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the network. Thus, eight wires can be used to simulate one link able to handle up to 800Mbps and load balance data across those links, as shown in Figure 10.7.

Figure 10.7: Eight equal−cost links between two switches, creating a bundle of eight channels acting as a single link.

Let’s take a look at what occurs during a link failure in an EtherChannel bundle. We’ll also examine the Port Aggregation Protocol (PAgP).

Link Failure

Fast EtherChannel provides redundancy in the event of a link failure. The EtherChannel bundle is managed by the Fast EtherChannel process and the Ethernet Bundle Controller (EBC). Should one link in the bundle fail, the EBC informs the Enhanced Address Recognition Logic (EARL) ASIC of the failure. The EARL ASIC immediately ages out all addresses learned on that link. The EBC and the EARL then recalculate in the hardware, sending queries to the other switches and learning the destination link based on the responses. The data traffic is rerouted on one of the other links in just a few milliseconds, making the convergence transparent to the user.

EtherChannel Administrative Groups

You can define an EtherChannel administrative group to identify groups of ports that are allowed to form an EtherChannel bundle together. When you create an EtherChannel port bundle, an administrative group is defined automatically. Administrative group membership is limited by hardware restrictions.

The administrative group can be any value between 1 and 1,024, inclusive. It is defined using the

set port channel <port list> <administrative group number>

command. To view the configured administrative groups, use this command:

show channel group <administrative group number>

Modifying a member port of an EtherChannel administrative group will cause the port to be removed from the group when STP realizes that a change has occurred in the network topology. The modified port that was a member of the EtherChannel administrative group must go through listening and learning mode again before it can return to forwarding mode and rejoin the EtherChannel bundle.

Port Aggregation Protocol

The Port Aggregation Protocol (PAgP) is used to manage the Fast EtherChannel bundles and aids in the automatic creation of Fast EtherChannel links. PAgP packets are sent between Fast EtherChannel−capable ports. PAgP learns of the neighbors and their group capabilities dynamically and then informs its neighbors of the local group capabilities. After the protocol determines all the paired, point−to−point, or bi−directional links, it groups into a single channel those ports that have the same neighbor device ID and neighbor group

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