Generic Routing Encapsulation
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OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
Alcatel-Lucent
GRE OVERVIEW
GRE is a simple, stateless protocol that allows for the tunneling of traffic. IP is
used as transport for GRE. GRE tunnels can be used to form VPNs, connecting
remote sites using private IP addresses via a public network. Typically, GRE
tunnel is run between the customer edge routers and are transparent to the rest of
the network. GRE tunnels are used to carry non-IP traffic (like IPX, Appletalk,
DECnet from legacy networks) over an IP backbone.
GRE TUNNEL SETUP
A GRE tunnel is configured by specifying two endpoints, one local and the other
remote. In order to establish a tunnel, a GRE tunnel must be configured from the
remote endpoint. No intermediary routers need to be configured, and the tunnel
rides on top of the standard IP. The only requirement is that the tunnel must be
configured in a context where the remote endpoint is reachable.
If the remote address of a GRE tunnel is not reachable then any circuit associated
with that tunnel is brought down. Any interface bound to a GRE circuit is also
marked in a down state, and any route to the tunnel interface is withdrawn. This
prevents the “blackholing” of traffic caused by network instability, where traffic is
sent through a tunnel that can no longer reach the remote endpoint.
Public addresses must be used for tunnel endpoint addresses. It is possible to
use private IP addresses as the GRE tunnel interface IP address allowing a
private address VPN to be carried over a public network.
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