3Com CoreBuilder 3500, Руководство по внедрению, Страница: 320 / 592

320 C
HAPTER
12: V
IRTUAL
R
OUTER
R
EDUNDANCY
P
ROTOCOL
(VRRP)
After the original Master again become operational, it begins again to
broadcast advertisements to the other virtual routers if preempt mode is
enabled. Packet forwarding responsibility then shifts back to the original
Master router.
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 62 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 63 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 63
Multiple Virtual Routers Backing Up Each Other
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)