Hi OSPF gurus,
1) Say I have 2 neighbours on 1GE, where "A" has MTU 9000 and "B" has MTU 1600, is the risk of mtu-ignore being enabled (so OSPF goes State=FULL) that if "A" has a large LSDB it's DBD pkts might be greater than 1600 bytes and "A" will think it doesn't have to fragment & "B" will drop the DBD?
2) If I have the same huge LSDB and DBD but MTU 1500 both sides does this work because the interfaces know to fragment?
Thanks very much for any clarification.
ps: This is between Cisco c6509 and Nortel 8600. With Nortel 8600 the only size that DBD MTU checking can work at is 1500. If Cisco isn't set to MTU 1500 Nortel must have mtu-ignore enabled.
I'm planning to have Nortel & Cisco MTU 1950 (the Nortel default) and hence must enable mtu-ignore.
1) It will happen also with data traffic
2) The router will generate several DBD packets with max size = interface IP MTU
This command was initially added to support OSPF adj between routers on different L2 networks (FDDI/ethernet for example). The switch took care about fragmentation if required.
Now, this command could be useful if you want to have an adj over GRE tunnel for example.
The important point is you have the same MTU on all devices sharing the same L2 segment so what you want to do is correct