Design Reference

• Simpler configuration — A Routed SMLT Layer 2 Edge configuration only requires enabling
RSMLT on a VLAN. VRRP requires virtual IP configuration along with other parameters.
For connections in pure Layer 3 configurations using a static or dynamic routing protocol, use a
Layer 3 RSMLT configuration instead of SMLT with VRRP. RSMLT configuration provides faster
failover than VRRP.
Important:
In an SMLT-VRRP environment that uses VRRP critical IP within both vIST core switches,
routing between directly connected subnets ceases to work when connections from each of the
switches to the exit router (the critical IP) fail. Do not configure VRRP critical IPs within SMLT or
RSMLT environments because SMLT operation automatically provides the same level of
redundancy.
Do not use VRRP BackupMaster and critical IP at the same time; use one or the other. Do not
use VRRP in RSMLT environments.
The VRRP Master typically forwards traffic for a given subnet. Use BackupMaster on the SMLT
aggregation switch with a destination routing table entry and the Backup VRRP switch also routes
traffic. The VRRP BackupMaster uses the VRRP standardized backup switch state machine. This
makes the VRRP BackupMaster compatible with standard VRRP. This capability prevents the traffic
from edge switches from unnecessarily utilizing the vIST to deliver frames destined for a default
gateway. In a traditional VRRP implementation, this operates only on one of the aggregation
switches.
The BackupMaster switch routes all traffic received on the BackupMaster IP interface according to
the switch routing table. The BackupMaster switch does not perform Layer 2 switching for the traffic
to the VRRP Master.
Ensure that both SMLT aggregation switches can reach the same destinations using a given routing
protocol. Configure individual VLAN IP addresses on both SMLT aggregation switches for routing
purposes. Introduce an additional subnet on the vIST that has a shortest-route path to avoid issuing
Internet Control Message Protocol (ICMP) redirect messages on the VRRP subnets. To reach the
destination, ICMP redirect messages are issued if the router sends a packet back out through the
same subnet on which it is received.
SMLT and IEEE 802.3ad interaction
Virtual Services Platform 4000 fully supports IEEE 802.3ad LACP on MLT and distributed MLT links.
On a pair of SMLT switches:
• MLT peer and SMLT client devices can be network switches, a server, or a workstation that
supports link bundling through IEEE 802.3ad.
• Multilink SMLT solutions support dual-homed connectivity for more than 350 attached devices,
which allow dual-homed server farm solutions.
Only dual-homed devices benefit from LACP and SMLT interactivity.
SMLT and IEEE link aggregation supports all known SMLT scenarios where an IEEE 802.3ad SMLT
pair can connect to SMLT clients or where two IEEE 802.3ad SMLT pairs can connect to each other
in a square or full-mesh topology.
Known SMLT and LACP failure scenarios include
• wrong ports connected
• LACP is disabled on the SMLT edge switch
Layer 2 switch clustering and SMLT
52 Network Design Reference for Avaya VSP 4000 Series June 2015
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