Design Reference

increasing ND traffic, especially when many hosts try to determine the reachability of one of more
routers.
To provide fast failover of a default router for IPv6 LAN hosts, the switch supports the Virtual Router
Redundancy Protocol (VRRP) for IPv6
VRRP for IPv6 provides a faster switchover to an alternate default router than is possible using the
ND protocol. With VRRP for IPv6, a backup router can take over for a failed default router in
approximately 3 seconds (using default parameters). This failover is accomplished without host
interaction and with a minimum amount of VRRP traffic.
IPv6 VRRP operation is similar to the IPv4 VRRP operation, including support for the holddown
timer, critical IP, fast advertisements, and backup master. With backup master enabled, the backup
switch routes all traffic according to its routing table. The backup master switch does not perform
Layer 2 switching for the traffic to the VRRP master.
New to the IPv6 implementation of VRRP from the IPv4 implementation, you must specify a link-
local address to associate with the virtual router. Optionally, you can also assign global unicast IPv6
addresses to associate with the virtual router. Network prefixes for the virtual router are derived from
the global IPv6 addresses assigned to the virtual router.
VRRP backup master with triangular SMLT:
The standard implementation of VRRP supports one active master switch for each IPv6 subnet. All
other VRRP interfaces in a network are in backup mode.
A deficiency occurs when VRRP-enabled switches use SMLT. If VRRP switches are aggregated
into two SMLT switches, the end host traffic is load-shared on all uplinks to the aggregation switches
(based on the Multilink Trunk [MLT] traffic distribution algorithm).
However, VRRP usually has only one active routing interface enabled. All other VRRP routers are in
backup mode. Therefore, all traffic that reaches the backup VRRP router is forwarded over Virtual
Inter-Switch Trunk (vIST) toward the master VRRP router. In this case, vIST potentially does not
have enough bandwidth to carry all the aggregated traffic.
To resolve this issue, assign the backup router as the backup master router. The backup master
router can actively load-share the routing traffic with a master router.
Because the two VRRP peer nodes exchange MAC address tables, the VRRP backup master can
forward traffic directly, on behalf of the master router. The switch in the backup master state routes
all traffic received on the backup master IP interface according to its routing table. The backup
master switch does not perform Layer 2 switching for the traffic to the VRRP master.
If you enable SMLT on the backup master router, the incoming host traffic is forwarded over the
SMLT links as usual.
Important:
Do not use VRRP backup master and critical IP at the same time. Use one or the other.
IPv6 VRRP and ICMP redirects:
In IPv6 networks, do not enable ICMP redirects on VRRP VLANs. If you enable this option (using
the ipv6 icmp redirect-msg command), VRRP cannot function. The option is disabled by
default.
Layer 3 network design
72 Network Design Reference for Avaya VSP 4000 Series January 2015
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