OSPF MTU value over the physical L2 path between L3 endpoint

Thiyagarajan K · ‎02-09-2017

Hello All,

I'm working on OSPF configuration and I have query on setting the MTU over the L2 path between L3 endpoints.

Topology:

Firewall(L3 SVI with an MTU of 1500 bytes) <=> Nexus 9K (L2 trunk with and MTU of 9000 buytes) <=> Nexus 9K (L2 trunk with and MTU of 9000 buytes) <=> Router (L3 SVI with an MTU of 1500 bytes)

In the above topology:

1- Does OSPF will form the full adjacency as the L2 trunks are set with an MTU of 9000 bytes whereas the SVI configured devices are set with an MTU of 1500 bytes?

2- Could you please share any link for OSPF troubleshooting on Nexus 9K?

3- Does OSPF has path MTU feature to identify the MTU value set the over the path to the other end points?

Regards,

Thiyagu

Ganesh Hariharan · ‎02-20-2017

Hi Thiyagu,

Interface MTU is defined as: “The size in bytes of the largest IP datagram that can be sent out the associated interface, without fragmentation”. So, routers attached to a link exchange their interface MTU value in DBD packets when the OSPF adjacency is initialized.

OSPF checks whether neighbors are using the same MTU on a common interface. This check is performed when neighbors exchange DBD packets. If the receiving MTU in the DBD packet is higher than the IP MTU configured on the incoming interface, OSPF adjacency will not be established.

If the ip ospf mtu-ignore command is enabled for a virtual network and you want to disable it, use the disable keyword in virtual network interface configuration mode.

Examples

The following example disables OSPF MTU mismatch detection on receiving DBD packets:

interface serial 0/0
 ip ospf mtu-ignore

Hope It Helps..

-GI