Dynamic Mtu Discovery - Avaya G450 Manual

retries. The number of retries after which the source interface declares that the tunnel is
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down. The default value is 3.
The following example configures Tunnel 1 to send keepalive packets every 20 seconds. If the
tunnel's destination interface fails to respond to three consecutive packets, the tunnel's source
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.
G450-001# interface tunnel 1
G450-001(if:Tunnel 1)# keepalive 20 3
Done!
Note:
You do not have to configure tunnel keepalive on both sides of the tunnel.
Note:

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
Use the tunnel path-mtu-discovery command in the GRE Tunnel interface context to
enable dynamic MTU discovery by the tunnel. Use the no form of this command to deactivate
the 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 its default.
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The default value of this parameter is 10 minutes.
infinite. The tunnel does not update the MTU, and its value remains permanent
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Configuring fragmentation
Configuring GRE tunneling
on page 502.
Issue 1 January 2008 461