Lets say 2 routers A and B connected each other, the MTU of Router A is larger than that of Router B and the router ID of Router B is larger than that of Router A.


After Router A and Router B establish two-way communication, they negotiate a master/slave relationship and determine initial DD sequence numbers for exchanging DD packets. The negotiation result is that Router B acts as the master. After Router A receives an initial DD packet from Router B, it sets the status of Router B to exchange and also sends a DD packet carrying the DD sequence number negotiated for Router B. After Router B receives the DD packet from Router A, it finds that the MTU (carried in the DD packet) of Router A mismatches its own MTU and thus ignores the received DD packet. Router B then re-sends a DD packet. On Router B, you can view that Router A is stuck in the Exstart state.

helpful note

nice doc

good note. It was helpful