On R2, rather than one interface on 172.16.1.1/21, can you make two loopbacks, one on 172.16.1.1/22 and the other on 172.16.5.1/22. Leave the ip summary-address eigrp 172.16.0.0 255.255.248.0 on R1 F0/1. I think R1 should have three routes:
172.16.0.0/22 via F0/1 with AD=90
172.16.4.0/22 via F0/1 with AD=90
172.16.0.0/21 via Null0 with AD=5, the discard route.
And you shoud be able to ping both the R2 loopbacks from R3.
Then remove the loopback on R2 with the 172.16.4.0/22, and change the mask on ,the 172.16.0.0/22 loopback to /21. Does the AD=5 discard route in R1 disappear and get replace by 172.16.0.0/21 via F0/1 at AD=90?
Can you still ping the remaining loopback?
Apologies for the delay, got sidetracked with work !!
It happens exactly as you suggested it would. Once you remove the 172.16.5.1/22 loopback and change the subnet mask on the 172.16.1.1/22 loopback to 255.255.248.0 then R1 removes
ip route 172.16.0.0 255.255.248.0 Null0 and replaces it with
ip route 172.16.0.0 255.255.248.0 192.168.10.57 ie. the route being received from R2.
Does seem to prove the case.
Thank you very much for doing that lab experiment - I have learned something useful from it. I hope I shall be able to return the compliment some day.
For a moment there I was wondering whether you could get into trouble by summarising the same way two different places. I'm talking about real summaries here, with a mask that is shorter than the routes it is summarising.
If you do summarize the same way in two different places, I guess router A has the discard route, and router B gets the propagated summary from router A, and therefore supresses the discard route as we saw here. Effectively, router B does not need a discard route cos it gets router A to discard the unknown subnets for it.
The AD=5 was a bit of a distraction, and I realise now I had misunderstood it. If I understand correctly now, the discard route goes into the local table at AD=5, and gets distributed to the other routers where it is picked up as a normal internal route and handled at AD=90.
Jon, what I was interested in knowing was what happens if you put a summary on R1 for the 172.16.0.0/21. Does it sink all the traffic on its way to R2?
I know that is a bit of an artifical test, but it could happen in the real world with a slightly different addressing scheme. Suppose, for example, R2 in your branch office had split 172.16.0.0/21 into two parts, 172.16.0.0/22 and 172.16.4.0/22, and you had summarised to 172.16.0.0/21 on R1 at HQ for administrative reasons. That would work. Then one day the local NetAdmin at the branch office decides to merge the two halves and gives you a route to 172.16.0.0/21 instead.