background image
L
AN segments are very reliable when compared to their wide
area counterparts. However, failure occurs on these segments as well, mak-
ing fault tolerance an important issue. The most well-known fault-tolerance
mechanism on a LAN consists of the dual rings encountered in Fiber Distrib-
uted Data Interface (FDDI) networks. This FDDI technique occurs at the
Physical layer of the OSI model.
In this chapter, you will learn about fault-tolerance methods that occur
beyond the Physical layer--in particular, fault tolerance that occurs at the
Data Link and Network layers of the OSI model. The types of fault tolerance
covered here include Proxy Address Resolution Protocol (ARP), Internet
Control Message Protocol (ICMP), ICMP Router Discovery Protocol (IRDP),
dynamic routing protocols, and the Hot Standby Routing Protocol (HSRP).
The Hot Standby Routing Protocol (HSRP) is a proprietary protocol
developed by Cisco to support fault tolerance on multi-access media. This
protocol provides high network availability and transparent network topol-
ogy changes.
The specifications for HSRP were published in March 1998 as RFC 2281.
A month later, the open standards implementation, called the Virtual Router
Redundancy Protocol (VRRP), was published in RFC 2338. VRRP and
HSRP perform the same essential function.
The purpose of HSRP is to allow hosts to appear to use a single router and
still maintain connectivity, even if the actual next hop router (default gate-
way) they are using fails. This is accomplished by creating a virtual router
and having the physical routers communicate in such a way that the virtual
router is always available.
Copyright ©2000 SYBEX , Inc., Alameda, CA
www.sybex.com