When a Router adds the EIGRP summary address interface command, it will inject a route for the summary address to interface Null0 into its global routing table with a default administrative distance (AD) of 5. This is known as the discard route, and it's not exclusive to EIGRP. However, this occurs because if the Router were to lose reachability to a more specific route that fell within the summary, the Router would wind up using the shorter summary route to Null0 for forwarding the traffic, thereby preventing packets from being sent to any neighbors that may no longer have the route.
If the Router did not have such a mechanism and instead forwarded the traffic anyway, then there could be a risk of a next hop neighbor having connectivity to some path that actually leads back to the Router - creating a loop!
In this scenario, the Provider sends a default route to R1 via eBGP with an AD of 20. R1's fa0/1 interface is then configured to send an EIGRP summary route. With no AD configured, a route to the EIGRP summary via interface Null0 with an AD of 5 is injected into the global routing table, thereby removing the previously accepted default route to 10.1.1.100 via eBGP with the AD of 20.
R2 learns the advertised route via EIGRP, and installs the route in its global routing table with the AD of 90. At this point, any destinations that R1 receives packets for that are not specifically in the global routing table will take the default route to Null0.
In this scenario, the Provider sends a default route to R1 via eBGP with an AD of 20.
Provider Sends Default route to R1:
router bgp 100 neighbor 10.1.1.1 remote-as 200 neighbor 10.1.1.1 default-originate
Resulting routing table for R1:
R1#show ip route | in 0.0.0.0Gateway of last resort is 10.1.1.100 to network 0.0.0.0B* 0.0.0.0/0 [20/0] via 10.1.1.100, 00:00:10
R1's fa0/1 interface is then configured to send an EIGRP summary route.
R1 Sends EIGRP Summary to R2:
interface fa0/1 ip summary-address eigrp 100 0.0.0.0 0.0.0.0
R1#show ip route 0.0.0.0 | include distance Known via "eigrp 100", distance 5, metric 2169856, candidate default path, type internal
With no AD configured, a route to the EIGRP summary via interface Null0 with an AD of 5 is injected into the global routing table, thereby removing the previously accepted default route to 10.1.1.100 via eBGP with the AD of 20.
Resulting routing table for R1:
R1# show ip route | include 0.0.0.0Gateway of last resort is 0.0.0.0 to network 0.0.0.0D* 0.0.0.0/0 is a summary, 00:21:30, Null0
R2 learns the advertised route via EIGRP, and installs the route in its global routing table with the AD of 90.
Resulting routing table for R2:
R2#show ip route | in 0.0.0.0Gateway of last resort is 10.12.1.1 to network 0.0.0.0D* 0.0.0.0/0 [90/2681856] via 10.12.1.1, 00:23:21, Fa0/1
At this point, any destination that R1 receives packets for that are not specifically in the global routing table will take the default route to Null0.
Although not always recommended and depending on your needs, you can remove the Null0 route by adding the AD of 255. However, in this case, a high AD of 220 is configured.
R1 Sends EIGRP Summary to R2:
interface fa0/1 ip summary-address eigrp 100 0.0.0.0 0.0.0.0 220
This allows the previously accepted route with the AD of 20 back to the global routing table, but still maintains a floating route to Null0 in the event the default eBGP route is lost.
Resulting routing table for R1:
R1# clear ip route*R1# show ip route | in 0.0.0.0 Gateway of last resort is 10.1.1.100 to network 0.0.0.0B* 0.0.0.0/0 [20/0] via 10.1.1.100, 00:00:08
R2 has no change, and still learns the advertised route via EIGRP
Resulting routing table for R2:
R2#show ip route | in 0.0.0.0Gateway of last resort is 10.12.1.1 to network 0.0.0.0D* 0.0.0.0/0 [90/2681856] via 10.12.1.1, 00:33:21, Fa0/1
