Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 117)

Configuration considerations for tunnel loopback ports
NOTE
The configuration considerations for tunnel loopback ports are only required for Generation 2 modules
supported on FSX devices.
NOTE
When a tunnel loopback port is configured, it is automatically added to the default vrf.
Consider the following when configuring tunnel loopback ports for GRE tunnels:
• For multicast traffic over a GRE tunnel, each PIM-enabled tunnel interface must have a dedicated
tunnel loopback port.
• For unicast traffic, a tunnel loopback port can be oversubscribed, meaning multiple GRE tunnels (up
to the maximum supported) can use the same tunnel loopback port for traffic. When oversubscribed,
proper traffic classification on the tunnel loopback port is necessary in order to avoid traffic
congestion. In this case, Brocade recommends that you configure the trust level at the DSCP level
for QoS by adding an ACL that maps DSCP 46 to priority 5. Otherwise, loss of loopback packets may
flap the tunnel interface.
• By default, when you create a tunnel loopback port for a GRE tunnel on a port that is part of the
default VLAN, the port will stay in the default VLAN. Before configuring a port as a tunnel loopback
port for a GRE tunnel, if the port is in the default VLAN (VLAN 1), first create a VLAN, then add the
port to the VLAN. Otherwise, an error message such as the following will appear on the console
when you attempt to configure a router interface for the default VLAN.
ERROR: Router-interface cannot be applied because of GRE loopback port 1/2
• Configuration of tunnel loopback ports are not applicable on the SX-FI48GPP interface module.
GRE MTU configuration considerations
When jumbo is enabled, the default Ethernet MTU size is 9216 bytes. The maximum Ethernet MTU size
is 10200 bytes for ICX 6610 devices, 10178 bytes for ICX 6450 devices and 10218 bytes for other
devices. The MTU of the GRE tunnel is compared with the outgoing packet before the packet is
encapsulated. After encapsulation, the packet size increases by 24 bytes. Therefore, when changing
the GRE tunnel MTU, set the MTU to at least 24 bytes less than the IP MTU of the outgoing interface. If
the MTU is not set to at least 24 bytes less than the IP MTU, the size of the encapsulated packet will
exceed the IP MTU of the outgoing interface. This will cause the packet to either be sent to the CPU for
fragmentation, or the packet will be dropped if the DF (Do-Not-Fragment) bit is set in the original IP
packet, and an ICMP message is sent.
NOTE
The fragmentation behavior depends on the mtu-exceed setting on the router. This feature is not
supported on FSX devices.
Configuration tasks for GRE tunnels
Perform the configuration tasks in the order listed.
Configuration considerations for tunnel loopback ports
FastIron Ethernet Switch Layer 3 Routing Configuration Guide 117
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