Design Reference

Figure 8: Problem description (2 of 2)
However, if you use VLACP to detect far-end failures and allow MLT to fail over when end-to-end
connectivity is not guaranteed for links in an aggregation group, VLACP prevents the failure
scenario in the preceding figure.
Avaya recommends that you use the following guidelines for VLACP implementation:
• Do not use VLACP on configured LACP MLTs because LACP provides the same functionality
as VLACP for link failure. Avaya Virtual Services Platform 4000 Series does not support
VLACP and LACP on the same link.
• Use the following best practice standard settings for VLACP:
- a short timer—no less than 500 milliseconds (ms)
- a time-out scale of 5
Tip:
The VSP 4000 supports both faster timers and lower time-out scales, but if VLACP flapping
occurs, increase the short timer and the time-out scale to their recommended values: 500
and 5, respectively. Although the software configuration supports VLACP short timers of
less than 30 ms, the platform does not support using values less than 30 ms in practice.
The shortest (fastest) supported VLACP timer is 30 ms with a time-out of 3, which achieves
sub-100 ms failover.
• Do not configure VLACP timers to less than 100 ms if you plan to use a Layer 3 core with
Equal Cost Multipath (ECMP).
Note:
This recommendation assumes a combination of basic Layer 2 and Layer 3 with Open
Shortest Path First (OSPF). If you have more complex configurations, you can require
higher timer values.
• Ensure that the VLACP configuration at the port level is consistent, that both sides of the point-
to-point connection are either enabled or disabled. If a VLACP-enabled port does not receive a
VLACP protocol data unit (PDU), it enters the disabled state. However, occasions exist when a
VLACP-enabled port does not receive a VLACP PDU but remains in the forwarding state. You
can avoid this situation with consistent port-level VLACP configuration.
• Configure VLACP on an individual port basis.
The port can be either an individual port or an MLT member. Each VLACP-enabled port
periodically sends VLACP PDUs. This action allows the exchange of VLACP PDUs from an
Physical layer redundancy
December 2014 Network Design Reference for Avaya VSP 4000 Series 41
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