Avaya 8800 Planning And Engineering, Network Design page 83

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
In this case, LACP on other switches detects the remote failure, and all links connected to the
failed system are removed from the link aggregation group. This process allows failure
recovery to a different network path.
SMLT and LACP System ID
Since Release 4.1.1, an administrator can configure the LACP SMLT System ID used by SMLT core
aggregation switches. Prior to Release 4.1.1, if the SMLT core aggregation switches did not know
and were unable to negotiate the LACP system ID, data could be lost. Avaya recommends that you
configure the LACP SMLT system ID to be the base MAC address of one of the aggregate switches,
and that you include the SMLT-ID. Ensure that the same System ID is configured on both of the
SMLT core aggregation switches.
An explanation of the importance of configuring the System ID is as follows.
The LACP System ID is the base MAC address of the switch, which is carried in Link Aggregation
Control Protocol Data Units (LACPDU). When two links interconnect two switches that run LACP,
each switch knows that both links connect to the same remote device because the LACPDUs
originate from the same System ID. If the links are enabled for aggregation using the same key,
then LACP can dynamically aggregate them into a LAG (MLT).
When SMLT is used between the two switches, they act as one logical switch. Both aggregation
switches must use the same LACP System ID over the SMLT links so that the edge switch sees one
logical LACP peer, and can aggregate uplinks towards the SMLT aggregation switches. This
process automatically occurs over the IST connection, where the base MAC address of one of the
SMLT aggregation switches is chosen and used by both SMLT aggregation switches.
However, if the switch that owns that Base MAC address restarts, the IST goes down, and the other
switch reverts to using its own Base MAC address as the LACP System ID. This action causes all
edge switches that run LACP to think that their links are connected to a different switch. The edge
switches stop forwarding traffic on their remaining uplinks until the aggregation can reform (which
can take several seconds). Additionally, when the restarted switch comes back on line, the same
actions occur, thus disrupting traffic twice.
The solution to this problem is to statically configure the same SMLT System ID MAC address on
both aggregation switches.
For more information about configuring the LACP SMLT system ID, see Avaya Ethernet Routing
Switch 8800/8600 Configuration — Link Aggregation, MLT, and SMLT, NN46205-518.
June 2016 Planning and Engineering — Network Design
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Network redundancy
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