TMS zl Management and Configuration Guide ST.1.2.100916

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Virtual Private Networks
Generic Routing Encapsulation (GRE) Concepts
In fact, a GRE tunnel will indicate that it is up before the other side of the
tunnel is configured. This means that the local tunnel endpoint routes packets
across the GRE tunnel even when the other endpoint is unreachable and the
packets are lost.
The TMS zl Module supports a GRE tunnel keepalive mechanism, which
enables each GRE tunnel endpoint to verify that the other tunnel endpoint is
reachable. If the remote endpoint does not respond to a certain number of
keepalives (the number being configurable per-tunnel), the module considers
the GRE tunnel to be down.
When the GRE tunnel is down, any static routes pointing out of the GRE tunnel
interface are removed from the routing table. In this way, the module avoids
sending packets across the GRE tunnel when the remote endpoint is unreach-
able. However, the module does continue sending keepalives so that when the
remote tunnel endpoint again becomes reachable, the GRE tunnel will go
back up.
Note The tunnel interface on the TMS zl Module will respond to keepalives from
the remote tunnel gateway even when you do not enable keepalives on that
interface. Therefore, you can set keepalives on one side of the tunnel but not
the other if you want. However, only the side of the tunnel that sends keepal-
ives will use the keepalives to determine the status of the tunnel; the other
side will always consider the tunnel to be up.
Redundant GRE
Redundant GRE is a strategy for minimizing tunnel downtime by using one
GRE tunnel as the primary link between sites and using a second GRE tunnel
as the back-up in case the primary GRE tunnel fails.