EtherChannel Port Aggregation Protocol (PAgP)

Port Aggregation Protocol or PAgP is an EtherChannel technology that is a Cisco proprietary protocol. It is a form of logical aggregation of Cisco Ethernet switch ports, and it enables data/traffic load balancing. PAgP EtherChannel can combine a maximum of 8 physical links into a single virtual link. We also have an IEEE open standard, Link Aggregation Control Protocol, LACP.


PAgP Initial Configuration Check

Since PAgP is a Cisco proprietary protocol, we need to make sure that all of the interfaces have the same following configurations in our Cisco network devices:

  1. Speed and Duplex
  2. Operational State (all Access or all Trunking)
  3. Access VLAN on access interfaces
  4. Native VLAN and Allowed VLANs on trunk interfaces
  5. STP interface settings


The 2 Port Aggregation Protocol Configuration Modes

There are two Cisco EtherChannel Port Aggregation Protocol modes, which we can implement as a part of the port configuration:

Auto mode– interface can respond to PAgP packet negotiation but will never start one on its own.

Desirable mode– interface actively attempts a negotiating state for PAgP packet negotiation.


Cisco PAgP Mode Combinations

Here are the combinations of the different PAgP modes (auto mode and desirable mode) and if link aggregation will work or not between the Cisco devices:

Auto Desirable
Auto No Yes
Desirable Yes Yes


How to Configure Port Aggregation Protocol?

We’ll use the network topology below for our example. We have two Cisco switches to be configured with PAgP.


Switch 1 Configuration:

Switch 1#conf t
Switch 1(config)#interface range fa0/1 - 2
Switch 1(config-if-range)#speed 100
Switch 1(config-if-range)#duplex full
Switch 1(config-if-range)#switchport mode trunk
Switch 1(config-if-range)#channel-group 1 mode desirable
Switch 1(config-if-range)#end


Switch 2 Configuration:

Switch 2#conf t
Switch 2(config)#interface range fa0/1 - 2
Switch 2(config-if-range)#speed 100
Switch 2(config-if-range)#duplex full
Switch 2(config-if-range)#switchport mode trunk
Switch 2(config-if-range)#channel-group 1 mode auto
Switch 2(config-if-range)#end


Switch logs showing Port-Channel1 comes up:

%LINK-5-CHANGED: Interface Port-channel1, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to up


Port Aggregation Protocol Verification

We can issue the ‘show etherchannel <channel-group number> port-channel’ command to check if the Port-channel is active or not. We can verify which of the two protocols is used, PAgP or LACP, and the interfaces participating in the aggregation.

Switch1#show etherchannel 1 port-channel
                 Port-channels in the group:
Port-channel: Po1
Age of the Port-channel = 0d:00h:04m:07s
Logical slot/port   = 16/0        Number of ports = 2
GC                  = 0x00010001    HotStandBy port = null
Port state          = Port-channel Ag-Inuse 
Protocol            = PAgP
Port security       = Disabled
Ports in the Port-channel:
Index    Load    Port     EC state       No of bits
0        00      Fa0/1    Automatic-Sl       0
0        00      Fa0/2    Automatic-Sl       0
Time since last port bundled: 0d:00h:00m:26s Fa0/2


Next, ‘show etherchannel summary’ will show us a quick overview of the EtherChannel status.

Switch 1#show etherchannel summary
Flags: D - down           P - bundled in port-channel
       I - stand-alone    s - suspended
       H - Hot-standby (LACP only)
       R - Layer3         S - Layer2
       U - in use         N - not in use, no aggregation
       f - failed to allocate aggregator
       M - not in use, minimum links not met
       m - not in use, port not aggregated due to minimum links not met
       u - unsuitable for bundling
       w - waiting to be aggregated
       d - default port
       A - formed by Auto LAG
Number of channel-groups in use: 1
Number of aggregators: 1
Group  Port-channel Protocol     Ports
1      Po1(SU)         PAgP      Fa0/1(P) Fa0/2(P) 


Lastly, we can issue the ‘show interfaces fa0/1 etherchannel’ command. We can see both local and neighbor interface information, and also the Cisco PAgP mode used.

Switch 1#show interfaces fa0/1 etherchannel
Port state    = Up Mstr In-Bndl
Channel group = 1           Mode = Automatic-Sl      Gcchange = 0
Port-channel  = Po1         GC = 0x00010001          Pseudo port-channel = Po1
Port index    = 0           Load = 0x00              Protocol = PAgP
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.
        d - PAgP is down.
Timers: H - Hello timer is running.        Q - Quit timer is running.
        S - Switching timer is running.    I - Interface timer is running.
Local information:
                                Hello    Partner  PAgP     Learning   Group
Port      Flags State   Timers  Interval Count   Priority   Method   Ifindex
Fa0/1     SAC   U6/S7   HQ      30s      1        128        Any       10
Partner's information:
          Partner              Partner           Partner        Partner Group
Port      Name                 Device ID         Port       Age Flags   Cap.
Fa0/1     Switch2              5000.0002.8000    Fa0/1      18s SC      10001
Age of the port in the current state: 0d:00h:06m:54s

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