HP TMS zl Module Security Administrator's Guide

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Configuring a VPN on the HP TMS zl Module
GRE Tunnels
Figure 4-237. Routes for an L2TP over IPsec Client-to-Site VPN
GRE Tunnels
Generic Routing Encapsulation (GRE) is a Layer 2 protocol that can encapsu-
late any protocol that Ethernet can encapsulate. GRE tunneling establishes a
virtual point-to-point connection between two devices across an intervening
network. For example, you could use GRE to tunnel FTP or HTTP traffic
between two networks across an intervening network.
When the TMS zl Module selects traffic for the GRE tunnel, it encapsulates
the traffic with a GRE header and a new IP header. The new IP header includes
the destination address of the remote tunnel endpoint.
GRE can tunnel both multicast and unicast traffic. Therefore, one of the
primary reasons to use GRE is to tunnel multicast traffic through an environ-
ment (such as the Internet) that does not allow multicasts. In fact, you can