3Com CoreBuilder 3500, Implementation Manual

Page: 316 / 592

316 C
HAPTER
12: V
IRTUAL
R
OUTER
R
EDUNDANCY
P
ROTOCOL
(VRRP)
VRRP Overview
A critical component of IP networking is the way in which hosts and
routing devices find the next-hop address in a connectionless
environment. There are several different ways of determining the
next-hop address, but they all fall into two basic categories:
■
Router to Router
■
Host to Host and Host to Gateway
Router to Router Router-to-router communication is usually accomplished by means of a
routing protocol such as Routing Information Protocol (RIP) or Open
Shortest Path First (OSPF), or by static routes. Routers consult their own
routing tables to make intelligent next hop decisions for the forwarding
of IP packets.
Host to Host and
Host to Gateway IP host-to-host communication typically begins with an ARP request to
the destination host address, providing that the destination resides on the
same subnet as the sending device. If the destination address resides on a
non-local subnet, then the sending device must use one of the following
methods to learn the route to the remote network:
■
Routing protocol
■
ICMP Router Discovery
■
Static route
■
Default gateway
Routing Protocols
Routing protocols provide dynamic updates to end stations in the event
of a network failure, but they are typically not used on most hosts
because they require additional setup, processing power and, in some
cases, additional software.
ICMP Router Discovery
ICMP Router Discovery directs a host to use the router with the highest
preference level as the default gateway. Internet Control Message
Protocol (ICMP) does this by enabling hosts that are attached to multicast
or broadcast networks to discover the IP addresses of their neighboring
routers and determine which router to use for a default gateway. If you
prefer, you can make this default gateway choice yourself.