GRE Tunnel Parameters

GRE Link Keepalive

When GRE tunnels are used in combination with static routing or policy-based routing, and a dynamic routing protocol such as RIP, BGP, or OSPF is not deployed over the GRE tunnel, a configured tunnel 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.

Maximum Number of GRE Tunnels Supported

Use the following table to determine how many GRE tunnels are supported. You can configure the device to support up to the maximum number of GRE tunnels as displayed in the following table.

Number of GRE Tunnels Supported for ICX Devices

Device Maximum Number of GRE Tunnels Default Number of GRE Tunnels
ICX 7550 64 16
ICX 7650 64 16
ICX 7850 64 16
ICX 8200 64 16
Note: You must save the configuration using the write memory command and reload the software to ensure the change is made.

GRE MTU

You can set an MTU value for packets entering the tunnel. When jumbo packet size is enabled, the default Ethernet MTU size is 9216 bytes. The maximum Ethernet MTU size is 10178 bytes. 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: For ICX 7850 devices, the maximum jumbo frame size supported is 9380. When jumbo mode is enabled, the maximum ethernet MTU size is 9358 bytes.

Parent topic: IPv4 Point-to-Point GRE Tunnels