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EIGRP/stub and Query

Hello

There is LAN segment with R1,R2,R3 running EIGRP. R3 is configured as stub. R1 and R2 are connected to other LAN segment. R1's connection to that segment is goin down. R1 send update that it has no longer that prefix. R3 receives it and let's assume that it has not feasible successor for that route. Will R3 query both R1 and R2 ? Does stub functionality changes this behavior or not ?

Thanx

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Accepted Solutions
Cisco Employee

Re: EIGRP/stub and Query

Hello,

The EIGRP Stub router feature has these properties:

1.) For any query received, a stub router replies immediately with "destination unreachable" without propagating the query further

2.) Any network that is learned via EIGRP a stub router is not advertised to any further neighbor

3.) Only a specific subset of locally generated or connected networks is advertised by a stub router to its neighbors (the options are connected, summary, static, any combination thereof, or none)

4.) Neighbors of stub routers do not send any queries to stub routers

However, by itself, an EIGRP Stub feature does not prevent the stub router itself to send queries.

Imagine a simple dual-homed stub router connected to two different routers at the HQ site, one of them being significantly faster and therefore preferred. Now, imagine this faster link going down. The stub router will most certainly lose the successor for majority of its networks and the neighbor at the slower link may not be identified as the feasible successor because of the relation between the feasible and reported distance. If the stub router was now prevented from sending queries, it would not use the slower backup link to reach the networks at HQ. However, there is no limitation on sending queries, even by a stub router. In this case, the stub router will send a query to its neighbor on the slower link and after it replies, the stub router will identify it as the backup route to destinations at HQ.

Best regards,

Peter

3 REPLIES
Cisco Employee

Re: EIGRP/stub and Query

Hello,

The EIGRP Stub router feature has these properties:

1.) For any query received, a stub router replies immediately with "destination unreachable" without propagating the query further

2.) Any network that is learned via EIGRP a stub router is not advertised to any further neighbor

3.) Only a specific subset of locally generated or connected networks is advertised by a stub router to its neighbors (the options are connected, summary, static, any combination thereof, or none)

4.) Neighbors of stub routers do not send any queries to stub routers

However, by itself, an EIGRP Stub feature does not prevent the stub router itself to send queries.

Imagine a simple dual-homed stub router connected to two different routers at the HQ site, one of them being significantly faster and therefore preferred. Now, imagine this faster link going down. The stub router will most certainly lose the successor for majority of its networks and the neighbor at the slower link may not be identified as the feasible successor because of the relation between the feasible and reported distance. If the stub router was now prevented from sending queries, it would not use the slower backup link to reach the networks at HQ. However, there is no limitation on sending queries, even by a stub router. In this case, the stub router will send a query to its neighbor on the slower link and after it replies, the stub router will identify it as the backup route to destinations at HQ.

Best regards,

Peter

Silver

Re: EIGRP/stub and Query

Why wasn't R2 also advertising the "other" lan segment to R3?

R1 and R2 know that R3 is stub so they won't query R3 but anyhow it's not needed here.

R3 will query R2 for the lost route.

Cisco Employee

Re: EIGRP/stub and Query

Jorgemario,

I think that the original author of this question did not assert that the R2 has not advertised the "other" network to R3. He just assumed that from R3's viewpoint, the R1 is a successor to that network while the R2 is not even a feasible successor which may happen quite easily, depending on the EIGRP metrics of R2's interfaces.

Best regards,

Peter

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