Avaya 8800 Planning And Engineering page 85

If an IST link fails, the SMLT client switches do not detect a failure and continue to communicate
as usual. Normally, more than one link in the IST is available (the interswitch trunk is itself a
distributed MLT). Thus, IST traffic resumes over the remaining links in the IST.
Finally, if all IST links are lost between an aggregation switch pair, the aggregation switches
cannot communicate with each other. Both switches assume that the other switch has failed.
Generally, a complete IST link failure causes no ill effects in a network if all SMLT client switches
are dual-homed to the SMLT aggregation switches. However, traffic that comes from single
attached switches or devices no longer predictably reaches the destination. IP forwarding may
cease because both switches try to become the VRRP Master. Because the wiring closets
switches do not know about the interswitch trunk failure, the network provides intermittent
connectivity for devices that are attached to only one aggregation switch. Data forwarding,
while functional, may not be optimal because the aggregation switches may not learn all MAC
addresses, and the aggregation switches can flood traffic that would not normally be flooded.
SMLT and IEEE 802.3ad interaction
The Avaya Ethernet Routing Switch 8800/8600 switch fully supports the IEEE 802.3adLink
Aggregation Control Protocol (LACP) on MLT and distributed MLTlinks, and on a pair of SMLT
switches. Be aware of the following information:
• MLT peer and SMLT client devices can be network switches or any type of server/
workstation that supports link bundling through IEEE 802.3ad.
• Single-link and multilink SMLT solutions support dual-homed connectivity for more than
350 attached devices, thus allowing you to build dual-homed server farm solutions.
Only dual-homed devices benefit from LACP and SMLT interactivity.
SMLT/IEEE link aggregation supports all known SMLT scenarios where an IEEE 802.3ad
SMLT pair can be connected to SMLT clients, or where two IEEE 802.3ad SMLT pairs can be
connected to each other in a square or full-mesh topology.
Known SMLT/LACP failure scenarios include:
• Wrong ports connected
• Mismatched SMLT IDs assigned to SMLT client
SMLT switches detect inconsistent SMLT IDs. In this case, the SMLT aggregation switch
that has the lowest IP address does not allow the SMLT port to become a member of the
aggregation group.
• SMLT client switch has LACP disabled
SMLT aggregation switches detect that aggregation is disabled on the SMLT client, thus
no automatic link aggregation is established until the configuration is resolved.
• Single CPU failure
Planning and Engineering — Network Design
Network redundancy
November 2010 85