This problem is common when running OSPF between a switch (i.e 3550 or 3560) and a router. The error message that is generated when this problem occurs is:
%OSPF-5-ADJCHG: Process 1, Nbr 126.96.36.199 on Vlan258 from DOWN to DOWN, Neighbor Down: Dead timer expired
%OSPF-5-ADJCHG: Process 1, Nbr 188.8.131.52 on Vlan258 from EXSTART to DOWN, Neighbor Down: Too many DBD retransmitions
The resolution is to either adjust the system MTU on the switch or have OSPF ignore the MTU.
A) Change the system MTU on the switch (system mtu 1500 or system mtu routing 1500)
B) Have the router and/or switch ignore the MTU (ip ospf mtu-ignore)
C) Change the interface MTU on the router (GigE only)
Note that using the system mtu 1500 option requires a reboot of the switch but the system mtu routing 1500 does not require a reboot.
About Brian Dennis, CCIE #2210:
Brian Dennis has been in the networking industry for more than 22 years, with a focus on Cisco networking for the past 16 years. Brian achieved his first CCIE in Routing & Switching in 1996, and is currently the only ten year CCIE that holds five CCIE certifications. Prior to working with INE, Brian taught and developed CCIE preparation courses for various well known training organizations. Brian not only brings his years of teaching experience to the classroom, but also years of real world enterprise and service provider experience.
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