Quiz: Redistributing Static Routes in EIGRP. Which of these is better ?

Consider the following

I have a network with the following configuration that is used to redistribute static routes into an EIGRP network.

router eigrp 1
 redistribute static metric 32 2000 255 1 1500
 network 10.0.0.0
 auto-summary
!
ip route 30.0.0.0 255.255.255.0 10.0.0.2
ip route 40.0.0.0 255.255.255.0 10.0.0.2
!

We have a requirement to do some route slugging on a router in the high availability path. We need to lower the metric of the static route 40.0.0.0/24 this are being redistributed so that this becomes the preferred router for that route.

Question: What is the difference between the following two solutions:

Solution 1

router eigrp 1
 redistribute static metric 32 2000 255 1 1500 route-map metricmod
 network 10.0.0.0
 auto-summary
!
ip route 30.0.0.0 255.255.255.0 10.0.0.2
ip route 40.0.0.0 255.255.255.0 10.0.0.2
!
logging alarm informational
access-list 10 permit 40.0.0.0 0.0.0.255
!
route-map metricmod permit 10
 match ip address 10
 set metric 10000 10 255 1 1500
!
route-map metricmod permit 20

Solution 2

router eigrp 1
redistribute static  route-map metricmod
network 10.0.0.0
auto-summary
!
ip route 30.0.0.0 255.255.255.0 10.0.0.2
ip route 40.0.0.0 255.255.255.0 10.0.0.2
!
logging alarm informational
access-list 10 permit 40.0.0.0 0.0.0.255
!
route-map metricmod permit 10
match ip address 10
set metric 10000 10 255 1 1500
!
route-map metricmod permit 20
set metric 32 2000 255 1 1500

Hint

Would you like to a hint ? [slider title="Click Here"]Solution 1 is much better in a real network[/slider]

I will post the answer in a week or so. Answers in the comments and I will say why its the best solution.

Answer

Solution 2 will require you to issue the following:

router eigrp 1
no redistribute static

and then

redistribute static route-map metricmod

to make these changes. Of course this removes all the routes from the routing table and will cause an outage. Whereas Solution 1 will not cause a routing tables to change and thus not cause an impact to services.

Sermon

For anyone who is learning Cisco networking, this might help to understand why detailed knowledge, “more than one way” and memorisation is important. By knowing it backwards, you spend your time thinking about other things rather than thinking only about the redistribution. Typically, people with limited skills will be happy with ANY solution, and not attempt to think of other ways of doing something that might be a better choice.

For mendicants in CCIE study: this iss why you need to know four different ways to do same thing. You learn to keep thinking about options, and the best choice. It’s been a valuable lesson for me.

Postscript

In this particular case (as Ivan Pepelnjak points out in the comments) the router had a couple of hundred static routes. When you delete and add that many routes you create a CPU and bandwidth hit as the routes are deleted and propagated. †In networks with slow bandwidth, this can cause service impacts as the routing updates can suck up bandwidth.

It’s a big knock on effect. You need to think of these things in real life to keep the network up all the time.

About Greg Ferro

Greg Ferro is a Network Engineer/Architect, mostly focussed on Data Centre, Security Infrastructure, and recently Virtualization. He has over 20 years in IT, in wide range of employers working as a freelance consultant including Finance, Service Providers and Online Companies. He is CCIE#6920 and has a few ideas about the world, but not enough to really count.

He is a host on the Packet Pushers Podcast, blogger at EtherealMind.com and on Twitter @etherealmind and Google Plus

You can contact Greg via the site contact page.

  • http://blog.ioshints.info Ivan Pepelnjak

    I know I’ll sound like an American, but I can’t help myself … amazing.

    I also like your summary :) BTW, you should have explained to the non-mendicants what the removal of numerous EIGRP routes means in a large non-summarized network with no stub routers and low-speed lossy links :D

  • http://blog.ioshints.info Ivan Pepelnjak

    It’s worse than what you’re describing in your Postscript. Due to the DUAL algorithm, the queries for the lost routes are spread to every single router in the network (unless they are configured as stub routers, see http://www.nil.com/ipcorner/EigrpStub/ ). If your network has low-speed lossy links it takes a while for the situation to stabilize.

    BTW, summarization does not help much, as the queries go one step beyond the summarization boundary (and I am not sure whether external EIGRP routes can be summarized).

    Fortunately the EIGRP code has been fixed years ago, so we don’t have to worry about the dreadful Stuck-in-Active events any more.

    • http://etherealmind.com Greg Ferro

      Yep. I wanted to add something, but didn’t have the time to do the topic in full detail. Maybe in an upcoming post. As you know, some of these things are really quite difficult to explain…… I appreciate your comments though.

  • dnt have

    it was so good

  • dnt have

    that was that i need it

  • dffd

    worse one

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