EIGRP internals and getting hands dirty in debugging routing adjacency and solving EIGRP neighboring issues.
What is sequence TLV and Conditional Receive CR-mode and CR flag
Couple of days ago I got a strange network behavior in my CCIE lab. Something was wrong between a router and L3 switch connection and there was EIGRP neighbor relationship reset every few minutes. It was happening all the time so I decided to debug a little.
It was looking something like this and I was very confused about it:
*Mar 1 01:00:32.135: EIGRP: Received Sequence TLV from 18.104.22.168
*Mar 1 01:00:32.135: 22.214.171.124
*Mar 1 01:00:32.139: address matched
*Mar 1 01:00:32.139: clearing CR-mode
*Mar 1 01:00:32.139: EIGRP: Received CR sequence TLV from 126.96.36.199, sequence 5
*Mar 1 01:00:32.139: EIGRP: Received UPDATE on FastEthernet0/0 nbr 188.8.131.52
*Mar 1 01:00:32.139: AS 1, Flags 0xA, Seq 5/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1, not in CR-mode, packet discarded
*Mar 1 01:10:00.123: EIGRP: FastEthernet0/0 multicast flow blocking cleared
The thing that was happening here is basically just some EIGRP internals doing their job. To be precise this was reliable routing information delivery over both multicast and unicast doing his reliable update delivery.
Normal EIGRP synchronization is done using multicast.
If we have two EIGRP routers that are trying to make initial sync and become neighbors they will intentionally see each other as “laggard” in the beginning. The term laggard is important here as it symbolize a neighbor to witch we are sending EIGRP updates in separate unicast communication.
If one EIGRP router is sending updates to, let’s say, 5 other neighbors. It will send that update to address 184.108.40.206 and it will include the update sequence number inside. Let’s say Seq=25.
When it sends the update Seq=25 to the network segment it will get ready to send next update with Seq=26 and wait for the acknowledgement of sequence 25 update. The problem is that the router will put newly prepared update Seq=26 onto the transmission lists for all 5 neighbors and it will not send it out to anybody until he acknowledges sequence 25 update from all 5 neighbors. That means that if one of 5 routers does not send back the acknowledgement for Seq=25 update our router will not continue sending multicast update Seq=26 to anybody and he will lose the neighbors after hold timer expires.