Routing Loop Labbing Question...

[deanwebb]

If I wanted to make a lab that involved creating a routing loop, what are the commands I would use to detect the loop? What are the counters I would want to examine?

[icecream-guy]

Mutual redistribution is an easy way to create a routing loop.  redistribute EIGRP into OSPF and then OSPF into EIGRP, your OSPF routes never make it into the routing table due to EIGRP having a better administrative distance.

use 'show ip route', 'show ip route eigrp', and 'show ip route ospf' to troubleshoot.

not really looking at counters, looking where routes are sourced from and where they are going to .

[that1guy15]

The fun ones are when prefixes pop in and out of routing tables continuously but they are sometimes difficult to reproduce intentionally.

INE has an amazing series on redistribution that I highly recommend spending some time with. They have some of it on youtube here:
https://www.youtube.com/results?search_query=INE+redistribution

But their 3 part blog series on the topic by  Petr Lapukhov was a cornerstone for my CCIE studies. This gives you several good scenarios.
http://blog.ine.com/2008/02/09/understanding-redistribution-part-i/
http://blog.ine.com/2008/02/19/understanding-redistribution-part-ii/
http://blog.ine.com/2008/03/17/understanding-redistribution-part-iii/


Commands:
debug ip route
show ip bgp|ospf|eigrp (look for age of route)
traceroute and ping

[deanwebb]

Good resources, all... I'm wanting to create a CCNA-level lab, but this takes it to a new level. I'm wanting to create some free CCNA-level resources with the interest of getting people to use them to learn how to think through these questions instead of hitting the dumps. There are lots of guys that are desperate for a cert that have no money, so if we can produce something of quality that allows them to get into networking, we do a service to our profession.

[that1guy15]

Redistribution and routing loops IMO are CCNP level topics. Maybe simple redistribution from IGP to BGP but thats it.

Redistribution requires you to have a firm understanding of each routing protocol and then fully understand how routes are shared and the translation of the metric. It goes a little deeper than I would expect a CCNA to know.

[deanwebb]

Redistribution and routing loops IMO are CCNP level topics. Maybe simple redistribution from IGP to BGP but thats it.

Redistribution requires you to have a firm understanding of each routing protocol and then fully understand how routes are shared and the translation of the metric. It goes a little deeper than I would expect a CCNA to know.

Right, but a routing loop *is* a CCNA concept, so I'd like to have a lab that's easy to set up and then demonstrate, "this is one such routing loop, others are possible."

[that1guy15]

ah gotcha. yeah that makes sense.

I know it would have been hell-a valuable during my CCNA days to have someone show me hands on a routing loop and how to troubleshoot it.

I like the idea!!

[LynK]

There is a problem with this at the CCNA level dean, let me explain.

EIGRP and OSPF have built-in loop prevention features. For example. EIGRP has a different Administrative distance for its internal routes (90) and its external routers (170). So if you have two EIGRP zones, or are redistributing OSPF into EIGRP, there is no way it can get confused (without altering Administrative distance).

The same applied for OSPF (sort-of). OSPF doesn't have different AD, but rather the SPF logic dictates that OSPF internal routes are preferred over over OSPF inter area routes.

Not only that, but OSPF inter area routes are preferred over OSPF external routes (and even further into that E1 > E2 | N1 > N2).


The easiest way to each routing loops would be to redistribute twice, so when a packet circles around and re-advertises, it can cause a loop.








Edit:

Actually there is a VERY easy way for CCNA to understand basic routing loops. Have two routers with two default routes pointing to each other. This IS COVERED in the CCNA and it will still provide a good learning tool for the site.

[deanwebb]

Actually there is a VERY easy way for CCNA to understand basic routing loops. Have two routers with two default routes pointing to each other. This IS COVERED in the CCNA and it will still provide a good learning tool for the site.

Indeed, that's what I am going to go with. Shows how routing loops == BAD and then it will help make sense of why routing protocols do the things they do to avoid creating routing loops.