Avaya 8800 Planning And Engineering, Network Design page 82

Redundant network design
SMLT failure and recovery
Traffic can cease if an SMLT link is lost. If a link is lost, the SMLT client switch detects the loss and
sends traffic on the other SMLT links, as it does with standard MLT. If the link is not the only one
between the SMLT client and the aggregation switches in question, the aggregation switch also
uses standard MLT detection and rerouting to move traffic to the remaining links. However, if the link
is the only route to the aggregation switch, the switch informs the other aggregation switch of the
SMLT trunk failure. The other aggregation switch then treats the SMLT trunk as a regular multilink
trunk. In this case, the MLT port type changes from splitMLT to normalMLT. If the link is
reestablished, the aggregation switches detect it and move the trunk back to regular SMLT
operations. The operation mode changes from normalMLT back to splitMLT.
Traffic can also cease if an aggregation switch fails. If an aggregation switch fails, the SMLT client
switch detects the failure and sends traffic out on other SMLT links, as in standard MLT. The
operational aggregation switch detects the loss of the partner IST. The SMLT trunks are modified to
regular MLT trunks, and the operation mode is changed to normalMLT. If the partner switch IST
returns, the operational aggregation switch detects it. The IST again becomes active, and after full
connectivity is reestablished, the trunks are moved back to regular SMLT.
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.
June 2016 Planning and Engineering — Network Design
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