Fabric Extend Tunnel IS-IS MTU

Table 1. Fabric Extend Tunnel IS-IS MTU Configuration product support
Feature	Product	Release introduced
Fabric Extend Tunnel IS-IS MTU configuration	5320 Series	Fabric Engine 8.10.1
	5420 Series	Fabric Engine 8.10.1
	5520 Series	Fabric Engine 8.10.1
	5720 Series	Fabric Engine 8.10.1
	7520 Series	Fabric Engine 8.10.1
	7720 Series	Fabric Engine 8.10.1
	VSP 4900 Series	VOSS 8.10.1
	VSP 7400 Series	VOSS 8.10.1

You can configure a unique IS-IS MTU value for each Fabric Extend (FE) tunnel. The IS-IS MTU gets applied to IS-IS packets that get sent through the interface.

You can configure each IS-IS logical interface with a unique IS-IS MTU value for each FE tunnel to improve fragmentation and reassembly in WAN connectivity. This configuration can be applied to standard WAN connections such as MPLS IP WAN as well as SD-WAN. To configure the MTU, see Configure Fabric Extend. For more information about Fabric Extend options for SD-WAN, see SD-WAN.

Note

FE Tunnel IS-IS MTU is an optional configuration that applies to IS-IS packets only. The MTU for data packets is configured separately. For example, with SD-WAN deployments, the MTU for data packets is managed using the SD-WAN Appliance.
The FE Tunnel IS-IS MTU configuration is supported only for Fabric Extend tunnels that run over a Layer 3 core network (FE-IP). This configuration is not supported for Fabric Extend tunnels over a Layer 2 core network.

IS-IS Packet Fragmentation and Reassembly

Fragmentation and reassembly of IS-IS packets is based on the IS-IS MTU value configured for the IS-IS logical interface that connects to the FE tunnel. For Layer 3 IS-IS logical interfaces, you can change the IS-IS MTU configuration at any time. The supported range, in bytes, is 750 to 1600 with a default of 1600.

For example, if you configure an FE tunnel with an IS-IS MTU of 900, and a packet size of 1950 is received on UNI with the destination on the FE tunnel, the system fragments the original 1950-sized packet into the three packets (900, 900, 150) with a packet size equal to or less than 900. The system transmits the three fragmented packets over the IS-IS logical interface of the FE tunnel. After the fragmented packets are received at the destination, the system reassembles the original 1950-sized packet.

Note that for reassembly to succeed, the fragments must arrive at the destination switch in the correct order (based on the offset). Otherwise, the fragments are discarded.

Fabric Extend Tunnel IS-IS MTU Considerations

Consider the following interactions between route MTU and FE Tunnel IS-IS MTU configurations:

If route MTU is not configured, the MTU value for each FE tunnel is applicable to ingress and egress traffic on the tunnel.
If route MTU is configured, the MTU value for each FE tunnel is applicable for ingress traffic on the tunnel. The route MTU value applies to all egress traffic.

Note

System MTU maximum is a separate configuration. You can configure a system maximum MTU size of 1522, 1950, or 9022. The default value is 1950.