TMS zl Management and Configuration Guide ST.1.1.100430

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Virtual Private Networks
Generic Routing Encapsulation (GRE) Concepts
Figure 7-153. Redundant GRE
Figure 7-153 shows redundant GRE tunnels between the TMS zl Module at Site
A and the Secure Routers at Site B. The tunnels allow the workstations in
VLAN10 at Site A to access the servers in VLAN8 at Site B. The primary GRE
tunnel has the TMS zl Module’s address in VLAN99 as the local gateway and
one Secure Router's public IP address as the remote gateway. In this example,
the secondary tunnel has the TMS zl Module’s address in VLAN100 as the local
gateway and the other Secure Router's public IP address as the remote
gateway. (It is also possible to create two GRE tunnels that use the same
module IP address as the local gateway.)
Floating Static Routes
Whenever you configure a GRE tunnel, you must configure routes to subnets
behind the remote tunnel endpoint; the forwarding interface for these routes
should be the tunnel interface. These routes can be static routes that you add
manually (including default routes), or you can configure a routing protocol
on the GRE tunnel to enable the module to discover dynamic routes.