Brocade Communications Systems 53-1002163-02 manual Link Aggregation Control Protocol

8 Link aggregation overview

Link Aggregation Control Protocol

Link Aggregation Control Protocol (LACP) is an IEEE 802.3ad standards-based protocol that allows two partner systems to dynamically negotiate attributes of physical links between them to form logical trunks. LACP determines whether a link can be aggregated into a LAG. If a link can be aggregated into a LAG, LACP puts the link into the LAG. All links in a LAG inherit the same administrative characteristics. LACP operates in two modes:

•Passive mode—LACP responds to Link Aggregation Control Protocol Data Units (LACPDUs) initiated by its partner system but does not initiate the LACPDU exchange.

•Active mode—LACP initiates the LACPDU exchange regardless of whether the partner system sends LACPDUs.

There are multiple ASIC limitations related to LAGs:

•Multicast traffic uses the primary link of standard LAG only.

•If more than 10Gb of traffic hashes to a single link of a standard LAG, the traffic is dropped (even though the other links are not used.)

•Brocade trunk can handle only up to 15G traffic coming from another single chip. If traffic comes from multiple chips then it can use the full bandwidth. If the traffic comes from same chip on which the Brocade trunk is present, then it can also use the full bandwidth.

Dynamic link aggregation

Dynamic link aggregation uses LACP to negotiate which links can be added and removed from a LAG. Typically, two partner systems sharing multiple physical Ethernet links can aggregate a number of those physical links using LACP. LACP creates a LAG on both partner systems and identifies the LAG by the LAG ID. All links with the same administrative key and all links that are connected to the same partner switch become members of the LAG. LACP continuously exchanges LACPDUs to monitor the health of each member link.

Static link aggregation

In static link aggregation, links are added into a LAG without exchanging LACPDUs between the partner systems. The distribution and collection of frames on static links is determined by the operational status and administrative state of the link.

Brocade-proprietary aggregation

Brocade-proprietary aggregation is similar to standards-based link aggregation but differs in how the traffic is distributed. It also has additional rules that member links must meet before they are aggregated:

•The most important rule requires that there is not a significant difference in the length of the fiber between the member links, and that all member links are part of the same port-group. The ports that belong to port-group 1, port-group 2, and port-group 3 are te0/0 to te0/7, te0/8 to te0/15, and te0/16 to te0/23, respectively.

•A maximum of four Brocade LAGs can be created per port-group.

82	Converged Enhanced Ethernet Administrator’s Guide
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