A P P E N D I X B - Link Aggregation Control Protocol

A P P E N D I X B

Link Aggregation Control Protocol

This appendix describes the Link Aggregation Control Protocol (LACP), which has been added to this release. This appendix includes the following sections:

Overview

LACP Benefits

Marker Protocol

Overview

Sun provides a fully compliant IEEE 802.3ad implementation including the LACP protocol and associated Marker Responder.

Link Aggregation Control Protocol

The following quotation is taken directly from the 802.3ad specification,

The Link Aggregation Control Protocol (LACP) provides a standardized means for exchanging information between Partner Systems on a link to allow their Link Aggregation Control instances to reach agreement on the identity of the Link Aggregation Group to which the link belongs, move the link to that Link Aggregation Group, and enable its transmission and reception functions in an orderly manner.

Information from received LACP messages is used to determine which physical ports to bind to which Aggregators and to control the operation of the Distributors and Collectors within each Aggregator.

Each device refers to itself as an Actor (that is, the device taking action) and to the device at the other end of the link as its Partner: LACP compares Actor information with that of the Partner to determine what action to take.

LACP ensures that aggregations are created only among links that interconnect the same pair of devices. An Actor will only consider aggregating links when received LACP information indicates that the same system is the Partner for all links in the potential aggregation.

LACP Benefits

The Sun implementation of LACP provides two basic benefits:

At the time that a user is requesting a link to aggregate, LACP will communicate with the Partner system to determine that the port is suitable for aggregation. If not, the port will remain associated with the aggregation yet there will be no traffic on this link until such time that the partner system information becomes suitable for aggregation or the link is unaggregated. The nettr(1M) command will return success in either case.

After a link has been successfully aggregated, LACP will detect if a Partner link is moved or cannot be unaggregated in some other way. If the link cannot be unaggregated, LACP will disable the port's Collector_Distributor and the link status will be continually monitored. Data traffic will not flow on the port until such time that the link can be aggregated. The user will be notified of such events. A few examples follow:

An example where LACP is beneficial is when a user decides to dynamically add a NIC to a system and request that it join an aggregation. LACP will dynamically determine if the NIC is plugged into the appropriate switch and add it to the aggregation if everything is correct.

Another example is where network interfaces on a Sun system are connected across layer 2 switches. When a user requests to aggregate, LACP will prevent links with a different System ID (MAC) from transmitting data. Without LACP, links that are statically aggregated would cause spanning tree to break a link Additionally, the illegal configuration would succeed.

Preventing traffic from flowing on illegal configurations in the first place is helpful. Subsequently, when encountering illegal configurations due to modifications, disconnecting the transmit/receive functions of the offending links and issuing user warnings also provides benefit.

Marker Protocol

The Marker Protocol includes the Marker Responder as well as the Marker Generator and Receiver for functionally correct load distribution schemes. Trunking 1.3 implements the Marker Responder as specified in 802.3ad. The 802.3ad standard also provides two methods to ensure that packets are not disordered when moving conversations. They are time-outs and the Marker Generator. Either a time-out can be used or the Distributor can send an explicit Marker PDU after the last frame in a conversation. Sun uses the timeout method with an initial default value of 2500Us prior to moving a conversation to another link.

Note - LACP will never aggregate a link that has not been previously configured through the nettr(1M) utility.

Dynamic link aggregation is disabled by default. This is called LACP_OFF. If enabled, the values can be either LACP_PASSIVE or LACP_ACTIVE. This can be controlled by a nettr(1M) option to turn on LACP. This option can also be modified after links have been aggregated through the -mod option.

System priorities are configured on all systems and switches that are running 802.3ad. A lower numerical value, such as 0, has a higher system priority than a higher numerical value. A system with a higher System Aggregation Priority can dynamically modify it's operational Key values.

Sun Trunking 1.3 has a default System Priority value of 0x1000. This is a very low system priority. Switches must retain the ability to dynamically alter their Actor Operational parameters such as key values. The link aggregation configuration on the Sun system on the other hand, is statically determined at the time the aggregation is specified through nettr(1M).