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EIGRP Stub Routing question

Community,

I am studying for the CCNP ROUTE exam and reading about stub routing. I'm reading in the "old school" BSCI book the following:

"A stub router indicates in the hello packet to all neighboring routers its status as a stub router. Any neighbor that receives a packet informing it of the stub status does not query the stub router for any routes. Therefore, a router that has a stub peer does not query that peer. Stub routers are not queried. Instead, hub routers connected to the stub router answer the query on behalf of the stub router." 

My question is this: If Queries are typically multicasted, then how can the router sending the query control which router receives the query? Is it dependent on the WAN technology being leveraged? I could maybe see in Frame Relay how the Hub router might be intelligent enough to not send a query on a certain DLCI or something.

 

I read that queries are usually multicasted but can sometimes be unicasted. I guess im confused as to in what circumstances a query would be multicasted and in what circumstances a query would be unicasted. 

 

Any help to clear up this confusion would be most helpful. I hate learning how to configure something without REALLY knowing how it works.

 

Thanks!

 

 

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Re: EIGRP Stub Routing question

Community,

I just labbed this up in GNS3 using a hub router and 3 spokes all connected to the hub via PPP serial links. I got EIGRP neighborships going between the hub and all 3 spokes and then configured the spokes as Stub routers. I then simulated a network failure on the Hub (admin down a loopback) to see if it would Mcast queries to the spokes and the hub DID NOT generate any queries to the spoke routers. So I think this answers my question, at least for p2p links. I am still not sure how it works on broadcast multi-access segments though. Ill have to try that out. Again, any feedback is also helpful. 

Thanks!

VIP Advocate

Re: EIGRP Stub Routing question

Hi,

The stub routing feature is most commonly used in hub-and-spoke networks. This feature is configured only on the spoke routers as you tested in your lab. When configured on the spoke router, the router announces its stub router status using a new TLV in the EIGRP Hello messages. The hub router upon receiving the Hello packet with the new TLV, the router will not query the stub router about the status of any prefixes. This is the default mode of operation.

 

Regards,
Deepak Kumar,
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