Gre mtu configuration considerations – Brocade Communications Systems Layer 3 Routing Configuration ICX 6650 User Manual
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Brocade ICX 6650 Layer 3 Routing Configuration Guide
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IPv4 point-to-point GRE tunnels
NOTE
The above features are supported on VLANs that do not have VE ports.
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Whenever multiple IP addresses are configured on a tunnel source, the primary address of the
tunnel is always used for forming the tunnel connections. Therefore, carefully check the
configurations when configuring the tunnel destination.
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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.
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.