Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

EIGRP Summary and Query

Hi Everybody,

I am having the confusion how EIGRP Summary works with query. The example topology is given below.

In the illustrated attached network, all routers are configured to run EIGRP on all links.

Router B is configured to only send a summary route to D,E and F. If the link between A and B fails, what is the maximum number of queries C will receive for 192.168.1.0/24, given that all the packets transmitted during convergence are transmitted once ? Will D, E or F receive any queries

Can you please clarify me how it works.

regards,

R.B.Kumar

1 ACCEPTED SOLUTION

Accepted Solutions

Re: EIGRP Summary and Query

Hi Kumar

Here i have difference of opinion based on the knowledge i have. Feasible successor is not taken into account because it is not populated in the routing table. For example, if the Router is replying that it is having the route to destination being queried, and if that route is in topology table instead of routing table, then what is the use?

You're close, but with EIGRP query to EIGRP neigbor router. The EIGRP neigbor router is going to check its EIGRP topology table to give a reply and not its routing table. So as far as the router is concerned a Successor or Feasible successor will do. Because the purpose of the Feasible Successor is to provide a backup when the Successor is no longer available.

HTH

Lejoe

5 REPLIES

Re: EIGRP Summary and Query

When the link between Router B and A goes down, Router B will send query messages to all its neighbors (D,E,F,C). However, D,E,F, which have summarized routes and no specific routes to 192.168.1.0/24, will reply immediately. Router C, on other hand is likely to so as well because it learns of that route via Router B.

Please note, if you didnt want D,E,F to receive queries, you need to configure them as stub routers. Non stub routers do not send Query messages to stub routers. Route summarisation on D,E,F just speeds up the reply to a query.

HTH

Lejoe

New Member

Re: EIGRP Summary and Query

Hi Lejoe,

Thanks for your response.

If Router D,E,F has summary route for 192.168.1.0 learned from Router B, I think it will be the same case for Router C too. Then what is the difference?

Secondly, I thought Routers D,E,F will also send query to Router C for network 192.168.1.0. Similarly whether Router C will send query to Routers D,E,F?

Can you please spend some time in explaining this case

R.B.Kumar

Re: EIGRP Summary and Query

Hi Kumar,

If Router D,E,F has summary route for 192.168.1.0 learned from Router B, I think it will be the same case for Router C too. Then what is the difference?

As long as you have configured summarisation on Router B's interface connecting to Router C as well.

Secondly, I thought Routers D,E,F will also send query to Router C for network 192.168.1.0. Similarly whether Router C will send query to Routers D,E,F?

Lets assume you send a summary 192.168.0.0/16 to Routers D,E,F, in which case when Router B sends a query for route 192.168.1.0/24, a more specific route. Router D,E,F see that it is more specific route and reply immediately that they do not have a route to it.

As for Router C, it will see it has successor for the route,Router B, which has not changed, so it will immediately reply with details it has regarding the route. i.e I use next-hop Router B to reach 192.168.1.0/24 (assuming you havent summarized routes to Router C)

Edit:

This how router's respond to query messages

If the router does no have route to destination being queried or a more specific route (if it has summarized route), it sends a reply stating it has no route to it

If the router's successor to that destination being queried about hasnt changed or there's a Feasible successor, it will send back reply message with details of the route it has.

If the above steps are not met, then the router being queried will actively query its neighbors.

HTH

Lejoe

New Member

Re: EIGRP Summary and Query

Hi Lejoe,

Good. Now i am getting clarity on the concept. But..You mentioned as below that

"f the router's successor to that destination being queried about hasnt changed or there's a Feasible successor, it will send back reply message with details of the route it has."

Here i have difference of opinion based on the knowledge i have. Feasible successor is not taken into account because it is not populated in the routing table. For example, if the Router is replying that it is having the route to destination being queried, and if that route is in topology table instead of routing table, then what is the use?

Hope you understood what i am communicating? Is this correct

R.B.Kumar

Re: EIGRP Summary and Query

Hi Kumar

Here i have difference of opinion based on the knowledge i have. Feasible successor is not taken into account because it is not populated in the routing table. For example, if the Router is replying that it is having the route to destination being queried, and if that route is in topology table instead of routing table, then what is the use?

You're close, but with EIGRP query to EIGRP neigbor router. The EIGRP neigbor router is going to check its EIGRP topology table to give a reply and not its routing table. So as far as the router is concerned a Successor or Feasible successor will do. Because the purpose of the Feasible Successor is to provide a backup when the Successor is no longer available.

HTH

Lejoe

122
Views
0
Helpful
5
Replies