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1992-2013 Cisco Systems Inc. All Rights Reserved. Generated on 2013-09-05-07:00

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EIGRP and Default Routes

[email protected] 3 posts since Nov 2, 2008

EIGRP and Default Routes Sep 3, 2013 1:23 PM

Hey guys,

I know there are several ways to skin a cat but wanted to get some thoughts on the solution

below:

I wanted to get some of your takes on if this is a legitimate way to advertise a default routeinto EIGRP. I have seen many folks do different variations but after some testing I landed

on the following. It seems to work in my lab. I am using a default route to Null0 on my core

router so that EIGRP will advertise network 0.0.0.0.

CoreRouter Config

router eigrp 1

network 10.0.1.0 0.0.0.3

network 172.25.1.0 0.0.0.3

network 172.30.1.0 0.0.0.3

network 192.168.1.0

network 0.0.0.0

no auto-summary

!

ip forward-protocol nd
http://localhost/var/www/apps/conversion/tmp/scratch_4/people/jengels%40communitycoffee.comhttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/jengels%40communitycoffee.comhttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/jengels%40communitycoffee.comhttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/jengels%40communitycoffee.com

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ip route 0.0.0.0 0.0.0.0 Null0

ip route 0.0.0.0 0.0.0.0 10.0.1.1

My understanding:

Null0's Administrative Distance is 255 so it is never used for routing in this case as the

10.0.1.1 route is an AD of 1. The Null0 route is only used to **** EIGRP into advertising

the 0.0.0.0 network. This is so that I don't have to specify and interface on my next hop for

10.0.1.1. I prefer to use the IP so that in the event of a failure, my standby ASA will take

over IP 10.0.1.1.

My thought process:

I prefer not to "redistribute static" due to some other static routes that I do not want to

redistribute for some VPN stuff which I won't go into here. The other option was to use a

route-map in the redistribute statement but being the Null0 setup was so easy and seems

to work I wanted to see if this is a legitimate way to install a default-route. (I also am aware

and familiar with the "default-network" command but wanted to try this first before adding

another subnet to the mix)

Your thoughts are greatly appreciated.

John E. Heyer23 posts since Jan 13, 2009

1. on page 2 Re: EIGRP and Default Routes Sep 3, 2013 1:36 PM

Main caveat I can foresee is always advertising a default route even when the default route

is unreable. I avoid using "default-information originate always" in OSPF for the same

reason.

[email protected] posts since Nov 2, 2008
http://localhost/var/www/apps/conversion/tmp/scratch_4/people/jengels%40communitycoffee.comhttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/jengels%40communitycoffee.comhttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/johnnylingohttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/johnnylingo

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in response to John E. Heyer

Yeah, that is one angle I did think about. So help me think through this:

Here are my thoughts on that angle: Thinking about a situation where the true default

route would go down; In the situation where the device or subnet containing 10.0.1.1 died

(became non-existent, in my case the ASA) then I would still be advertising a default route

as you said above. So at this point all default route traffic would still be directed to the core

of my network:

Would this be a bad thing? My initial thought is: My gateway of last resort is down at this

point so does it matter that all my traffic is directed to the Core and then dropped or is theresome other impact I am not thinking about?

(I will have and Active and Standby ASA, hence the reason for using the Null0 and IP

10.0.1.1 idea)

John E. Heyer23 posts since Jan 13, 2009


in response [email protected]

Do you ony have a single core router? Or is this a pair?

rboldy191 posts since Jun 3, 2011


in response [email protected]

If you don't have another way out your network there's no technical reason that it would be

bad for this traffic to be null routed at egress point so long as you plan for the bandwidth andextra load to process and discard these packets. You may see an increase here due to TCP

retransmissions so make sure you have some head-room. In your case so long as you're

running at less than 60% utilization on bandwidth and 40-50% average on CPU/memory it's

a good idea to do this because a traceroute will stop at the place where the issue exisits -

i.e. at the router that has lost it's default-route. This makes troubleshooting a lot easier.
http://localhost/var/www/apps/conversion/tmp/scratch_4/people/CSCO10168280http://localhost/var/www/apps/conversion/tmp/scratch_4/message/329512#329512http://localhost/var/www/apps/conversion/tmp/scratch_4/message/329512#329512http://localhost/var/www/apps/conversion/tmp/scratch_4/people/CSCO10168280http://localhost/var/www/apps/conversion/tmp/scratch_4/people/CSCO10168280http://localhost/var/www/apps/conversion/tmp/scratch_4/message/329512#329512http://localhost/var/www/apps/conversion/tmp/scratch_4/message/329512#329512http://localhost/var/www/apps/conversion/tmp/scratch_4/people/johnnylingohttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/johnnylingohttp://localhost/var/www/apps/conversion/tmp/scratch_4/message/329508#329508http://localhost/var/www/apps/conversion/tmp/scratch_4/message/329508#329508

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Now, the only issue you may come across is if you lost other (more-specific) routes in your

internal network. Then the traceroute would take the default path and may cause some

confusion because it might be that traffic should never usually go there. This could lead to

wasted time looking at the router when infact it's something else that stopping this morespecific route closer to the affected device. However you'll have this problem regardless of

how you inject a default-route so long as you have one.

Brian2,964 posts since Aug 17, 2009


the null 0 route does not have an AD of 255, unless you set it manually. i do not see that

you did this. Also, since you are using the "network 0.0.0.0" command you are enabling

EIGRP on "all" interfaces. this may or may not be problem depending on what othersinterface are configured and whether you want EIGRP to be running on those interfaces.

the proper way is to use a default route and redistribute into EIGRP. use a route map and

call an ip prefix-list to allow only the default route. like so

ip prefix-list DEFAULT seq 5 permit 0.0.0.0/0

!

route-map DEFAULT_ONLY permit 10

match ip address prefix-list DEFAULT

!

HTH

Brian
http://localhost/var/www/apps/conversion/tmp/scratch_4/people/b.schoonoverhttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/b.schoonover

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Adeel180 posts since Apr 19, 2011


in response to Brian

Brian very well explained.
http://localhost/var/www/apps/conversion/tmp/scratch_4/message/329610#329610http://localhost/var/www/apps/conversion/tmp/scratch_4/message/329610#329610http://localhost/var/www/apps/conversion/tmp/scratch_4/people/ilyas_adeelhttp://localhost/var/www/apps/conversion/tmp/scratch_4/people/ilyas_adeel

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