Technical Tip: OSPF Interoperability with Cisco: Setting up an area ID greater than 255

Description

This article describes how to configure an area number greater than 255 on the FortiGate OSPF setting to match that of Cisco.

The area ID in OSPF is represented in a dotted decimal format, similar to an IP address. While Cisco routers allow a single decimal value for configuring an area, a FortiGate requires the area to be configured in a dotted decimal format.

Scope

FortiOS v4.0, v5.0, v5.2.

Solution

The decimal area that is configured on the Cisco will have to be converted into a dotted decimal format and then used on the FortiGate.

Example 1:
Area number configured on the Cisco 256.
Corresponding dotted decimal format for FortiGate is: 0.0.1.0.

Example 2:
Area number configured on Cisco 257.
Corresponding dotted decimal format for FortiGate is: 0.0.1.1.

Using the sniffer:

The other way, in case a conversion is not wanted, would be to run a sniffer on proto 89 and look at the OSPF header. The command is:

diagnose sniffer packet "proto 89" 6 0 a

Once the packets are captured, convert them using help from the related KB article, Technical Tip: How to import 'diagnose sniffer packet' data to WireShark - Ethereal application.

An example is shown below:

Related article:

Technical Tip: How to import 'diagnose sniffer packet' data to WireShark - Ethereal application