As per the attached image, i've got three routers connected in a square triangle
.
Configs are as below, quite simple setup.
I start off without the manual summary configured on R1, everything is fine, all routing is as expected.
I add the manual summary to R1 and the following events occur.
1) R1 sends a flash update to R3, prefix 15.15.0.0/16(1), metric in brackets! R3 will update its RIB and send a flash update to R2(2), it won't send a flash update back to R1 due to split-horizon.
2) R2 receives the prefix 15.15.0.0/16(2) from R3, updates it's RIB and sends a flash update 15.15.0.0/16(3) to R1. No flash update is sent to R3 due to split-horizon.
3) When R1 receives the prefix 15.15.0.0/16(3) from R2, it updates it's RIB, and sends a poison prefix 15.15.0.0/16(16) to R3.
4) R3 will receive the poisoned route, the prefix is removed from the RIB and R3 will send a flash update with the poisoned prefix to both R1 and R2. This poisoned triggered updates will propagate throughout the network until all ribs are flushed of 15.15.0.0/16.
5) Eventually, R1 will send the summary to R3 via a standard update and this cycle will continue again.
Step 3) is the interesting part!
I found that this behaviour occurs only if a prefix received on R1 matches the interface summary. The prefix will be poisoned out the summary interface. If the poisoned route makes its way back via an alternative path to R1, the cycle will continue. If no alternate path existed, R1 would keep the RIB entry i.e. prefix received by R2 and on the next update send the summary to R3, i.e. stable network.
Looks like a summary route feedback protection mechanism i never knew about!
R1#sh ip int bri | ex una
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.1.9 YES manual up up
FastEthernet0/1 192.168.1.1 YES manual up up
Loopback0 15.15.15.15 YES manual up up
R1#sh runn | sec router rip
router rip
version 2
passive-interface Loopback0
network 15.0.0.0
network 192.168.1.0
no auto-summary
R1#sh runn int fa0/1
Building configuration...
Current configuration : 144 bytes
!
interface FastEthernet0/1
ip address 192.168.1.1 255.255.255.252
ip summary-address rip 15.15.0.0 255.255.0.0
duplex auto
speed auto
end
!
R2#sh ip int bri | ex una
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.1.10 YES manual up up
FastEthernet1/0 192.168.1.5 YES manual up up
R2#sh runn | sec router rip
router rip
version 2
network 192.168.1.0
no auto-summary
R3#sh ip int bri | ex una
Interface IP-Address OK? Method Status Protocol
FastEthernet0/1 192.168.1.2 YES manual up up
FastEthernet1/0 192.168.1.6 YES manual up up
R3#sh runn | sec router rip
router rip
version 2
network 192.168.1.0
no auto-summary
R3#
