Administration of the Avaya G350 Media Gateway - Avaya Support

Configuring the routerDynamic MTU discoveryThe size of packets that can travel through a GRE tunnel is limited by the lowest MTU of anyrouter along the route through the tunnel. When dynamic MTU discovery is enabled, the tunnelmaintains an MTU limit.When a large packet is sent from the host with the DF bit on, and a router in the tunnel path hasan MTU that is smaller than the size of the packet, since the DF bit is set, the router sends anICMP unreachable message back in the originator (in this case, the GRE router). The GRErouter then updates the tunnel’s MTU limit accordingly. When a packet larger than the MTUarrives at the tunnel, if the packet is marked do not fragment, the tunnel’s source interfacesends the packet back to the host requesting the host to fragment the packet. When dynamicMTU discovery is disabled, the tunnel’s source interface marks each packet as may befragmented, even if the packet’s original setting is do not fragment. For more information onMTU and fragmentation, refer to Overview of fragmentation on page 201.Use the tunnel path-mtu-discovery command in the context of the GRE tunnel interfaceto enable dynamic MTU discovery by the tunnel. Use the no form of this command to deactivatethe feature.The tunnel path-mtu-discovery command includes the following parameters:●age-timer — how long, until the local tunnel endpoint returns the tunnel MTU to itsdefault. The default value of this parameter is 10 minutes.Setting up a GRE tunnelTo set up a GRE tunnel:1. Use the interface tunnel command, followed by a number identifying the tunnel, tocreate the new tunnel interface. If you are changing the parameters of an existing tunnel,use the interface tunnel command, followed by a number identifying the tunnel, toenter the context of the tunnel. For example:G350-001(super)# interface tunnel 2G350-001(super-if:Tunnel 2)#2. In the interface context, use the tunnel source command, followed by the public IPaddress of the local tunnel endpoint, to set the source address of the tunnel. For example:G350-001(super-if:Tunnel 2)# tunnel source 70.70.70.2Done!G350-001(super-if:Tunnel 2)#172 Administration of the Avaya G350 Media Gateway