Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 124)

does not have the ability to bring down the line protocol of either tunnel endpoint, if the far end
becomes unreachable. Traffic sent on the tunnel cannot follow alternate paths because the tunnel is
always UP. To avoid this scenario, enable GRE link keepalive, which will maintain or place the tunnel
in an UP or DOWN state based upon the periodic sending of keepalive packets and the monitoring of
responses to the packets. If the packets fail to reach the tunnel far end more frequently than the
configured number of retries, the tunnel is placed in the DOWN state.
To enable GRE link keepalive, configure it on one end of the tunnel and ensure the other end of the
tunnel has GRE enabled.
NOTE
Keepalives are not supported when a tunnel interface is not within the default-VRF.
To configure GRE link keepalive, enter commands such as the following.
device(config)# interface tunnel 1
device(config-tnif-1)# keepalive 12 4
These commands configure the device to wait for 4 consecutive lost keepalive packets before bringing
the tunnel down. There will be a 12 second interval between each packet. Note that when the tunnel
comes up, it would immediately (within one second) send the first keepalive packet.
Syntax: [no] keepalive seconds retries
Use the no form of the command to disable the keepalive option.
The seconds variable specifies the number of seconds between each initiation of a keepalive
message. The range for this interval is 2 - 32767 seconds. The default value is 10 seconds.
The retries variable specifies the number of times that a packet is sent before the system places the
tunnel in the DOWN state. Possible values are from 1 through 255. The default number of retries is 3.
Use the show interface tunnel and show ip tunnel traffic commands to view the GRE link keepalive
configuration.
Configuring Path MTU Discovery (PMTUD)
PMTUD is enabled by default on tunnel interfaces. This section describes how to disable and re-
enable PMTUD on a tunnel interface, change the PMTUD age timer, manually clear the tunnel
PMTUD, and view the PMTUD configuration.
NOTE
For the SX-FI8GMR6, SX-FI2XGMR6, SX-FI624HF, SX-FI624C, SX-FI624P, and the SX-FI62XG
modules, all fragments will carry the same DF bit as the incoming packet. For the SX-FI-24GPP, SX-
FI48GPP, SX-FI-24HF, SX-FI-2XG, and SX-FI-8XG modules and the FCX modules, the DF bit on the
outer IP header after encapsulation will be set if the PMTU is enabled. If PMTU is disabled, the DF bit
will be unset irrespective of the DF bit of the incoming packet.
Disabling and re-enabling PMTUD
PMTUD is enabled by default. To disable it, enter the following command:
device(config-tnif-1)# tunnel path-mtu-discovery disable
To re-enable PMTUD after it has been disabled, enter the following command:
device(config-tnif-1)# no tunnel path-mtu-discovery disable
Configuring Path MTU Discovery (PMTUD)
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