Lab 42 - Summarization with a Floating Default Route

Prerequisites: CCNP level skills.

Personal Note!
The example used in this post is completely ludicrous. It is designed to show a certain behavior of EIGRP and the way to resolve the issue. It is not a best practice example (on the contrary). Nor is the CCIE lab.

EIGRP (as per CCNP track) offers a plethora of ways to summarize the prefixes:

1. Automatic Summarization.
2. Summarization ('ip summary-address).
3. Summarization with a Default Route.
4. Summarization with a Floating Default Route (floating summary route).
5. Summarization with Administrative Distance 255.
6. Selective Summarization with a Leak Map.

Topology

Pic. 1 - EIGRP NBMA Topology.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task 1

EIGRP is running on all interfaces of all routers shown in the topology (pic. 1). Check R3's EIGRP routing table. On R2 summarize all EIGRP prefixes so that EIGRP on R1 receives only a default route from R2. On R3 check the reachability to 192.168.2.2 and 192.168.3.2.

Task 2
On R1 summarize all EIGRP perfixes so that EIGRP on R3 receives only a default route from R1. On R3 check the reachability to 192.168.2.2 and 192.168.3.2. In case of problems fix the issue but do not remove the summarization on R1 and R2.

Lab Solution

Task 1

EIGRP is running on all interfaces of all routers shown in the topology (pic. 1). Check R3's EIGRP routing table. On R2 summarize all EIGRP prefixes so that EIGRP on R1 receives only a default route from R2. On R3 check the reachability to 192.168.2.2 and 192.168.3.2.

Pic. 2 - Routing Table on R3.

R2 Configuration:

!

interface FastEthernet1/0
 ip address 10.1.12.2 255.255.255.0
 ip summary-address eigrp 1 0.0.0.0 0.0.0.0 5

!

Verification:

Pic. 3 - R1's Routing Table.

Pic. 4 - R3's Routing Table.

Pic. 5 - Ping from R3 to R2's Loopbacks.

Task 2

On R1 summarize all EIGRP perfixes so that EIGRP on R3 receives only a default route from R1. On R3 check the reachability to 192.168.2.2 and 192.168.3.2. In case of problems fix the issue but do not remove the summarization on R1 and R2.

R1 Configuration:

!

interface Serial0/1
 ip address 10.1.13.1 255.255.255.0
 ip summary-address eigrp 1 0.0.0.0 0.0.0.0 5

!

Verification:

Pic. 6 - Ping on R3 to R2's Loopbacks.

Note!

Ping does NOT get through. Checking the routing table on R3 shows the default route towards R1.

Pic. 7 - R3's Routing Table.

Pic. 8 - R1's Routing Table.

Note!

The discard (summary) route gets installed and the one propagated by R2 is gone. Since it is pointing to Null0 interface packets to R2 are being discarded by R1. The reason is that summary route has the default Administrative Distance of 5. The default route advertised by R2 has the Administrative Distance of 90.

I'm aware of two ways of fixing it. First one is presented below, the next one in the next post. Order of operations matters. First, I must remove the summary route on R1 (simple overwriting does not work). Then re-create it with Administrative Distance higher than 90.

R1 Configuration:

!

R1(config-if)#no ip summary-address eigrp 1 0.0.0.0 0.0.0.0
R1(config-if)# ip summary-address eigrp 1 0.0.0.0 0.0.0.0 250

!

Verification:

Pic. 9 - R1 Installs the Default Route with AD 90.

Pic. 10 - R3 Reachability.

The second method is to poison the discard route on R1 by increasing its AD to the maximum 255 (look at the next post).