Michael McNamara

Virtual Link Aggregation Control Protocol (VLACP) is extension of the Link Aggregation Control Protocol (LACP) developed by Nortel to detect end-to-end failure over an Ethernet network. We've been deploying VLACP within our network for the past year with great success. We were eager to deploy VLACP because the Nortel Ethernet Switch 470 Gigabit Ethernet fiber ports (GBIC) did not support autonegotiation and are required to be hard set to 1000/Full Duplex when connecting to a Nortel Ethernet Routing Switch 8600. Without autonegotiation there is no mechanism to provide link failure notification (RFI, FEFI) on the specific interface. The problem can arise if you have a GBIC malfunction or a single fiber strand breaks leaving one side of the link up and the other side down. VLACP mitigates this problem by providing a mechanism to detect the path failure and can be applied to provide end-to-end failure notification over a telco carrier network.

Here's what Nortel has to stay in their document, "Link Aggregation Control Protocol (LACP) 802.3ad and VLACP Technical Configuration Guide" dated August 2007;

Virtual LACP (VLACP) is an extension to LACP, used to detect end-to-end failure. VLACP takes the point-to-point hello mechanism of LACP and uses it to periodically send hello packets to ensure end-to-end reachability and provide failure detection (across any L2 domain). When Hello packets are not received, VLACP transitions to a failure state and the port will be brought down. The benefit of this over LACP is that VLACP timers can be reduced to 400 milliseconds between
a pair of ERS8600 switches. This will allow for approximately one second failure detection and switchover. Note that the lowest VLACP timer on an ES460/470 is 500ms. VLACP can also be used with Nortel’s proprietary aggregation mechanism (MLT) to complement its capabilities and provide quick failure detection. VLACP is recommended for all SMLT access links when the links are configured as MLT to ensure both end devices are able to communicate. By using VLACP over Single-Port SMLT, enhanced failure detection is extended beyond the limits of the number of SMLT or LACP instances that can be created on the ERS8600. VLACP can also be used as a loop prevention mechanism in SMLT configurations and should be used when setting up the IST. It also protects against CPU failures by causing traffic to be switched or rerouted to the SMLT peer in the case the CPU fails or gets hung up. Please refer to the Technical Configuration Guide for Switch Clustering using Split-Multilink Trunking (SMLT) with ERS8600 for more details.

NOTE: In regards to the ERS8600, although either the CLI or JDM interface allows you to configure the short timers to less than 400ms, Nortel does not support this configuration unless the ERS8600 is equipped with the SuperMezz daughter module for the 8692SF. The SuperMezz allow for very quick sub 100ms failure detection.

Although functions such as Remote fault indication (RFI) or Far-end fault indication (FEFI) can be used to indicate link failure, there are some limitations with these mechanisms. The first limitation is that with either of these mechanisms, they terminate at the next Ethernet hop. Hence, failures cannot be detected on an end-to-end basis over multiple hops such as LAN Extension services. The second limitation is both of these mechanisms required Auto-Negotiation to be enabled on the Ethernet interface. Hence, if an Ethernet interface does not support Auto-Negotiation; neither of these mechanisms can be used. The third limitation is if an Ethernet interface should fail and still provide a transmit signal, RFI nor FEFI will be able to detect a failure. Hence, the far-end interface will still think the link up and continue to transmit traffic. VLACP will only work for port-to-port applications when there is a guarantee for a logical port-port match. It will not work in a port-to-multi-port scenario where there is no guarantee for a pointpoint match.

NOTE: Please note that VLACP does not perform link aggregation. Is it simply used to detect end-to-end link failures and can be enabled over single links or even MLT trunks. VLACP does not require LACP to be enabled; LACP and VLACP are independent features.

NOTE: When configuring VLACP, both ends of the link must be configured with the same EtherType, Multicast MAC address, and same timers. By default, the VLACP parameters across all ES and ERS switches are the same with the exception of the FastPeriodicTimer which is set to 200ms on the ERS8600 and 500ms on all other switches. When connecting, for example, an ERS8600 to and ERS5500, the recommendation is to use 500ms FastPeriodicTimers with ShortTimeout in order to achieve fast failover. Also, when using the ES460/470 in the 3.6.x software release, the VLACP EtherType must be configured with a different value on each MLT link. The EtherType must match the EtherType value at the far end of the MLT link.

NOTE: If VLACP is used with LACP, there is no difference in how VLACP and LACP bring down a port if no LACP or VLACP PDUs are received. VLACP will declare the VLACP status as down and will report the event in the log file whereas LACP will not synchronize, not activate Collecting and Distributing on this port, and not report a message in the log file. The end result is the same where the port will block traffic; the physical layer for this port will remain up. Although you can enable VLACP with LACP, there is no practical reason why you would do so.

There was an interim solution before VLACP developed by Nortel called Single Fiber Fault Detection (SFFD) specifically designed to allow remote fault detection on Gigabit Ethernet fiber ports that did not support autonegotiation. Unfortunately we had some issues with SFFD and never really deployed the feature beyond our testlab environment.

Ethernet Routing Switch 5510
Here's how you would configure VLACP on the MLT uplinks to an ERS 8600 Switch. You'll need to connect to the 5510 switch and enter the "Command Line Interface" if you have the menu up.

5510> enable
5510# configure terminal
5510(config)# interface fastEthernet 47,48
5510(config-if)# vlacp port 47,48 timeout short
5510(config-if)# vlacp port 47,48 enable
5510(config-if)# exit
5510(config)# vlacp enable
5510(config)# exit

Ethernet Routing Switch 8600
Here's how you would configure VLACP on the MLT uplinks to the ERS 5510 Switch above.

ERS-8610:6# config ethernet 1/1, 2/1 vlacp enable
ERS-8610:6# config ethernet 1/1, 2/1 vlacp timeout short
ERS-8610:6# config ethernet 1/1, 2/1 vlacp fast-periodic-time 500
ERS-8610:6# config vlacp enable

In this example we're using ports 1/1 and 2/1 as the uplinks to ports 47 and 48 on the ERS 5510 respectively. The VLACP short timeout timers on the ERS 8600 default to 200ms so we need to configure them to match the minimum possible with the ERS 5500 series switches of 500ms.

If the interface appears to be bouncing you should definitely check the timers.

Cheers!