Cisco 2950G 24 Software Configuration Guide - Page 555
Physical Learners and Aggregate-Port Learners, PAgP Interaction with Other Features
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Chapter 27 Configuring EtherChannels Understanding EtherChannels Physical Learners and Aggregate-Port Learners Network devices are classified as PAgP physical learners or aggregate-port learners. A device is a physical learner if it learns addresses by physical ports and directs transmissions based on that knowledge. A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports. When a device and its partner are both aggregate-port learners, they learn the address on the logical port-channel. The device sends packets to the source by using any of the interfaces in the EtherChannel. With aggregate-port learning, it is not important on which physical port the packet arrives. The switch uses source-MAC address distribution for a channel if it is connected to a physical learner even if the user configures destination-MAC address distribution. These frame distribution mechanisms are possible for frame transmission: • Port selection based on the source-MAC address of the packet • Port selection based on the destination- MAC address of the packet The switch supports up to eight ports in a PAgP group. PAgP Interaction with Other Features The Dynamic Trunking Protocol (DTP) and Cisco Discovery Protocol (CDP) send and receive packets over the physical interfaces in the EtherChannel. Trunk ports send and receive PAgP protocol data units (PDUs) on the lowest numbered VLAN. Spanning tree sends packets over a single physical interface in the EtherChannel. Spanning tree regards the EtherChannel as one port. PAgP sends and receives PAgP PDUs only from interfaces that are up and have PAgP enabled for the auto or desirable mode. Understanding Load Balancing and Forwarding Methods EtherChannel balances the traffic load across the links in a channel by reducing part of the binary pattern formed from the addresses in the frame to a numerical value that selects one of the links in the channel. EtherChannel load balancing can use either source-MAC or destination-MAC address forwarding. With source-MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on the source-MAC address of the incoming packet. Therefore, to provide load balancing, packets from different hosts use different ports in the channel, but packets from the same host use the same port in the channel (and the MAC address learned by the switch does not change). With destination-MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on the destination host's MAC address of the incoming packet. Therefore, packets to the same destination are forwarded over the same port, and packets to a different destination are sent on a different port in the channel. You configure the load balancing and forwarding method by using the port-channel load-balance global configuration command. In Figure 27-3, an EtherChannel of four workstations communicates with a router. Because the router is a single-MAC-address device, source-based forwarding on the switch EtherChannel ensures that the switch uses all available bandwidth to the router. The router is configured for destination-based forwarding because the large number of workstations ensures that the traffic is evenly distributed from the router EtherChannel. 78-14982-01 Catalyst 2950 Desktop Switch Software Configuration Guide 27-5