EIGRP auto summmary query

[steven.crutchley]

I have the following network:

All routers are configured in EIGRP AS 1 and the network 172.16.0.0 command (and the network 10.0.0.0 command for R1 and R4). All except for Router 1 have been configured with no auto. Router 1 has been left as the default of auto summarisation. The link between R2 and R3 is down.

My question is as follows:

What will R1 advertise to  R3 about 172.16.4.0/24 and why?

It will summarise at the classful boundary... meaning that it will send 172.16.0.0 to Router 4. However what will it send to R3?

After running this in GNS3 I find that R3 has 172.16.4.0/24 in its routing table. But why? Does it realise that the link to R3 is in the same classful network and not summarise?

[John Blakley · Accepted solution]

Steven,

"Does it realise that the link to R3 is in the same classful network and not summarise?"

This is correct. EIGRP summarizes at boundaries. What you would see is your 172.16.4.0/24 and 10.0.0.0/8 on R3. On R4, you should see the 172.16.0.0/16 and 10.1.1.0/24 (because it's connected. If you changed R2's link to 10.1.2.0/24, you'd see that 10.1.2.0/24 is on R4, but R3 would have a 10.0.0.0/8.

HTH,

John

[steven.crutchley]

Ah ok. I understand. Thank you.