A LAN client can use a
dynamic process or static configuration to determine which router should be the
first hop to a particular remote destination. The client examples of dynamic
router discovery are as follows:
client uses Address Resolution Protocol (ARP) to get the destination it wants
to reach, and a router responds to the ARP request with its own MAC address.
protocol—The client listens to dynamic routing protocol updates (for example,
from Routing Information Protocol [RIP]) and forms its own routing table.
IRDP (ICMP Router
Discovery Protocol) client—The client runs an Internet Control Message Protocol
(ICMP) router discovery client.
The drawback to
dynamic discovery protocols is that they incur some configuration and
processing overhead on the LAN client. Also, in the event of a router failure,
the process of switching to another router can be slow.
An alternative to
dynamic discovery protocols is to statically configure a default router on the
client. This approach simplifies client configuration and processing, but
creates a single point of failure. If the default gateway fails, the LAN client
is limited to communicating only on the local IP network segment and is cut off
from the rest of the network.
The Virtual Router
Redundancy Protocol (VRRP) feature can solve the static configuration problem.
VRRP is an IP routing redundancy protocol designed to allow for transparent
failover at the first-hop IP router. VRRP enables a group of routers to form a
. The LAN clients can then be configured with the virtual router
as their default gateway. The virtual router, representing a group of routers,
is also known as a
Figure 1 shows a LAN topology in which VRRP
is configured. In this example, Routers A, B, and C are
(routers running VRRP) that compose a virtual router. The IP address of the
virtual router is the same as that configured for the interface of Router A
Figure 1. Basic VRRP
Because the virtual
router uses the IP address of the physical interface of Router A, Router A
assumes the role of the
and is also known as the
IP address owner.
As the master virtual router, Router A controls the IP address of the
virtual router and is responsible for forwarding packets sent to this IP
address. Clients 1 through 3 are configured with the default gateway IP address
Routers B and C
routers. If the master virtual router fails, the router configured with the
higher priority becomes the master virtual router and provides uninterrupted
service for the LAN hosts. When Router A recovers, it becomes the master
virtual router again.
We recommend that you disable Spanning Tree Protocol (STP) on switch
ports to which the virtual routers are connected. Enable RSTP or rapid-PVST on
the switch interfaces if the switch supports these protocols.