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Author Topic: 5520 OSPF cisco mtu issue  (Read 2521 times)

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Offline JonathanP

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5520 OSPF cisco mtu issue
« on: June 08, 2014, 04:57:42 PM »
I have a couple of 5520 stacks connected to each other by fibre. Each stack is then linked on to a couple of Cisco 2901 routers connected to our MPLS. We are running OSPF with the Ciscos and the Nortels talking to each other and the two Nortels talking to each other via OSPF.
In order to make the OSPF work our MPLS provider has had to set the mtu on the Cisco interfaces connected to the Nortels down to 1500. They would like to run with a higher mtu so that they can pass MPLS packets across the fibre link between the two Nortels on a seperate vlan. When they raise the mtu the OSPF stops working. Is there any way to change the mtu for an interface on a 5520 switch? The only setting I can find relating to mtu is jumbo frames which is on globally.

Offline Telair

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Re: 5520 OSPF cisco mtu issue
« Reply #1 on: June 17, 2014, 11:57:45 AM »
You have to enable Jumbo frames globally first.  Then if you have a routed interface on the switch, I think you should be able to edit the MTU value of that interface.  Unfortunately I don't have one right here to poke at to verify it.  Otherwise, doesn't Cisco have a command to ignore MTU sizes on OSPF frames specifically to get around this issue?

Offline Michael McNamara

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Re: 5520 OSPF cisco mtu issue
« Reply #2 on: June 22, 2014, 10:16:26 AM »
I believe you need to be running software release 6.0 or higher in order to support Jumbo frames, then you need to confirm that it's enabled globally and you should be all set. The support for Jumbo frames only supports frames greater than 1500 bytes (less then 9000 bytes). You need to change the actual MTU size on the endpoints, the devices sending and receiving the traffic.

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Offline TankII

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Re: 5520 OSPF cisco mtu issue
« Reply #3 on: July 08, 2014, 11:51:36 AM »
Jumbo frames is enabled by Default since version 4 code on 5500.
I don't have this configuration in our environment, and haven't seen any 5500 OSPF issues since 6.0 and up was GA, so I cannot help you.
I don't see how that small of an MTU change would impact performance, especially since 90% of all packets are going to have a payload less than 1472 anyway.