Technical Tip: How is MTU mismatch identified in OSPF

ranand · ‎11-26-2021

Description

This article discusses why the OSPF neighborship is stuck in an EXSTART state.

Scope

FortiGate.

Solution

The OSPF Neighborship can get stuck in the Exstart/Exchange state, especially when OSPF is negotiated over an IPsec tunnel.

How is this MTU mismatch identified:

During the Exstart State, DBD packets are exchanged, which contain Interface MTU information.
As per RFC 2328, Section 10.6. If the Interface MTU field in the Database Description packet indicates an IP datagram size larger than the router can accept on the receiving interface without fragmentation, the Database Description packet is rejected.

Solution:
Enable mtu-ignore on the OSPF interface or manually set the mtu to be the same on both sides.

Configuring MTU.

config router ospf
config ospf-interface
        edit int_1
set mtu xxxx<----- Desired MTU size.
        next
   end
end

Enable mtu-ignore on the OSPF interface.

config router ospf
config ospf-interface
        edit int_1
            set mtu-ignore enable
        end
   end

Note:

To form the OSPF neighborship the MTU also should match on both ends or Ignore the MTU using the command set mtu-ignore enable

Troubleshooting steps:

diagnose ip router ospf all enable

diagnose ip router ospf level info

diagnose debug console timestamp enable

diagnose debug enable

Here is the exmpale of the debug output: