I have two routers connected over a frame relay PVC using sub-interfaces. Router A has a HSSI interface with a larger MTU size. Router B has a serial interface with a normal 1500 MTU size. These two routers will not form an OSPF adjacency. I know I would normally use the ip ospf mtu-ignore command on the router with the smaller mtu size but the command is not available due to it being an older IOS. Is there another workaround? Each router has other PVCs to other routers with which they have OSPF adjacenys already as well. I would like to not upgrade the IOS for now unless I have to.
MTU values are part of the OSPF database description packets. Thus the only way in your case to get an OSPF adjacency seems to be: set the MTU size to the same value on both ends of the FR PVC.
Thanks for the reply. Yes, I went ahead and brought down the mtu size on the subinterface of the HSSI interface and I was able to get an adjacency. Thanks.
