Foundry Networks AR3202-CL - Manual (Page 225)

Foundry AR-Series Router User Guide
14 - 8 © 2004 Foundry Networks, Inc. June 2004
When multipath is disabled, Foundry selects the nexthop address with lowest ip address. For equal cost routes
the nexthops are stored in the increasing (ascending) order of IP address. show ip rpf command displays the
selected path, based on the configured multipath method and the nexthops of the best route to the IP address
passed.
Generic Routing Encapsulation (GRE)
Generic Routing Encapsulation (GRE) handles the transportation of IP multicast traffic between two sites that
have only IP unicast connectivity. Using tunnels in a VPN environment is important because IPSec encryption is
limited to IP unicast frames. IPSec over GRE tunneling allows for the encryption and the transportation of
multiprotocol traffic across the VPN since both unicast and multicast IP packets appear to the IPSec protocol as IP
unicast frame after GRE tunneling. If all connectivity must go through the home gateway router, tunnels also
enable the use of private network addressing across a service provider's backbone without the need for running
the Network Address Translation (NAT) feature.
The Foundry Network implementation of the Generic Routing Encapsulation (GRE) tunneling protocol is based on
standards RFC1701 and RFC2784.
GRE can encapsulate a wide variety of protocol packet types inside IP tunnels, creating a virtual point-to-point link
between routers at remote points over an IP network. An IP tunnel is a logical interface that provides a way to
encapsulate passenger packets inside a transport protocol. By connecting multiprotocol subnetworks in a single-
protocol backbone environment, IP tunneling using GRE allows network expansion across a single-protocol
backbone environment.
For more information on GRE, refer to the GRE Configuration Guide.