Common Causes of GRE Tunnel Up/Down State

BCD

A GRE tunnel, also known as a Generic Routing Encapsulation tunnel, is a way to create a virtual point-to-point connection between two endpoints over an existing IP network. GRE tunnels encapsulate traffic in a GRE header and are commonly used to connect remote sites over the internet or to connect a remote site to a central site in a WAN.

The up/down state of a GRE tunnel indicates that the tunnel interface is operational, but the protocol state is down, meaning that the tunnel is not forwarding traffic. Here are the three circumstances that can cause a GRE tunnel to be in an up/down state:

1. The tunnel interface IP address is misconfigured: If the tunnel interface IP address is misconfigured, the tunnel will not be able to establish a connection between the two endpoints. The tunnel interface IP address must be a unique IP address that is not in use by any other device on the network. If the IP address is misconfigured, the tunnel will be in an up/down state.

2. The tunnel source interface is down: The tunnel source interface is the interface on the local router that is used to initiate the tunnel. If the source interface is down, the tunnel will not be able to establish a connection. This can be caused by a physical link failure or by an administrative shutdown of the interface. If the source interface is down, the tunnel will be in an up/down state.

3. A valid route to the destination address is missing from the routing table: For a GRE tunnel to forward traffic, there must be a valid route to the destination address in the routing table. If there is no valid route, the tunnel will not be able to forward traffic, and it will be in an up/down state. The routing table must have an entry for the destination address with the tunnel interface as the next hop.

The following answer options are incorrect: 4. The tunnel address is routed through the tunnel itself: This is not a valid option as it is not possible for the tunnel address to be routed through the tunnel itself, as it would cause a routing loop.

5. The ISP is blocking the traffic: This is not a valid option as it is beyond the control of the local router.
6. An ACL is blocking the outbound traffic: This is not a valid option as an ACL can only block traffic that is already flowing, and if the tunnel is in an up/down state, traffic is not flowing.

In summary, a GRE tunnel can be in an up/down state due to misconfigured tunnel interface IP address, a down source interface, or a missing valid route to the destination address in the routing table.