Avaya G430 Manual page 439

interface concludes that the tunnel is down. The source interface continues to send keepalive
packets, but until it receives a response from the tunnel's destination interface, the tunnel
informs hosts that send packets to the tunnel that the tunnel is down.
Gxxx-001# interface tunnel 1
Gxxx-001(if:Tunnel 1)# keepalive 20 3
Done!
Keepalive command parameters
The keepalive command includes the following parameters:
seconds: The length, in seconds, of the interval at which the source interface sends keepalive
packets. The default value is 10.
retries: The number of retries after which the source interface declares that the tunnel is
down. The default value is 3.
Dynamic MTU discovery
The size of packets that can travel through a GRE tunnel is limited by the lowest MTU of any
router along the route through the tunnel. When dynamic MTU discovery is enabled, the tunnel
maintains an MTU limit.
When a large packet is sent from the host with the DF bit on, and a router in the tunnel path
has an MTU that is smaller than the size of the packet, since the DF bit is set, the router sends
an ICMP unreachable message back in the originator (in this case, the GRE router). The GRE
router then updates the tunnel's MTU limit accordingly. When a packet larger than the MTU
arrives at the tunnel, if the packet is marked "do not fragment", the tunnel's source interface
sends the packet back to the host requesting the host to fragment the packet. When dynamic
MTU discovery is disabled, the tunnel's source interface marks each packet as may be
fragmented, even if the packet's original setting is do not fragment. For more information on
MTU and fragmentation, refer to
Enabling and deactivating dynamic MTU discovery
Procedure
1. Use the tunnel path-mtu-discovery command in the GRE Tunnel interface
2. To deactivate the feature, use the no tunnel path-mtu-discovery
tunnel path-mtu-discovery parameters
The tunnel path-mtu-discovery command includes the following parameters:
age-timer: How long until the local tunnel endpoint returns the tunnel MTU to its default. The
default value of this parameter is 10 minutes.
Administering Avaya G430 Branch Gateway
context to enable dynamic MTU discovery by the tunnel.
command.
Fragmentation on page 477.
The router
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