Technical Tip: Basic OSPF configuration and MTU settings in OSPF
Description
- FortiOS supports OSPF routing protocol. This article is intended to demonstrate the minimal configuration required for OSPF and explain the default behavior of MTU values under OSPF configuration.
- MTU (Maximum Transmission Unit) is the size (in bytes or octets) of the largest protocol data unit that the layer can accept.
- Normal Ethernet frames accept an MTU value of ~1500 bytes.
Scope
FortiGate.
Solution
In FortiOS, OSPF configuration is simple and can be done in 3 sub hierarchies under the 'router ospf' hierarchy after defining the 'router-id'.
OSPF configuration consists of 3 basic hierarchies:
- Area Configuration.
- Interface Configuration.
- Network Configuration.
Area Configuration:
config area
edit <a.b.c.d>
next
end
Interface Configuration:
config ospf-interface
edit "new"
set interface "XXXX"
next
end
Network Configuration:
config network
edit X
set prefix <p.q.r.s> <subnet mask>
next
end
Default MTU value and CLI option to change it.
MTU value is one crucial element to be matched to establish the adjacency of OSPF. MTU values are exchanged in the Hello packets between 2 OSPF speaking routers.
OSPF Adjacency is established only when the MTU values of the two routers are matched. By default, the MTU configuration option is available under config router -> config ospf-interface hierarchy.
FortiOS supports a default MTU value of 1500 on all FortiOS versions.
Examples to manually configure MTU value in OSPF:
The default values of all the parameters can be seen by checking the 'full-configuration'. For example:
FGT (ospf) # show full-configuration
config ospf-interface
edit "ospf"
...
set mtu 0 <--
...
next
end
The default value of MTU size under the OSPF interface is seen as '0'. This implies that ospf-interface does not change the default MTU value of 1500.
To change this value, edit 'set mtu' under the ospf-interface hierarchy. For example:
config ospf-interface
set mtu <1436>
MTU can also be ignored:
config router ospf
config ospf-interface
edit int_1
set mtu-ignore enable
end
end
Configure the MTU value at the Interface:
Note:
This configuration is necessary when jumbo frame is enabled on the neighbor's interface. Since FortiGate does not support Jumbo frames by default (as the default MTU is 1500), it has to be enabled at the interface level by using an MTU size of 9000+ on the interface that is participating in the OSPF configuration.
config system interface
edit port <x>
set mtu-override enable
set mtu 9216 <----- MTU Value.
next
end
Related articles:
Technical Note: MTU size and Jumbo frames support on FortiGate devices
Technical Tip: Set up a jumbo frame in the IPsec VPN interface