Technical Tip: How is MTU mismatch identified in OSPF

[ranand]

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: