Technical Tip: Basic OSPF configuration and MTU settings in OSPF

[agodbole]

Description

This article describes basic OSPF configuration on FortiOS and the behavior of default MTU settings.

• 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

Default OSPF configuration.

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:

1. Area Configuration.
   
2. Interface Configuration.
   
3. 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) # sh 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

    edit "XXX"
        set mtu <1436>

MTU can also be ignored:

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