I have a unique issue that I ran into and I can't figure out what is happening.
I have a hub router with multiple frame pvc's over a single DS-3. At the remote ends I have a mix of 1602, 1720's and 2611's. They are all pretty much configured the same, but when I started reviewing the configs I noticed that all of the subinterfaces on the 7507 that were connected to the 1720's all had the mtu set at 1500. All of the other interfaces were set at the default 4470 for the physical interface. I removed the "ip mtu 1500" command from the subinterface and OSPF stopped working, but the subinterface showed "up/up". When I checked the OSPF neighbor relationship the subinterface was in EXSTART and would not change. I reapplied the mtu to 1500 and it came right back up.
I'm just trying to figure out why I have to have the mtu command configured on subinterfaces that connect to 1720's but not on a subinterface that connects to a different model.
In order for OSPF to form adjacencies, the MTU of each neighbor must be identical. Apparently, the 1720s are set to 1500 and the others are set to 4470. To verify this, do a show int on the devices in question and the MTU should be listed.
I understand that, but why is it that I have to configure the MTU on the links connecting to the 1720's, but not the 1602's or the 2611's? The only difference that I can find is that the 1720's are using a different (newer) version of code. If I look at the interfaces on the 1602/2611 the mtu is 1500, but the other end of the circuit on the 7507 has the mtu set at 4470. I'm just wondering if it is an IOS issue that is causing me to configure the mtu size for the 1720 links.
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