FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article describes a possible issue when IS-IS adjacency is not formed between FortiGate and Cisco. This issue might manifest when jumbo frames are enabled on the interface used for IS-IS.
Scope
FortiOS.
Solution
In case the jumbo frames are enabled on the Cisco side, Cisco starts sending the IS-IS hello packets with Ethernet type of 0x8870.
FortiGate will ignore this ethertype, and the adjacency is not formed.
Solution:
IS-IS hello padding must be disabled.
On FortiGate, the configuration is:
config router isis config isis-interface edit <interface_name> set hello-padding disable next end end
IS-IS hello padding must also be disabled on the Cisco side.
