Use Virtual Link Aggregation Control Protocol (VLACP) as an extension to LACP for end-to-end failure detection. VLACP is not a link aggregation protocol, it is a mechanism to periodically check the end-to-end health of a point-to-point connection. VLACP uses the Hello mechanism of LACP to periodically send Hello packets to ensure end-to-end communication. After Hello packets are not received, VLACP transitions to a failure state, which indicates a service provider failure and that the port is disabled.
The VLACP only works for port-to-port communications where there is a guarantee for a logical port-to-port match through the service provider. VLACP does not work for port-to-multiport communications where there is no guarantee for a point-to-point match through the service provider. You can configure VLACP on a port.
You can also use VLACP with MLT to complement its capabilities and provide quick failure detection.
VLACP trap messages are sent to the management stations if the VLACP state changes. If the failure is local, the only traps that are generated are port linkdown or port linkup.
The Ethernet cannot detect end-to-end failures. Functions such as remote fault indication or far-end fault indication extend the Ethernet to detect remote link failures. A major limitation of these functions is that they terminate at the next Ethernet hop. They cannot determine failures on an end-to-end basis.
For example, in Problem description (1 of 2), after the Enterprise networks connect the aggregated Ethernet trunk groups through a service provider network connection (for example, through a VPN), far-end failures cannot be signaled with Ethernet-based functions that operate end-to-end through the service provider network. The multilink trunk (between Enterprise switches S1 and S2) extends through the Service Provider (SP) network.
The following illustration shows an MLT running with VLACP. VLACP can operate end-to-end, but you can also use it as a point-to-point link.
In the following illustration, if the Layer 2 link on S1 (S1/L2) fails, the link-down failure is not propagated over the SP network to S2 and S2 continues to send traffic over the failed S2/L2 link.
Use VLACP to detect far-end failures, which causes MLT to failover if end-to-end connectivity is not guaranteed for links in an aggregation group. VLACP prevents the failure scenario.
The switch software uses the following VLACP timers:
fast periodic timer—100 to 20 000 ms; default 200 ms
slow periodic timer—10 000 to 30 000 ms; default 30 000 ms
Use the information in this section to understand the considerations while configuring VLACP into your network.
If a VLACP-enabled port does not receive a VLACP Data Unit (VLACPDU), it must enter the disabled state. There are occasions where a VLACP-enabled port does not receive a VLACPDU but remains in the forwarding state. To avoid this situation, ensure that the VLACP configuration at the port level is consistent. You must either enable or disable both sides of the point-to-point connection.
If VLACP is enabled on a MACsec Key Agreement-enabled link, it takes approximately 30 seconds for the VLACP session to begin.
You can configure VLACP on each port. The port can be either an individual port or an MLT member. VLACPDUs can be sent periodically on each port where VLACP is enabled to exchange VLACPDUs from an end-to-end perspective. If VLACPDUs are not received on a particular link, that link is taken down after the expiry timeout occurs (timeout scale x periodic time).