While PAgP allows for all links within the EtherChannel to be used to forward and receive user traffic, there are some restrictions that you should be familiar with regarding the forwarding of traffic from other protocols. DTP and CDP send and receive packets over all the physical interfaces in the EtherChannel. PAgP sends and receives PAgP Protocol Data Units only from interfaces that are up and have PAgP enabled for auto or desirable modes.
When an EtherChannel bundle is configured as a trunk port, the trunk sends and receives PAgP frames on the lowest numbered VLAN. Spanning Tree Protocol (STP) always chooses the first operational port in an EtherChannel bundle. The show pagp [channel number] neighbor command, which can also be used to validate the port that will be used by STP to send packets and receive packets, determines the port STP will use in an EtherChannel bundle, as shown in the following output:
| Switch-1#show pagp neighbor Flags: S – Device is sending Slow hello. C – Device is in Consistent state. A – Device is in Auto mode. P – Device learns on physical port.Channel group 1 neighbors Partner Partner Partner Partner Group Port Name Device ID Port Age Flags Cap. Fa0/1 Switch-2 0014.a9e5.d640 Fa0/1 2s SC 10001 Fa0/2 Switch-2 0014.a9e5.d640 Fa0/2 1s SC 10001 Fa0/3 Switch-2 0014.a9e5.d640 Fa0/3 15s SC 10001 |
Referencing the above output, STP will send packets only out of port FastEthernet0/1 because it is the first operational interface. If that port fails, STP will send packets out of FastEthernet0/2. The default port used by PAgP can be viewed in the show EtherChannel summary as illustrated in the following output:
| Switch-1#show EtherChannel summary Flags: D – down P – in port-channel I – stand-alone s – suspended H – Hot-standby (LACP only) R – Layer3 S – Layer2 u – unsuitable for bundling U – in use f – failed to allocate aggregator d – default portNumber of channel-groups in use: 1 Number of aggregators: 1Group Port-channel Protocol Ports ——+————-+———–+——————————————– 1 Po1(SU) PAgP Fa0/1(Pd) Fa0/2(P) Fa0/3(P) |
When configuring additional STP features such as Loop Guard on an EtherChannel, it is very important to remember that if Loop Guard blocks the first port, no BPDUs will be sent over the channel, even if other ports in the channel bundle are operational. This is because PAgP will enforce uniform Loop Guard configuration on all of the ports that are part of the EtherChannel group.
In production networks, you may run across the Cisco Virtual Switching System (VSS), which is comprised of two physical Catalyst 6500 series switches acting as a single logical switch. In the VSS, one switch is selected as the active switch while the other is selected as the standby switch. The two switches are connected together via an EtherChannel, which allows for the sending and receiving of control packets between them. Access switches are connected to the VSS using Multichassis EtherChannel (MEC). An MEC is simply an EtherChannel that spans the two physical Catalyst 6500 switches but terminates to the single logical VSS. Enhanced PAgP (PAgP+) can be used to allow the Catalyst 6500 switches to communicate via the MEC in the event that the EtherChannel between them fails, which would result in both switches assuming the active role (dual active), effectively affecting forwarding of traffic within the switched network. This is illustrated in the diagram below:
While VSS is beyond the scope of the SWITCH exam requirements, it is beneficial to know that only PAgP can be used to relay VSS control packets. Therefore, if implementing EtherChannels in a VSS environment, or an environment in which VSS may eventually be implemented, you may want to consider running PAgP instead of LACP, which is an open standard that does not support the proprietary VSS frames. VSS will not be described any further in this guide.
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