Configuration Guide User guide

FastIron Configuration Guide 1049
53-1002494-02
IPv4 point-to-point GRE tunnels
• When a GRE tunnel is configured, you cannot configure 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 configure the OSPF
protocol on the same tunnel and vice-versa. When a tunnel has OSPF configured, 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 flap and the Syslog
message TUN-RECURSIVE-DOWN to be logged. To resolve this issue, create a static route for
the tunnel destination.
Configuration considerations for tunnel loopback ports
NOTE
The configuration considerations for tunnel loopback ports are supported only on FESX and FSX
devices.
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
The default Maximum Transmission Unit (MTU) value for packets in a GRE tunnel is 1476 bytes, or
10194 bytes for jumbo packets. 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.