Routing loops occur when a router incorrectly forwards packets, that is: to a wrong next hop router. This is most easily detected with traceroute where you will see the same hops recurring over and over, or ping where you will see a message like "ttl expired in transit".
Routing loops are always caused by misconfiguration although it is not always as obvious as two static routes that point to eachother. Redistribution of routing information is a notorious source of routing loops.
Thanks for u r invaluable information.I just wanted to know how we can find the loops when a link is configured with EIGRP and OSPF.
I got to know from my senior that using Debug commands and Sh ip route command we can find the the loops.Is that true? and to stop the routing loop in EIGRP is there a command anything like #eigrp stub as we have split-horizon in RIP.
The traceroute is the best to find simple routing loops. Show ip route to a point will find the more complex ones. You hope to see routes coming in and out of the table very quickly. You will also see the routing processes running all the time.
A simple example if you have a lab of a way to create a difficult to find loop would be. You can actaully do this with 2 routers it is just to make the example more clear.
Router A talks to router b via OSPF
Router A talks to router C via EIGRP
ROuter B talks to router C via OSPF
Router C is running both OSPF and EIGRP and redistributes EIGRP routes into OSPF
Now say router A has a route
IP route 10.10.10.10 255.255.255.255 220.127.116.11 200
This has a high metric of 200 to illistrate this problem.
Now router A redistributes this static route into EIGRP. Router C receives it via EIGRP puts it in his routing table and redistributes in into OSPF.
Router B gets this route via OSPF and puts it in his routing table. He also forwards this to router A.
When router A gets this route via OSPF route it has a better administrative distance than the static so he removes the static and puts the OSPF route in the routing table.
Now since the static route was just removed from the routing table router A takes it out of EIGRP which means router C stops sending the OSPF route to router B. Since router B no longer has the OSPF router he no longer tells router A about the route. Router A removes the ospf and puts the static back in since it now is the only route.
And you start all over again.
This type of routing loop goes so fast it is very hard to tell. Traffic will work sometime and fail others. These type of loops are best just avoided by design.
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