3Com 4007, Implementation Manual, Page: 402 / 684

402 C
HAPTER
17: V
IRTUAL
R
OUTER
R
EDUNDANCY
P
ROTOCOL
(VRRP)
For this scheme to work, the association between VRIDs and IP addresses
must be coordinated among all VRRP routers across the LAN: otherwise,
the backup router does not have a valid set of IP addresses to use.
Virtual Router
Decision-making
The example in Figure 45 shows only two routers, so there is no
ambiguity as far as which router should have assumed responsibility upon
a failure. However, there can be more than one virtual router on a
network because there can be more than one backup router for each
static gateway. This is because a single backup router, at the time of
assuming primary router responsibilities, becomes the single point of
failure.
See Figure 46 for an example of a network topology that:
■
Allows all routers on the LAN to be backed up by more than one
virtual router
■
Allows hosts on any subnetwork to reach destinations on any other
subnetwork in the extended network
Figure 46 Multiple Virtual Routers Backing Up Each Other
The parallel design in Figure 46 takes advantage of the capabilities of
VRRP. This design can be extended to include more routers and more
subnetworks. In a more complex virtual router scheme with many backup
routers, this method ensure that all routers have adequate backup in the
event of a failure.
99.99.1.5
99.99.2.7
99.99.2.5
99.99.2.6 99.99.2.7
99.99.1.0
99.99.2.0
99.99.1.6
L3
L3 L3
Router A
(Backup B
and C) Router B
(Backup A
and C)
Router C
(Backup A
and B)