3Com CoreBuilder 3500, Implementation Manual

Page: 317 / 592

VRRP Overview 317
Static Route
A static route is an IP address that is user-configured and fixed. Static
routes are useful if the host only needs to access a few networks; in this
case, static routes actually require less overhead than dynamic routing
protocols. However, in today’s networking environment in which traffic
patterns are less predictable, many routes are usually required, and static
routes then become prohibitive to maintain.
Default Gateway
Most host stations today use a default gateway to facilitate routing. You
simply define for the host an IP address on the local subnet of a router
that is responsible for routing packets to their destinations. This approach
is widely deployed today; however, it has one major drawback: if the
default gateway becomes unavailable, then all routing to remote
networks stops, requiring manual intervention to restore connectivity
even if there are alternate paths available.
VRRP addresses this drawback by defining an election protocol that
dynamically assigns responsibility for a
virtual router to one of the VRRP
routers on a LAN. The election process automatically detects a failure of
the primary (Master) router, and transfers all traffic forwarding to the
backup router. All of this is done without your intervention, which
dramatically increases uptime in a Layer 3 IP network.
Example
In the simplest scenario, a VRRP configuration includes two routers, a
primary router (called the Master router) and a backup router. If the
Master router fails for any reason, the backup router assumes all
forwarding functions for the Master router. The backup router monitors
the network for hello packets, which are periodically sent by the Master
router (the default time period is 1 second). If the backup router misses
three hello packets in succession, that router assumes forwarding
functions for the Master.
See Figure 62 for a visual representation of a simple virtual router
configuration.