Pagp Interaction With Virtual Switches And Dual-Active Detection - Cisco Catalyst 2960 series Configuration Manual

loses hardware-signal detection. You can configure which port is always selected for packet transmission by
changing its priority with the pagp port-priority interface configuration command. The higher the priority,
the more likely that the port will be selected.
The switch supports address learning only on aggregate ports even though the physical-port keyword is
Note
provided in the CLI. The pagp learn-method command and the pagp port-priority command have no
effect on the switch hardware, but they are required for PAgP interoperability with devices that only
support address learning by physical ports, such as the Catalyst 1900 switch.
When the link partner of the switch is a physical learner, we recommend that you configure the switch as
a physical-port learner by using the pagp learn-method physical-port interface configuration command.
Set the load-distribution method based on the source MAC address by using the port-channel load-balance
src-mac global configuration command. The switch then sends packets to the physcial learner using the
same port in the EtherChannel from which it learned the source address. Only use the pagp learn-method
command in this situation.
Related Topics
Configuring the PAgP Learn Method and Priority , on page 348
EtherChannel Configuration Guidelines, on page 338
Default EtherChannel Configuration, on page 337
Monitoring EtherChannel, PAgP, and LACP Status, on page 358
Layer 2 EtherChannel Configuration Guidelines, on page 340

PAgP Interaction with Virtual Switches and Dual-Active Detection

A virtual switch can be two or more core switches connected by virtual switch links (VSLs) that carry control
and data traffic between them. One of the switches is in active mode. The others are in standby mode. For
redundancy, remote switches are connected to the virtual switch by remote satellite links (RSLs).
If the VSL between two switches fails, one switch does not know the status of the other. Both switches could
change to the active mode, causing a dual-active situation in the network with duplicate configurations
(including duplicate IP addresses and bridge identifiers). The network might go down.
To prevent a dual-active situation, the core switches send PAgP protocol data units (PDUs) through the RSLs
to the remote switches. The PAgP PDUs identify the active switch, and the remote switches forward the PDUs
to core switches so that the core switches are in sync. If the active switch fails or resets, the standby switch
takes over as the active switch. If the VSL goes down, one core switch knows the status of the other and does
not change its state.
PAgP Interaction with Other Features
The Dynamic Trunking Protocol (DTP) and the Cisco Discovery Protocol (CDP) send and receive packets
over the physical ports in the EtherChannel. Trunk ports send and receive PAgP protocol data units (PDUs)
on the lowest numbered VLAN.
In Layer 2 EtherChannels, the first port in the channel that comes up provides its MAC address to the
EtherChannel. If this port is removed from the bundle, one of the remaining ports in the bundle provides its
MAC address to the EtherChannel.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
Information About EtherChannels
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