• When a GRE tunnel is congured, you cannot congure the same routing protocol on the tunnel through which you learn the
route to the tunnel destination. For example, if the FastIron learns the tunnel destination route through the OSPF protocol, you
cannot congure the OSPF protocol on the same tunnel and vice-versa. When a tunnel has OSPF congured, the FastIron
cannot learn the tunnel destination route through OSPF. This could cause the system to become unstable.
• The tunnel destination cannot be resolved to the tunnel itself or any other local tunnel. This is called recursive routing. This
scenario would cause the tunnel interface to ap and the Syslog message TUN-RECURSIVE-DOWN to be logged. To resolve
this issue, create a static route for the tunnel destination.
Conguration considerations for tunnel loopback ports
NOTE
The conguration considerations for tunnel loopback ports are only required for Generation 2 modules supported on FSX
devices.
NOTE
When a tunnel loopback port is congured, it is automatically added to the default
vrf.
Consider the following when conguring tunnel loopback ports for GRE tunnels:
• For multicast trac over a GRE tunnel, each PIM-enabled tunnel interface must have a dedicated tunnel loopback port.
• For unicast
trac, a tunnel loopback port can be oversubscribed, meaning multiple GRE tunnels (up to the maximum
supported) can use the same tunnel loopback port for trac. When oversubscribed, proper trac classication on the tunnel
loopback port is necessary in order to avoid trac congestion. In this case, Brocade recommends that you congure 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
ap 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 conguring a port as a tunnel loopback port for a GRE tunnel, if the port is in the default VLAN
(VLAN 1), rst 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 congure a router interface for the default VLAN.
ERROR: Router-interface cannot be applied because of GRE loopback port 1/1/2
• Conguration of tunnel loopback ports are not applicable on the SX-FI48GPP interface module.
GRE MTU conguration 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.
IPv4 point-to-point GRE tunnels
FastIron Ethernet Switch Layer 3 Routing
53-1003627-04 107
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