How to view a routing loop?

mikeybinec

Using real equipment, how do I view a routing loop? Debug?

Thanks

networker050184

What do you mean by "view" it? You can always just look at the routing table. Or traceroute etc.

mikeybinec

I guess a better way to explain my question is to see the TTL decrement

thanks

networker050184

Traceroute would be the most obvious way. Debugs start to get a bit hard to use for this sort of thing depending where the router sits in the path since packets forwarded in hardware aren't going to show in debugs.

OfWolfAndMan

If you're not using RIP/IGRP for your routing protocol, then the best way to create a routing loop is via redistribution.

i would view a case study on routing loops and recreate one yourself.

networker050184

If you just want to watch one point two defaults at each other.

mikeybinec

Since you guys opened the door, you mean point a default in the wrong direction

Thanks

OfWolfAndMan

What he means is point a default to one router, and then on the router you point the default to, you would configure a default pointing back to the first router you created a default on.

Funny story, we used to play games to mess with each other in our old networking shop. Since we used static routes years ago to point everything around the network, I added a x.x.x.x 255.255.255.255 route one day for a buddy I worked with using his leased IP at the time back to the first hop his computer goes to. Along with renaming his profile cache with a .old extension. Let's just say he almost reimaged his computer LOL

theodoxa

Wireshark using Embedded Packet Capture on one of the routers or a Catlayst switch with Port Mirroring (SPAN) placed in between. You could also use a dumb hub in between the routers, but those are getting difficult to find.

dppagc

Is he confusing layer 2 broadcast storm.
There are no loops in layer 3 since there is routing protocol.

OfWolfAndMan

dppagc wrote: »

There are no loops in layer 3 since there is routing protocol.

Lol. You sure about that? If you're using OSPF or EIGRP as a single IGP, then yes I could agree. Otherwise, it's debatable.

dppagc

Oh yeah I was too simple minded (at least at CCNA level).
There could be routing loops if there are different IGP and redistribution occurs between them.

theodoxa

dppagc wrote: »

Is he confusing layer 2 broadcast storm.
There are no loops in layer 3 since there is routing protocol.

I have seen routing loops created (accidentally and more than once) when someone decided to use static routes. Routing loops can also be caused when there are multiple points of redistribution between 2 routing protocols. The latter can be easily prevented by tagging routes and checking the tags before redistributing them. You could also technically disable Split Horizon in a Distance Vector routing protocol.

OfWolfAndMan

We did something something recently during a migration we had to migrate part of the infrastructure from EIGRP to OSPF, and did so by redistributing OSPF into EIGRP but not vice versa. Instead, we advertised a default route into OSPF using the default information originate command going to the point of redistribution. Worked like a charm without causing any loops with the way routing was setup. It was suboptimal from OSPF to EIGRP, but temporary.