20111027_cht_tl_教育訓練 day4

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2011 Juniper Networks, Inc. All rights reserved. | www.juniper.net

Day 4

Layer 3 Routing(EGP - BGP)

Johnson Liu

[email protected] Oct. 27, 2011

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Border Gateway Protocol(BGP)

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2011 Juniper Networks, Inc. All rights reserved. www.juniper.net | 3

BGP is the core routing protocol within the Internet

What Is BGP?

AS 65501

AS 65503

AS 65502

AS 65504

Note: BGP Is an IETF standard defined in RFC 4271 (supersedes RFC

1771).

BGP is a path-vector protocolused for interdomain routing.

BGP views the Internet as acollection of autonomous systems.

BGP

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BGP is typically used in large enterpriseenvironments where multiple ISP connections exist,and in all service provider environments

When Should I Use BGP?

AS 65501

ISP B

ISP A

Static Routing

Customer A

Customer B

Single-homed customers typicallyuse a default route to the Internet.

BGP

AS 65502

AS 65503

Multihomed customers use BGP tocontrol inbound and outbound traffic.

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BGP peers can reside in different ASs or the sameAS

Peers in different ASs use the external session type(EBGP)

Peers in the same AS use the internal session type(IBGP)

BGP Peers (1 of 2)

IGP

AS 65501

IGPIBGP

AS 65503

IGPIBGP

AS 65502

IGP

AS 65504

EBGP

IBGP is not used because

a single BGP speakerexists.

IBGP is used becausemultiple BGP speakers exist.

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BGP peering sessions are manually defined andrely on TCP connections(port 179) No automatic neighbor discovery

BGP Peers (2 of 2)

R1 R2

BGP Neighbor States

TCP Connectivity BGP Connectivity

TCP Connectivity

BGP Connectivity

Established Neighbors

(1) Idle

(2) Connect

(4) OpenSent

(5) OpenConfirm

(6) Established(3) Active

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BGP Message Types

BGP messages are used to establish and maintainBGP peering sessions

All BGP messages use a common header

BGP Message TypesOpen Keepalive

Update Notification

Refresh

R1 R2

TCP Connectivity

BGP Connectivity


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BGP Update Messages

BGP update messages include pathadvertisements and their associated attributes

Can also list withdrawn routes that are no longerreachable

R1 R2 R3

Established Neighbors Established Neighbors

Router compares attributes associatedwith update messages to select the bestpath

Route 129.1.0.0/16 Route 129.1.0.0/16

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High-Level BGP Operation

Customer ACustomer B(AS 65501)

ISP A(AS 65001)

ISP C(AS 65003)

Customer A is single-homed to ISP A anduses 172.20.21.0/24 subnet, which was

assigned by ISP A

Static defaultroute to ISP A

Static route to Customer A

ISP B(AS 65002)

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ISP As Network

Customer A

ISP A(AS 65001)

ISP C(AS 65003)

I can reach172.20.0.0/16

(BGP Aggregate

Route)

R1

R2

R3

R4

Static route for 172.20.21.0/24 toCustomer A

I can reach172.20.21.0/24(Static => IGP)

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ISP As Aggregate


ISP A(AS 65001) ISP C(AS 65003)

ISP B(AS 65002)

172.20.0.0/16 is reachablethrough AS 65001

172.20.0.0/16 isreachable through

AS 65002 andAS 65001


AS 65003, AS65002 and AS

65001

ISP A advertises an aggregate of172.20.0.0/16 through BGP to ISP B

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Customer Bs Aggregate


ISP A(AS 65001)

ISP C(AS 65003)

ISP B(AS 65002)

172.31.128.0/20 isreachable through AS65003 and AS 65501

172.31.128.0/20 is reachablethrough AS 65002, AS 65003

and AS 65501

Default staticroute

172.31.128.0/20is reachablethrough AS

65501

Customer B advertises its172.31.128.0/20 network

through BGP to ISP C

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Customer B Connects to ISP B


ISP A(AS 65001)

ISP C

(AS 65003)

ISP B(AS 65002)

172.31.128.0/20 is reachablethrough AS 65003 and AS 65501


AS 65501

Customer B advertises its172.31.128.0/20 network through

BGP to ISP B and ISP C

172.31.128.0/20 is reachablethrough AS 65002 and AS 65501

ISP B chooses the bestpathand

advertises only that path


AS 65501

Default staticroute

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BGP Attributes Table

BGP Attributes

Name Type

AS Path Well-known mandatory

Local Preference Well-known discretionary

MED Optional nontransitive

Origin Well-known mandatory

Next Hop Well-known mandatory

Community Optional transitive

Aggregator Optional transitive

Atomic Aggregator Well-known discretionaryCluster List Optional nontransitive

Originator ID Optional nontransitive

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BGP attributes are included in the updatemessages and describe the BGP prefixes receivedfrom a peer

Attributes are used to select the best path

Some common examples include:

BGP Attributes

Common BGP Attributes

NextHop

Local PreferenceASPath

Origin MED Community

R1 R2 R3


Route XRoute X


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The next-hop attribute is the IP address of the peer

advertising the prefix Next-hop address must be reachable for receiving peer to

install route in RIB-local (routing table)

BGP Attributes: Next Hop

Common BGP AttributesNextHop



R2(AS200)

R3(AS200)

Next-hop value is changed bydefault across EBGP links only

Next-hop value is not changed by defaultacross IBGP links; can be changed through

policy

EBGPRoute XNH: 8.1.1.1

R1(AS100)

8.1.1.1/30 8.1.1.2/30

IBGPRoute XNH: 8.1.1.1

16.6.6.1/30 16.6.6.2/30

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BGP Attributes: Local Preference

Determines the preferred path outofthe AS All BGP traffic in an AS flows toward the peer with the

highest local preference value Can be altered through BGP configuration or policy

Values are used only within an individual AS

Nothing is sent across EBGP links


NextHop



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IGPIBGP

MyNET wants to use ISP A for outbound traffic buthave ISP B available for backup outbound traffic

Local-Preference Example

MyNET (AS 65503)

ISP AAS 65501

ISP BAS 65502

R3

R1 R2

R1: local preference = 300 R2: local preference = 100

Default local preference = 100

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Used to indicate path back to the routes source

and to prevent routing loops Routes with the receiving routers AS number in the AS

path are considered looped and not advertised (prefershortest AS Path first)

Route X

BGP Attributes: AS Path

Route X Route X

Route X

AS 501 AS 645 AS 452 AS 521

AS Path = 452 645 501AS Path = 645 501


NextHop



AS Path = 501

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BGP Attributes: Origin

Added by the router that injected a route into BGPand describes from where the route informationwas received (prefer lowest origin code so thepriority is prefer IGP than EGP, or prefer EGP than

Incomplete) I = IGP (0)

E = EGP (1)

? = Incomplete (2)


NextHop



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BGP Attributes: Multi-Exit Discriminator

Used to help influence the preferred path back intoan AS when multiple links exist between the sametwo ASs

Can be altered through BGP configuration or policy

Lower values are betterRoute X (MED = 20)

Route X (MED = 10)prefix X

AS 65501 AS 65502

IGPIBGP

IGPIBGP

R1

R2

R1

R2


NextHop



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BGP Attributes: Community

Used to tag certain routes that can be identifiedeasily

Defined under [edit policy-options] hierarchy[edit policy-options]user@R1# show

policy-statement ibgp-export {

from neighbor 172.25.125.2;

then {

community set customer-routes;

}

}

community customer-routes members 64700:133;

Communities are set, added,or deleted through routingpolicy.

Community format is typically:AS-

number:community


NextHop



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Selecting the Active BGP Route(*)

Once BGP verifies next-hop reachability and thatno loops exist, it selects the active route as follows:

BGP Route Selection Summary

1. Prefer the highest local-preferencevalue

6. Prefer best exit from AS

2. Prefer the shortest AS-path length 7. For EBGP-received routes, prefer thecurrent active route; otherwise, preferroutes from the peer with the lowest

RID

3. Prefer the lowest origin value 8. Prefer paths with the shortest clusterlength

4. Prefer the lowest MED value 9. Prefer routes from the peer with thelowest peer ID

5. Prefer routes learned from anEBGP peer over an IBGP peer

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IBGP is used within an ASEBGP is used between ASs

IBGP Versus EBGP

IGP

AS 65501

IGPIBGP

AS 65503

IGPIBGP

AS 65502

IGP

AS 65504

EBGP

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IBGP sessions are usually established betweenloopback addresses

Uses IGP to maintain sessions regardless of physicaltopology

EBGP sessions are usually established using the IPaddresses of the physically connected interfaces

Loopback and Interface Peering

AS 65503

IGPIBGP

R3

R1

R2AS 65502

(.1) 172.24.1.0/30 (.2)

ge-0/0/1.0 ge-0/0/1.0

If failure occurs, loopback-based IBGPsessions stay up over working links

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To avoid loops, BGP speakers do not propagate

IBGP-received routes to other IBGP peers1. A full mesh is required to ensure all IBGP speakers

have consistent BGP routing information or

2. BGP Route Reflector(RR) or

3. BGP Confederation

IBGP Route Propagation

AS 65503

R3R1 R2

AS 65502

IBGP NeighborsIBGP Neighbors

Route X Route X

Rule prohibits R2 fromadvertising route X to R3

Solution is to have R1 and R3become IBGP neighbors

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By default, IBGP peers do not change the next hopfor routes received from EBGP peers

To make the next hop reachable, you can:

Put external interface in IGP using thepassiveoption, or

Use next-hop selfin a policy to cause the router to use itsown IP address as the next hop

IBGP Next-Hop Propagation

AS 65503

R1 R2AS 65502

IBGP Neighbors

Route X Route X

By default, the next-hop value for the routeX advertisement will remain as 172.24.1.1

(.1) 172.24.1.0/30 (.2)

How do I get to

172.24.1.1?

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Default BGP Advertisement Rules

AS 655011. IBGP advertises routes

learned from EBGP

2. EBGP advertises routeslearned from IBGP or

EBGP

4. IBGP does notadvertiseany routes learned from

IBGP

AS 65510

prefix X

Route X

EBGP IGPIBGP

IGP

IBGP

AS 65502

IGP

IBGP

AS 65503

3. IBGP advertises

routes learned fromEBGP

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Case Study: Monitoring BGP (1 of 3)

Use the show bgp summary command to showan overview of the systems BGP information:user@R1> show bgp summary

Groups: 2 Peers: 2 Down peers: 0

Table Tot Paths Act Paths Suppressed History Damp State Pending

inet.0 12 6 0 0 0 0

Peer AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#Act/Rec/Acc/Damped.172.30.1.2 65501 914 915 0 0 6:51:16 6/6/6/0 0/0/0/0

192.168.100.2 65503 978 983 0 0 7:19:03 0/6/6/0 0/0/0/0

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Use the show bgp neighbor command to showthe BGP neighbor database:user@R1> show bgp neighbor

Peer: 172.30.1.2+62790 AS 65501 Local: 172.30.1.1+179 AS 65503

Type: External State: Established Flags:

Last State: OpenConfirm Last Event: RecvKeepAlive

Last Error: NoneExport: [ adv-aggregate ]

Options:

Holdtime: 90 Preference: 170

Number of flaps: 0

Peer ID: 172.18.1.1 Local ID: 192.168.100.1 Active Holdtime: 90

Keepalive Interval: 30 Peer index: 0

BFD: disabled, down

Local Interface: ge-0/0/3.0

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Use the show bgp group command to show theBGP group database:user@R1> show bgp group

Group Type: Internal AS: 65503 Local AS: 65503

Name: int-65503 Index: 0 Flags:

Export: [ next-hop-self-policy ]

Holdtime: 0Total peers: 1 Established: 1

192.168.100.2+51067

inet.0: 0/6/6/0

Group Type: External Local AS: 65503

Name: ext-65501 Index: 1 Flags:

Export: [ adv-aggregate ]

Holdtime: 0

Total peers: 1 Established: 1

172.30.1.2+62790

inet.0: 6/6/6/0

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Case Study: Displaying BGP Routes (1 of 3)

Use show route protocol bgp to display BGProutes installed in the RIB-Local:user@R1> show route protocol bgp

inet.0: 15 destinations, 21 routes (15 active, 0 holddown, 0 hidden)

+ = Active Route, - = Last Active, * = Both

10.0.0.0/16 *[BGP/170] 1d 21:43:42, localpref 100

AS path: 64501 65500 65501 65502 65503 I

> to 172.30.1.2 via ge-0/0/3.0

[BGP/170] 1d 21:43:42, localpref 100, from 192.168.100.2

AS path: 64502 65400 65501 65502 65503 I

> to 172.24.1.2 via ge-0/0/1.0

Note: You can add options to filter the output by BGP attributes such as AS path,next hop, and community.

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Use show route receive-protocol bgpneighbor to display received routes (RIB-In):

user@R1> show route receive-protocol bgp 172.30.1.2


Prefix Nexthop MED Lclpref AS path

* 10.0.0.0/16 172.30.1.2 65501 65510 65515 65520 65525 I

* 10.1.0.0/16 172.30.1.2 65501 65510 65515 65520 65525 I

* 10.2.0.0/16 172.30.1.2 65501 65510 65515 65520 65525 I

Importpolicy

RIB-Local(Route table)

RIB-In

Routesfrom BGP

peers

Displays route entries in the RIB-Intable that have not yet been filtered

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Use show route advertising-protocolbgp neighbor to display advertised routes (RIB-

Out):user@R1> show route advertising-protocol bgp 172.30.1.2


Prefix Nexthop MED Lclpref AS path* 172.24.0.0/22 Self I

RIB-Local(Route table)

Exportpolicy

RIB-Out

Routes toBGP peers

Displays route entries in the RIB-Outtable that are not yet filtered

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Advanced BGP Featuresin JUNOS

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[edit protocols bgp group ext-peers]

type external;

peer-as 2;

neighbor 10.222.28.2;

neighbor 10.222.29.2;

}

user@router> show bgp summary

Peer AS InPkt OutPkt OutQ Flaps Last Up/Dwn State|#Active/Rec

10.222.28.2 2 7 7 0 0 00:00:02 4/4/0

10.222.29.2 2 8 10 0 0 00:00:06 0/4/0

BGP Multipath: Part 1BGP can ignore both router ID and peer ID

comparisons whenmultipathis configured withinBGP Can use:

Two peering sessions to the same router

Two peering sessions to different routers in the same AS

R2(AS 2)

R1(AS 1)

10.222.28.2/2410.222.28.1/24

10.222.29.2/2410.222.29.1/24 R3

(AS 2)

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BGP Multipath: Part 2

Routes from each peer contain a singlenext hop

user@R1> show route protocol bgp terse



A Destination P Prf Metric 1 Metric 2 Next hop AS path

* 172.16.20.4/30 B 170 100 >10.222.28.2 2 I

B 170 100 >10.222.29.2 2 I

* 172.16.20.8/30 B 170 100 >10.222.28.2 2 I

B 170 100 >10.222.29.2 2 I

* 172.16.20.12/30 B 170 100 >10.222.28.2 2 I

B 170 100 >10.222.29.2 2 I

* 172.16.20.16/30 B 170 100 >10.222.28.2 2 I

B 170 100 >10.222.29.2 2 I

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BGP Multipath: Part 3

Peer group on R1 configured with multipath

Active route receives two next hops

Forwarding table still maintains a single next hop perroute[edit protocols bgp group ext-peers]

type external;

peer-as 2;

multipath;

neighbor 10.222.28.2;

neighbor 10.222.29.2;

}

user@R1> show route protocol bgp terse



A Destination P Prf Metric 1 Metric 2 Next hop AS path

* 172.16.20.4/30 B 170 100 >10.222.28.2 2 I10.222.29.2

B 170 100 >10.222.29.2 2 I

* 172.16.20.8/30 B 170 100 >10.222.28.2 2 I

10.222.29.2

B 170 100 >10.222.29.2 2 I

* 172.16.20.12/30 B 170 100 >10.222.28.2 2 I

10.222.29.2

B 170 100 >10.222.29.2 2 I

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Multihop Peering

lo0: 192.168.3.4

EBGP sessions can peer with nonphysicaladdresses

R2(AS 2)

R1(AS 1)

10.10.1.1/2410.10.1.2/24

lo0: 172.16.128.1

[edit protocols bgp group ext-peers]

type external;

local-address 192.168.3.4;

neighbor 172.16.128.1 {

multihop ttl 1;

}

[edit routing-options]

static {

route 172.16.128.1 next-hop [ 10.10.1.1 10.10.2.1 ];

}

10.10.2.1/2410.10.2.2/24

Step 1

Step 2

Step 3

A TTL value of 1 accommodatespeering to a loopback address on adirectly connected peerhigher valuesare needed for peers that are notdirectly connected

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passive keeps BGP from sending open message

To have the router not send active BGP openmessages to the neighbor, include the passive

statement.

The router instead waits for the peer to send anopen message first before sending one.[edit protocols bgp]group ext-peers {

type external;peer-as 2;neighbor 10.10.10.1 {

passive;}

}

Peer Configuration Options: Part 1

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prefix-limit allows a specified amount of

prefixes to be received

[edit protocols bgp]group ext-peers {

type external;

peer-as 2;family inet {

unicast {prefix-limit {

maximum 25000;}

}}

neighbor 10.10.10.1;}

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hold-timealters the value used in the sessionnegotiation process(lowest win)

JUNOS default : keepalive 30/hold-time 90

IOS default : keepalive 60/hold-time 180

[edit protocols bgp]group ext-peers {

type external;hold-time 45;peer-as 2;neighbor 10.10.10.1;

}

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GR allows a router undergoing a restart event toinform its neighbors and request a grace periodduring which it can recover from that restart event

Forwarding through existing paths can continue during

restart

R1s neighbors hide the failurefrom other routers in the network.R2, R4, and R5 are known ashelper routers in GR terminology.

Graceful Restart(GR)

R3 and R6 are not awarea restart event occurred.

Once R1 recovers from the restart event, R1synchronizes with its neighbors without disrupting

packet forwarding.

R1 informs all neighbors of a restartevent. R1 is known as therestarting router in GR terminology.

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Routers (restarting and helper routers) must have

GR enabled and be able to support nonstopforwarding

End-of-RIB markers sent for each NLRI

Notifies the neighbor that all current routing informationwas sent

Local router defers path selection algorithm until themarker is received

Configured globally within the [edit routing-options] hierarchy

GR Support and Requirements

Packet Forwarding

Engine

Routing Engine

Packets In Packets Out

Control Plane

Forwarding Plane

FT

During a restart event, forwardingcontinues based on existing forwardingtable entries.

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Configuring GR (1 of 2)GR helper mode is enabled by default

You can disable GR helper mode globally atthe [edit routing-options] hierarchy or on a

per-protocol, per-group, or per-neighbor basis(depending on the protocol)

[edit]

user@R1# show routing-optionsgraceful-restart {

disable;

}

[edit]

user@R1# show protocols bgp

graceful-restart;

group my-group {

type internal;local-address 192.168.1.1;

neighbor 192.168.1.2;

neighbor 192.168.2.2 {

graceful-restart {

disable;

}

}

}

Disables GR for BGP peer

Enables helper mode for BGP

Note: The most specific application ispreferred.

Disables helper mode globally forall protocols that support GR

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Configuring GR (2 of 2)

GR restarting router mode is not enabled by default You can enable this mode at the [edit routing-options] hierarchy and disable it on a per-protocol, per-

group, or per-neighbor basis (depending on the protocol)

Configuration options vary betweenthe supported protocols [edit]user@R1# show routing-options

graceful-restart;

[edit]

user@R1# show protocols bgp

graceful-restart;

group my-group {

type internal;

local-address 192.168.1.1;

neighbor 192.168.1.2;

neighbor 192.168.2.2 {

graceful-restart {

disable;

}

}

}

Disables GR for specificBGP peer

Enables restarting router mode forall protocols that support GR

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AS 65003192.168.19.0/24

AS 65002

local-preference

R1 R2

lo0=192.168.40.1

user@R2> show route advertising-protocol bgp 192.168.40.1


Restart Complete


* 192.168.19.0/24 Self 0 100 65003 I

[edit]

user@R2# set protocols bgp group int-peers local-preference 300

user@R2> show route advertising-protocol bgp 192.168.40.1


Restart Complete


* 192.168.19.0/24 Self 0 300 65003 I

Modifying Local Preference

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Modifying AS Path: remove-private

AS 1000

192.168.17.0/24 AS-Path 1000

192.168.18.0/24 AS-Path 1000

192.168.19.0/24 AS-Path 1000

remove-private

192.168.17.0/24

AS-Path 65001

192.168.19.0/24

AS-Path 65003

192.168.18.0/24

AS 65002 AS 65003192.168.19.0/24

AS 65001192.168.17.0/24

Internet

192.168.18.0/24

AS-Path 65002

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Modifying AS Path: local-as(Part 1)

172.16.10.0/24

AS-Path 1 222

172.16.12.0/24

AS-Path 1 333

172.16.10.0/24

AS-Path 222

172.16.12.0/24

AS-Path 333

AS 222172.16.10.0/24 172.16.12.0/24

AS 333

AS 777

AS 1EBGP

172.16.10.0/24

AS-Path 777 1 222

172.16.12.0/24

AS-Path 777 1 333

Internet

EBGP

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Modifying AS Path: local-as (Part 2)

172.16.10.0/24

AS-Path 222

172.16.12.0/24

AS-Path 333

172.16.10.0/24

AS-Path 777 1 222

172.16.12.0/24

AS-Path 777 1 333

172.16.10.0/24

AS-Path 1 222

172.16.12.0/24

AS-Path 1 333

Internet

AS 777

local-as 1

172.16.12.0/24

AS 333AS 222

172.16.10.0/24

IBGP

EBGP

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Modifying AS Path: local-as (Part 3)

172.16.10.0/24

AS-Path 777 222

172.16.12.0/24AS-Path 777 333

172.16.10.0/24

AS-Path 222

172.16.12.0/24

AS-Path 333

AS 222

172.16.10.0/24 172.16.12.0/24

AS 333

Internet

AS 777

local-as 1 private

IBGP

EBGP

172.16.10.0/24

AS-Path 222

172.16.12.0/24

AS-Path 333

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Modifying AS Path: as-override172.16.10.0/24

AS-Path 65022

172.16.10.0/24

AS-Path 65432 65432

as-override

10.222.4.1

10.222.4.2

user@AS65432> show route advertising-protocol bgp 10.222.4.2inet.0: 8 destinations, 8 routes (8 active, 0 holddown, 0 hidden)Prefix Nexthop MED Lclpref AS path

* 172.16.10.0/24 Self 65022 I

user@AS65022> show route receive-protocol bgp 10.222.4.1inet.0: 7 destinations, 7 routes (7 active, 0 holddown, 0 hidden)

[edit]user@AS65432# set protocols bgp group AS-65022 as-override

user@AS65432> show route advertising-protocol bgp 10.222.4.2inet.0: 8 destinations, 8 routes (8 active, 0 holddown, 0 hidden)Prefix Nexthop MED Lclpref AS path

* 172.16.10.0/24 Self 65022 I

user@AS65022> show route receive-protocol bgp 10.222.4.1inet.0: 8 destinations, 8 routes (8 active, 0 holddown, 0 hidden)Prefix Nexthop MED Lclpref AS path

* 172.16.10.0/24 10.222.4.1 65432 65432 I

AS 65022

172.16.10.0/24

AS 65432

AS 65022

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Modifying AS Path: loops

172.16.10.0/24: 65022 172.16.10.0/24: 65432 65022

user@AS65022> show route receive-protocol bgp 10.222.4.1


[edit]

user@AS65022# set routing-options autonomous-system 65022 loops 2

user@AS65022> show route receive-protocol bgp 10.222.4.1



* 172.16.10.0/24 10.222.4.1 65432 65022 I

AS 65022

172.16.10.0/24AS 65432 AS 65022

loops

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Scaling BGP

IBGP full-mesh peer requirement has an n2problem

Addition of a new router requires new peering with allcurrent IBGP speakers

Current IBGP speakers must update their configurations Two primary scaling mechanisms:

Route Reflection (RFC 4456)

Confederations (RFC 3065)

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Route Reflection Concepts

Allows an IBGP speaker to re-advertise anIBGP-learned route to anotherIBGP speaker

Route reflector only re-advertises the active routeto clients

Route reflector does not, by default, changeexisting IBGP attributes

Two new BGP attributes to prevent loops:

Cluster list Contains one or more cluster ID values

Originator ID

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New Cluster Attributes Prevent Loops Steps:

1. Client sends routes to RR

2. RR sends routes to all clients in the cluster and all RRs

3. Those RRs send the routes to all their peers forming a loop

4. If RR1 received the same cluster ID in cluster list then it dropthe route

10.10.10.0/24

RR

RR3

RR2

Clients

ClientsClients

RR1

10.10.10.0/24

10.10.10.0/24

10.10.10.0/24

10.10.10.0/24

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Route Reflection Attributes

Cluster list:

Operates like an AS path, used by RR for loop prevention

Also used in the route selection algorithm

Contains a sequence of cluster IDs

Cluster ID represents each RR cluster in the network RR drops routes that have already transited the cluster

Added to the cluster list when a RR touchesa route

Originator ID:

Identifies the first router to inject a route in an RR network

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Route reflector clients are configured in a separate

peer groupEach peer group uses the cluster keyword

Cluster ID uses unique 32-bit number Often the router ID of the RR is used

Clients only peer to their route reflectors

[edit protocols bgp]

group int-peers {type internal;

local-address 172.16.1.1;cluster 172.16.1.1;neighbor 172.16.2.2;

neighbor 172.16.3.3;neighbor 172.16.4.4;

}

[edit protocols bgp]

group int-peers {type internal;local-address 172.16.2.2;

neighbor 172.16.1.1;

}

Route Reflection Configuration

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Basic Route Reflection

Client > RR > Clients and Nonclients

Nonclient > RR > Clients Only

RR

RR

RR

RR

Client

Client

Client Client

Client

ClientClient

Client

ClientClient

Client

Client

IBGP Full MeshBetween Route

Reflectors

R P i

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Route Propagation

Steps:

1. Client sends routes to route reflector2. Route reflector sends routes to all clients in the cluster

and all peers3. Route reflector sends routes from peers to all clients in

the cluster

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RR

RR

RR

Clients

ClientsClients

RR

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10.10.10.0/24

10.10.10.0/24

10.10.10.0/24

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M dif i A ib h RR

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Modifying Attributes on the RR

Route reflector can modify any BGP attribute usinga routing policy

Presence of RRs should not affect forwarding paths Use of next-hop selfcan result in inefficient

forwarding paths In this example, the RR incorrectly overwrites the BGP

next hop for the 192.168.0.0/16 route Packets are now forwarded through the reflector instead of

directly between the clients

RR

172.16.1.1

Client172.16.3.3

Client172.16.2.2

192.168.0.0/16

BNH = 172.16.2.2

192.168.0.0/16

BNH = 172.16.1.1

S li BGP C f d ti

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Scaling BGPConfederations

Breaks a global AS into multiple pieces (sub-AS)

Within each sub-AS:

Use private AS numbers

An IBGP full-mesh topology is still required

Between each sub-AS: EBGP-type configurations are required

(multihop, and so forth)

Only the AS path attribute is changed

Prevents loops in the network Sub-AS networks are notused when comparing AS path lengths

Other BGP attributes are not modified by default

Next hop, local preference, and MED are all unaffected

C f d ti AS P th S t

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Confederation AS Path Segments

AS confederation sequence:

Each sub-AS is added to the AS path attribute

(65000 65001 65002) 100 200 shows a sequence

Used for loop prevention only

Sequence values are not counted as AS hopsAS confederation set is used when an aggregated

route loses the granularity of the sequence:

192.168.24.0/24 (65000 65001) 100

192.168.100.0/24 (65000 65002) 100

192.168.0.0/16 ({65000 65001 65002}) 100

C f d ti C fi ti

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Confederation Configuration

The global AS appears as a whole network whenviewed externally by peer networks

All routers remove all confederation information atthe edge of the global AS

Other AS peers do not see the details of theconfederation

No need for remove-private

[edit routing-options]user@router# show

autonomous-system 65000;

confederation 201 members [ 65000 65001 65002 65003 65004 ];

Confederation Peering

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Confederation PeeringCBGP

CBGP

CBGP

CBGP

CBGP

CBGP

AS 65004

AS 65003

AS 65002

AS 65001

AS 65000 AS 201

RRRR

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20111027_cht_tl_教育訓練 day4

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