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Port Channel Commands 613
Port Channels
Trunking, which is also called Port Channels or Link Aggregation, is initiated
and maintained by the periodic exchanges of Link Aggregation Control PDUs
(LACPDUs).
From a system perspective, a LAG is treated as a physical port. A LAG and a
physical port use the same configuration parameters for administrative
enable/disable, port priority, and path cost. When a physical port is
configured as part of a LAG, it no longer participates in forwarding operations
until the LAG becomes active.
A LAG failure of one or more of the links stops traffic on the failed link. Upon
failure, the flows mapped to a link are dynamically reassigned to the
remaining links of the LAG. Similarly when links are added to a LAG, the
conversations may need to be shifted to a new link.
LAG Hashing
The purpose of link aggregation is to increase bandwidth between two
switches. It is achieved by aggregating multiple ports in one logical group. A
common problem of port channels is the possibility of changing packets order
in a particular TCP session. The resolution of this problem is correct selection
of a physical port within the port channel for transmitting the packet to keep
original packets order.
The hashing algorithm is configurable for each LAG. Typically, an
administrator is able to choose from hash algorithms utilizing the following
attributes of a packet to determine the outgoing port:
• Source MAC, VLAN, EtherType, and incoming port associated with the
packet.
• Source IP and Source TCP/UDP fields of the packet.
• Destination MAC, VLAN, EtherType, and incoming port associated with
the packet.
• Source MAC, Destination MAC, VLAN, EtherType, and incoming port
associated with the packet.
• Destination IP and Destination TCP/UDP Port fields of the packet.
• Source/Destination MAC, VLAN, EtherType, and incoming port
associated with the packet.
2CSPC4.X8100-SWUM102.book Page 613 Friday, March 15, 2013 8:56 AM