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Port Trunking (Link Aggregation)
Ethernet bridges operate in accordance with the 802.1d spanning
tree/transparent bridge standard. The standard allows parallel links to
exist between switches, but only one link is permitted to be active.
The others are placed in a "blocking state" and only become active
when the active link goes down. This limits the link bandwidth
between switches to a single interface operating in full-duplex mode.
To address this problem, port trunking can be used. Figure 6.7
illustrates a single and trunked port switch configuration. Port
trunking provides the capability to group switch ports to form a single
link to another switch or computer equipped with a multilink interface.
Port trunking will work with Fast, Gigabit, or standard Ethernet half-
or full-duplex switch ports. Trunks can be constructed out of any
combination of port types (provided the port configurations match on
each end) and can consist of up to eight ports. Operationally speaking,
a trunked interface behaves like a standalone port. In each trunk, one
port in the group will function as the "lead" port. It is from the lead
port that the other members of the trunk derive their VLAN and
spanning-tree information.
Figure 6.7. A comparison of parallel links between
switches with and without port trunking.
With spanning tree enabled, it is possible to have two trunk interfaces
connecting to the same switch. One of the interfaces will be placed in
a blocking state. The spanning tree cost value is calculated in the total
aggregate bandwidth of the trunk link. The link with the highest
aggregate bandwidth will be the active link. In a scenario where both
trunks are configured identically, the trunk with the lowest port
number will be the primary link.