Cisco WS-C2980G-A Software Guide - Page 79
Understanding the PAgP, PAgP Modes
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Chapter 6 Configuring Fast EtherChannel and Gigabit EtherChannel Understanding the PAgP Understanding the PAgP Use the information in the following sections if you are configuring EtherChannel using PAgP. If you are using LACP, see the "Understanding the LACP" section on page 6-16. PAgP Modes The Port Aggregation Protocol (PAgP) facilitates the automatic creation of Fast EtherChannel and Gigabit EtherChannel links by exchanging packets between channel-capable ports. The protocol learns the capabilities of port groups dynamically and informs the neighboring ports. After PAgP identifies correctly paired channel-capable links, it groups the ports into a channel. The channel is then added to the spanning tree as a single bridge port. A given outbound broadcast or multicast packet is transmitted out one port in the channel only, not out every port in the channel. In addition, outbound broadcast and multicast packets that are transmitted on one port in a channel are blocked from returning on any other port of the channel. There are four user-configurable channel modes: on, off, auto, and desirable. PAgP packets are exchanged only between ports in auto and desirable mode. Ports that are configured in on or off mode do not exchange PAgP packets. The auto and desirable modes can be modified with the silent and non-silent keywords. Table 6-1 describes each mode. Table 6-1 Channel Modes Mode on off auto desirable Description Forces the port to channel without negotiation. PAgP packets are not exchanged. The port is channeling regardless of how the peer port is configured. If the peer port is in on mode, a channel is formed. In any other mode, the peer port is placed in the errdisable state due to a channel misconfiguration. Prevents the port from channeling. PAgP packets are not exchanged. The port is not channeling regardless of how the peer port is configured. No channel is formed. Places a port into a passive negotiating state, in which the port responds to PAgP packets it receives but does not initiate PAgP packet negotiation. A channel is formed only with another port group in desirable mode. (Default) Places a port into an active negotiating state, in which the port initiates negotiations with other ports by sending PAgP packets. A channel is formed with another port group in either desirable or auto mode. Use the silent keyword when you are connecting to a "silent partner" (a device that is not generating BPDUs or other traffic). An example of a silent partner is a traffic generator that is not transmitting packets. Use this keyword with the auto or desirable mode. If you do not specify silent or non-silent, silent is assumed. Use the non-silent keyword when you are connecting to a device that will transmit BPDUs or other traffic. Use this keyword with the auto or desirable mode. Both the auto and desirable modes allow ports to negotiate with connected ports to determine if they can form a channel, based on criteria such as port speed, trunking state, native VLAN, and so on. 78-15486-01 Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 6-5