EIGRP route distribution question

[john.dill]

I'm a software engineer, but I've been asked to study some router configuration files for setting up a lab environment and I'm responsible for troubleshooting network connectivity problems.  In the network, one component I'm studying is a pair of EIGRP networks that uses a tunnel over a BGP connection to connect to each other.  First question is whether EIGRP routers in separate groups share routing table information?

router eigrp 1 -> BGP (tunnel) -> router eigrp 2

For example, will the topology table for routers in eigrp 1 include routes from eigrp 2, and vice versa.  From my reading, the tunnel makes it look like the gateways of eigrp 1 and 2 are just one hop away, so they are considered adjacent.  I only have equipment for eigrp1 and nothing for eigrp2 so I can't test for myself.

Second question starts with a 'redistribute static metric' command that references a route-map that has a static ip route defining the tunnel.

router eigrp 1
 network ...
 network ...
 redistribute static metric 100000 5 255 1 1500 route-map LOCAL-DEFAULT

ip route 0.0.0.0 0.0.0.0 Tunnel xxx.xxx.xxx.xxx tag 200

route-map LOCAL-DEFAULT permit 10
 match tag 200

I've read that the 'tag 200' are used to mitigate router loops from occurring.  However, in the topology for eigrp 1 and 2 individually, there doesn't appear to be any loops; it's a straight connection of switch -> router -> gateway.  However, I do not know the topology of the BGP network.  Is it possible that a loop can occur in the gateways between eigrp 1 and 2 because of the BGP tunnel connecting them, and this tag prevents any potential redistribution that would induce a loop across this link?

[chrihussey]

First question is whether EIGRP routers in separate groups share routing table information? For example, will the topology table for routers in eigrp 1 include routes from eigrp 2, and vice versa.  From my reading, the tunnel makes it look like the gateways of eigrp 1 and 2 are just one hop away, so they are considered adjacent.

ANS: Yes, the tunnel makes it appear to the EIGRP 1 & 2 routers that they are 1 hop away from each other. Provided the both routers have the same EIGRP process number (autonomous system), they will peer and exchange routes with each other.

Is it possible that a loop can occur in the gateways between eigrp 1 and 2 because of the BGP tunnel connecting them, and this tag prevents any potential redistribution that would induce a loop across this link?

ANS: In this case the tag is locally significant to the router to identify which static routes are to be redistributed into EIGRP. So if you had multiple static routes, only the ones with the tag would be redistributed into EIGRP.

http://www.cisco.com/c/en/us/td/docs/ios/12_2/iproute/command/reference/fiprrp_r/1rfindp1.html

There is a possibility of a loop inside the tunnel, or across any network for that matter, in that both sides, R1 & R2, have the same route pointing to each other for the same destination. That all depends on how things are set up.

With tunneling, the BGP domain should not see the EIGRP routes or IPs of traffic passing between the two EIGRP routers. All traffic going through the tunnel is encapsulated (enveloped) inside the tunnel source and destination IPs. There is not so much the danger of a loop, as much as there is a possibility of recursive tunneling, where the destination of the tunnel is through the tunnel, which would cause it to fail.

Hope this helps