EIGRP Question

jediknight · August 2007

This is hopefully an easy question, but I'm asking as I want to get the terminology right.

Ok, you're running EIGRP and you have a Router that goes offline. The Router that was previous connected to the Router that went off line now needs to find a new path to the remote network. The router does not have any feasible successor routes, so it queries the neighbors.

Here's the question...

Let's say 2 other Routers reply with a new path to this remote network. Do these Routers reply with their successor route to this remote network or do they send the querying Router their feasible successor route that is in their topology table?

Netstudent · August 2007

A router will reply to the query with a route that has a lower advertised distance than the feasible distance (feasible successor) to the subnet and this will become the querying router's successor.

If the router that recieves the query does not have a route to the subnet because it's only route went down because of the failure, then the recieving router will recursively pass the query to the next hop.

Go here ftp://ftp.bookpool.com/sc/7X/158720147X.pdf And scroll down to diagram 3-6 and read all the bullets that explain the process very carefully. This will answer your question better than I can explain it.

jediknight · August 2007

Netstudent wrote:

A router will reply to the query with the lowest metric route to the subnet and this will become the querying router's feasible successor. If the router that recieves the query does not have a route to the subnet because it's only route went down because of the failure, then the recieving router will recursively pass the query to the next hop.

Thanks for the quick reply Netstudent! I'm still a little confused.

I understand that the lowest metric route will be sent to the querying Router and that recursion will take place if the neighbor router does not have a route to the subnet.

Basically what I don't understand is if the Router does have a route to the subnet that the offline router can no longer route to, does this Router send the querying router it's successor router or feasible successor route to this subnet? I know that the querying Router will look at the routes from each Router (lets say there was 2 Routers that had routes) and make one a successor route and the other a feasible successor.

Netstudent · August 2007

I'm pretty sure the replying router replies with a feasible successor.

Go here ftp://ftp.bookpool.com/sc/7X/158720147X.pdf And scroll down to diagram 3-6 and read all the bullets that explain the process very carefully. This will answer your question better than I can explain it.

On bullet 3 they typo-ed the feasible distances for the replying routers. They say the feasible distance is 21 and 21 but it should be 20 and 21. That is why the querying router chooses routerE's feasible successor route. Router E has a feasible distance of 20 VS RouterF's 21.

jediknight · August 2007

Netstudent wrote:

I'm pretty sure the replying router replies with a feasible successor.

Go here ftp://ftp.bookpool.com/sc/7X/158720147X.pdf And scroll down to diagram 3-6 and read all the bullets that explain the process very carefully. This will answer your question better than I can explain it.

On bullet 3 they typo-ed the feasible distances for the replying routers. They say the feasible distance is 21 and 21 but it should be 20 and 21. That is why the querying router chooses routerE's feasible successor route. Router E has a feasible distance of 20 VS RouterF's 21.

Thanks alot for taking the time to get this! This is exactly what I was looking for.

EIGRP Question

Comments