What is MTU size for OSPF?

1500
FortiOS supports default MTU value of 1500 on all FortiOS versions. The default value of MTU size under OSPF interface is seen as ‘0’. This implies that ospf-interface does not change the default MTU value of 1500.

Table of Contents

Does MTU affect OSPF?

OSPF uses IP MTU to peer neighborship and it cannot be set per interface. For example, if the max-frame-size = 1514, then the IP MTU = 1500. Considering the above, you need to increase the Media MTU value so as to get the Protocol MTU the same on all the peers.

What happens if MTU mismatch in OSPF?

Database Description packet is rejected. Basically this means that if a router tries to negotiate an adjacency on an interface in which the remote neighbor has a larger MTU, the adjacency will be denied.

What is the correct MTU size?

28 bytes – IP header and ICMP headers. 1460 + 28 = 1488 is the optimum MTU Setting.

What are the OSPF packet types?

Packet types for OSPF

Hello packet. This packet is sent by the OMPROUTED server to discover OSPF neighbor routers and to establish bidirectional communications with them.
Database description packet.
Link-state update packet.
Link-state request packet.
Link-state acknowledgment packet.

What is the size of OSPF header?

24 bytes
The length of an OSPF packet header is 24 bytes.

How does OSPF choose DR and BDR?

Two rules are used to elect a DR and BDR:

router with the highest OSPF priority will become a DR. By default, all routers have a priority of 1.
if there is a tie, a router with the highest router ID wins the election. The router with the second highest OSPF priority or router ID will become a BDR.

What is ip OSPF MTU ignore?

If you configure ip ospf mtu-ignore what happens is that OSPF ignores MTU as part of the state matchine checking if to form adjacency. The adjacency will then form but there may be other issues later. Many people believe that this is the solution but it may still be an issue when synchronizing the database.

How many packets are in OSPF?

OSPF uses five different packets in its protocols.

What is OSPF packet?

OSPF sends packets to neighbors to establish and maintain adjacencies, send and receive requests, ensure reliable delivery of Link-state advertisements (LSAs) between neighbors, and to describe link-state databases. Link-state databases are generated from all the LSAs that an area router sends and receives.

Can OSPF packages be larger than 1500 bytes?

After Cisco bug ID CSCse01519, OSPF in IOS can pack OSPF packets to be larger than 1500 bytes. This occurs if the MTU of the outgoing interface is larger than 1500 bytes. Transmissions are more efficient because more information can be packed into one larger packet.

Why does the OSPF adjacency fail when the packets are large?

After Cisco bug ID CSCse01519 and the command ip ospf mtu-ignore is not used, it is possible that the OSPF adjacency fails when the actual OSPF packets are larger than 1500 bytes. After Cisco bug ID CSCse01519 and with the command ip ospf mtu-ignore and before Cisco bug ID CSCsk86476, it could be that large (> 1500 bytes) OSPF packets are dropped.

How does OSPF handle large size LSA packets?

There is one exception to this: if the LSA is so big that one LSA holds more than 1500 bytes, then OSPF builds that packet, no matter what the size (OSPF cannot fragment one LSA). The IP stack of the router then fragments it to fit the MTU of the outgoing interface.

What was the size of OSPF DBD packets before cscse01519?

Before Cisco bug ID CSCse01519, the size of OSPF DBD packets was limited by 1500 bytes. That allowed OSPF to work normally even in a MTU-mismatch situation, when OSPF mtu-ignore functionality was used.