KB.15.15

Use the trunk option to establish a trunk group between a switch and another device, where the

other device's trunking operation fails to operate properly with LACP trunking configured on the

switches.

Viewing trunk data on the switch

Static trunk group Appears in the menu interface and the output from the CLI

show trunk and show interfaces commands.

Dynamic LACP trunk group Appears in the output from the CLI show lacp command.

Static non-protocolStatic LACP trunk groupDynamic LACP trunk

group

Interface option

YesYesNoMenu interface

YesYesNoCLI show trunk

YesYesNoCLI show interfaces

NoYesYesCLI show lacp

YesYesNoCLI show spanning-tree

YesYesNoCLI show igmp

YesYesNoCLI show config

Outbound traffic distribution across trunked links

The two trunk group options (LACP and trunk) use SA/DA pairs for distributing outbound traffic

over trunked links. That is, the switch sends traffic from the same source address to the same

destination address through the same trunked link, and may also send traffic from the same source

address to a different destination address through the same link or a different link, depending on

the mapping of path assignments among the links in the trunk. Likewise, the switch distributes traffic

for the same destination address but from different source addresses through links depending on

the path assignment.

The load-balancing is done on a per-communication basis. Otherwise, traffic is transmitted across

the same path as shown in Figure 62 (page 158). That is, if Client A attached to Switch 1 sends

five packets of data to Server A attached to Switch 2, the same link is used to send all five packets.

The SA/DA address pair for the traffic is the same. The packets are not evenly distributed across

any other existing links between the two switches; they all take the same path.

Figure 62 Example of single path traffic through a trunk

The actual distribution of the traffic through a trunk depends on a calculation using bits from the

SA/DA. When an IP address is available, the calculation includes the last five bits of the IP source

address and IP destination address; otherwise, the MAC addresses are used. The result of that

process undergoes a mapping that determines which link the traffic goes through. If you have only

two ports in a trunk, it is possible that all the traffic will be sent through one port even if the SA/DA

158 Port Trunking