126

ARP Throttling

3 Obtaining rewrite informationl L3 Engine sends an ARP Request for Host B and waits for ARP Replyl Throttling Adjacency While in glean state subsequent packets to that

host are dropped so that input queues do not fill and so the Layer 3 engine isnrsquot busy with duplicate ARP Requests (Note Ciscorsquos routers drop the first packet when there is no ARP entry while sending the ARP Request)

ARP Request

Drop packets until ARP Reply received (Throttling Adjacency)

Throttling Adjacency is removed when no ARP Reply is received in 2 seconds This allows for another packet to to initiate a new ARP Request

Throttling Adjacencyrelieves the Layer 3 Engine of excessive ARP processing or ARP-based DoS attacks

XX

X

64

127

ARP Throttling

4 Host B sends ARP Reply

ARP Reply

Drop packets until ARP Reply received (Throttling Adjacency)

XX

X

128

ARP Throttling

5 The Layer 3 Engine installs Adjacency for Host B and removes the throttling (drop) adjacency

Next Packet Rewrite (Coming)

Drop packets until ARP Reply received (Throttling Adjacency)

1020102Host Brsquos MAC Address

65

129

Packet Rewrite

Egress Packet

130

Packet Rewrite

The switch receives another packetl After a multilayer switch finds valid entries in the FIB and adjacency

tables a packet is almost ready to be forwarded l One step remainsmdashthe packet header information must be rewritten l Multilayer switching occurs as quick table lookups

Find the next-hop addressOutbound switch port

l The IP header must also be adjusted as if a traditional router had done the forwarding (TTL)

Default Gateway

Host A TTL

L3 ChecksumL2 Checksum

1020102Host Brsquos MAC Address

66

131

Packet Rewrite

The packet rewrite engine makes the following changes to the packet just prior to forwarding

l Layer 2 destination addressmdashChanged to the next-hop devices MAC addressl Layer 2 source addressmdash Changed to the outbound Layer 3 switch interfaces

MAC addressl Layer 3 IP Time To Live (TTL)mdash Decremented by one as one router hop has

just occurredl Layer 2 frame checksummdashRecalculated to include changes to the Layer 2 and

Layer 3 headersl Layer 3 IP checksummdash Recalculated to include changes to the IP header

Default Gateway

TTL

L3 ChecksumL2 Checksum

Host B MAC Add

TTL - 1

L2 Checksum L3 Checksum

Host AL3 switch outbound interface

1020102Host Brsquos MAC Address

132

Packet Rewrite

l A traditional router would normally make the same changes to each packet

l The multilayer switch must act as if a traditional router were being used making identical changes

The multilayer switchCan do this very efficiently with dedicated packet rewrite hardware and with address information obtained from table lookups

Default Gateway

TTL

L3 ChecksumL2 Checksum

Host B MAC Add

TTL - 1

L2 Checksum L3 Checksum

Host AL3 switch outbound interface

1020102Host Brsquos MAC Address

67

133

Packet Rewrite

l The switch performs a Layer 3 lookup and finds a CEF entry for Host B

l The switch rewrites packets per the adjacency information and forwards the packet to Host B on its VLAN

Default Gateway

TTL

L3 ChecksumL2 Checksum

Host B MAC Add

TTL - 1

L2 Checksum L3 Checksum

Host AL3 switch outbound interface

1020102Host Brsquos MAC Address

134

CEF

l Catalyst switches do not support routing of all types of frames in hardware l For example the following list details common frame types that are not supported by

hardware switchingPackets with IP header optionsPackets sourced from or destined to tunnel interfacesPackets using Ethernet encapsulation types other than ARPAPackets that require fragmentation (exceed MTU of the interface)

l Two types of CEFCentral CEF ndash Forwarding decisions done by ASIC that is central to all interfacesDistributed CEF (dCEF) ndash Forwarding decisions done on independently on interfaces or line modules (faster)

68

135

Switching Table Architectures

l Multilayer switches build the following tables for centralized or distributed switching in hardware using high-speed memory tables

Routing (CEF FIB and adjacency)BridgingQoSAccess Control ist (ACL) tables

136

Switching Table Architectures - Details

Multilayer switches deploy memory tables using specialized memory architectures

l CAM (content addressable memory) Provides only two results 0 (true) or 1 (false) For exact matches such as MAC address tables

l TCAM (ternary content addressable memory ) ndash Ternary Logic Provides three results 0 (donrsquot care) 1 (true) 2 (false) Ternary Logic Ternary number system (Base 3) - tritsFor longest matches such as IP routing tables organized by IP prefixes

CAM TCAM

69

137

CAM

l For Layer 2 switching tables l With CAM tables switches must find exact matches or the switches

use a default behavior l Switch must find an exact match to a destination MAC address or

the switch floods the packet out all ports in the VLAN

138

CAM

l The information a switch uses to perform a lookup in a CAM table is called a key

Destination MAC address VLAN ID

Key

VLAN ID

Key

70

139

TCAM

l TCAM is a specialized CAM designed for rapid table lookups

l For example the Catalyst 2950 3550 4500 and 6500 families of switches use TCAM to handle ACL lookups at line rate

l Thus applying ACLs does not affect the performance of the switch

l Single lookup provides the following information

Layer 2Layer 3ACL

140

TCAMl VMR (value mask and result) refers to the

format of entries in TCAM l The ldquovaluerdquo in VMR refers to the pattern that

is to be matchedExamples include IP addresses and protocol ports

l The ldquomaskrdquo refers to the mask bitsassociated with the pattern and determines the prefix

l The ldquoresultrdquo refers to the result or action that occurs in the case where a lookup returns a hit for the pattern and mask

This result might be a ldquopermitrdquo or ldquodenyrdquo in the case of a TCAM for ACLs Another example of a result is a pointer to an entry in the hardware adjacency table that contains the next-hop MAC rewrite information in the case of a TCAM used for IP routing

l If TCAM becomes full the wildcard entry will force the packet to route via the routing table

71

141

CEF-Based MLS Lookups

1 Layer 3 packets initiate TCAM lookup2 The longest match returns adjacency with rewrite information3 The packet is rewritten per adjacency information and forwarded

142

Inter-VLAN Routing Summary

l A router on a stick can be used to route between VLANs using either ISL or 8021Q as the trunking protocol

l A router on a stick requires subinterfaces one for each VLAN

l Verify inter-VLAN routing by generating IP packets between two subnets

l Multilayer switches can forward traffic both at Layer 2 and at Layer 3

l Multilayer switches rewrite the Layer 2 and Layer 3 header using tables held in hardware

72

143

Configuring Inter-VLAN Routing Through an SVI

Switch(config)ip routing

Step 1 Configure IP routing

Switch(config)router ip_routing_protocol ltoptionsgt

Step 4 Configure the IP routing protocol if needed

Switch(config)interface vlan vlan-idStep 2 Create an SVI interface

Switch(config-if)ip address ip-address mask

Step 3 Assign an IP address to the SVI

144

Configuring a Routed Port

Switch(config)ip routing

Step 1 Configure IP routing

Switch(config)router ip_routing_protocol ltoptionsgt

Step 4 Configure the IP routing protocol if needed

Switch(config-if)no switchportStep 2 Create a routed port

Switch(config-if)ip address ip-address mask

Step 3 Assign an IP address to the routed port

73

145

Enabling CEF

Switch(config-if)ip cef

Switch(config-if)ip route-cache cef

The commands required to enable CEF are platform dependent

ndash On the Cisco Catalyst 4000 switch

ndash On the Cisco Catalyst 3550 switch

146

Verifying CEFSwitchshow ip cef [type modport | vlan_interface] [detail]

Switch show ip cef vlan 11 detail

IP CEF with switching (Table Version 11) flags=0x010 routes 0 reresolve 0 unresolved (0 old 0 new) peak 013 leaves 12 nodes 14248 bytes 14 inserts 1 invalidations0 load sharing elements 0 bytes 0 referencesuniversal per-destination load sharing algorithm id 4B936A242(0) CEF resets 0 revisions of existing leavesResolution Timer Exponential (currently 1s peak 1s)0 in-place0 aborted modificationsrefcounts 1061 leaf 1052 node

Table epoch 0 (13 entries at this epoch)

1721611024 version 6 epoch 0 attached connected0 packets 0 bytesvia Vlan11 0 dependenciesvalid glean adjacency

74

147

Verify Layer 3 SwitchingSwitchshow interface type modport | port-channel number | begin L3

Switchshow interface fastethernet 33 | begin L3L3 in Switched ucast 0 pkt 0 bytes - mcast 12 pkt 778 bytes mcastL3 out Switched ucast 0 pkt 0 bytes - mcast 0 pkt 0 bytes

4046399 packets input 349370039 bytes 0 no bufferReceived 3795255 broadcasts 2 runts 0 giants 0 throttles

Switch

148

Displaying Hardware Layer 3 Switching StatisticsSwitchshow interfaces type modport | port-channel number include switched

Switchshow interfaces gigabitethernet 95 | include switchedL2 Switched ucast 8199 pkt 1362060 bytes - mcast 6980 pkt 371952 bytesL3 in Switched ucast 3045 pkt 742761 bytes - mcast 0 pkt 0 bytes mcastL3 out Switched ucast 2975 pkt 693411 bytes - mcast 0 pkt 0 bytes

75

149

Adjacency Information

Switchshow adjacency [type modport | port-channel number | detail | internal | summary]

Switchshow adjacency gigabitethernet 95 detailProtocol Interface AddressIP GigabitEthernet95 1722053206(11)

504 packets 6110 bytes00605C865B82000164F83FA50800ARP 034931

150

Debugging CEF Operations

Switchdebug ip cef drops | access-list | receive | events | prefix-ipc | tablebull Displays debug information for CEF

Switchdebug ip cef ipc | interface-ipc

bull Displays debug information related to IPC in CEF

Switchping ip

bull Performs an extended ping

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CEF Summaryl Layer 3 switching is high-performance packet switching in

hardwarel MLS functionality can be implemented through CEFl CEF uses tables in hardware to forward packetsl Specific commands are used to enable and verify

CEF operationsl Commands to enable CEF are platform dependentl CEF problems can be matched to specific solutionsl Specific commands are used to troubleshoot and solve CEF

problemsl Ordered steps assist in troubleshooting CEF-based problems

l SWITCH v60 MCMSN v50 (Cisco flash v50)l CCNP 3 version 5

Module 4 Implementing Inter-VLAN routing