PAgP Desirable and Auto Packet Capture

This packet capture contains two PAgP (Port Aggregation Protocol) Info PDUs, Cisco’s own protocol for building an EtherChannel. SW2 runs channel-group 1 mode auto, SW1 runs mode desirable, and you can see the difference in a single flag bit.

Each PDU also tells you who’s on the other end: device IDs, port numbers and the hostname and port name of the sender. The EtherChannel lesson explains PAgP and LACP modes, and the LACP PDU Packet Capture shows the standards-based alternative. More captures are in the Packet Capture Library.

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Capture overview

File	etherchannel-pagp-desirable-auto.pcapng
Packets	2
Protocol	PAgP (Cisco proprietary), Info PDU
Encapsulation	IEEE 802.3, LLC/SNAP with OUI 00:00:0c and PID 0x0104, destination 01:00:0c:cc:cc:cc
Devices	Cisco IOS switches: SW2 in auto mode (packet 1), SW1 in desirable mode (packet 2), port Et0/1 on both
Wireshark filter	pagp

Packets in this capture

No.  Time      Source             Destination            Protocol  Info
1    0.000000  aa:bb:cc:00:6a:10  CDP/VTP/DTP/PAgP/UDLD  PAGP      Info PDU; Flags 0x7; Local DevID: aa:bb:cc:80:6a:00, Partner DevID: aa:bb:c...
2    0.307492  aa:bb:cc:00:69:10  CDP/VTP/DTP/PAgP/UDLD  PAGP      Info PDU; Flags 0x5; Local DevID: aa:bb:cc:80:69:00, Partner DevID: aa:bb:c...

Packet walkthrough

Info PDU from SW2, auto mode (packet 1)

IEEE 802.3 Ethernet
    Destination: CDP/VTP/DTP/PAgP/UDLD (01:00:0c:cc:cc:cc)
    Source: aa:bb:cc:00:6a:10 (aa:bb:cc:00:6a:10)
    Length: 70
Logical-Link Control
    DSAP: SNAP (0xaa)
    SSAP: SNAP (0xaa)
    Control field: U, func=UI (0x03)
    Organization Code: 00:00:0c (Cisco Systems, Inc)
    PID: PAgP (0x0104)
Port Aggregation Protocol
    Version: Info PDU (0x01)
    Flags: 0x07, Slow Hello, Auto Mode, Consistent State
        .... ...1 = Slow Hello: Yes
        .... ..1. = Auto Mode: Yes
        .... .1.. = Consistent State: True
    Local Device ID: aa:bb:cc:80:6a:00 (aa:bb:cc:80:6a:00)
    Local Learn Capability: Arbitrary Distribution (0x02)
    Local Port Hot Standby Priority: 128
    Local Sent Port ifindex: 2
    Local Group Capability: 0x00010001
    Local Group ifindex: 6
    Partner Device ID: aa:bb:cc:80:69:00 (aa:bb:cc:80:69:00)
    Partner Learn Capability: Arbitrary Distribution (0x02)
    Partner Port Hot Standby Priority: 128
    Partner Sent Port ifindex: 2
    Partner Group Capability: 0x00010001
    Partner Group ifindex: 0
    Partner Count: 1
    Number of TLVs: 2
    TLV Entry #1
        Type: Device Name TLV (1)
        Length: 7
        Device Name: SW2
    TLV Entry #2
        Type: Physical Port Name TLV (2)
        Length: 9
        Physical Port Name: Et0/1

Let’s look at the important fields:

• 01:00:0c:cc:cc:cc with SNAP PID 0x0104: PAgP shares the Cisco multicast MAC with CDP, VTP and DTP. The PID tells them apart.
• Auto Mode: Yes: this port is in auto mode. It answers PAgP but doesn’t start the negotiation. Two auto ports never form a channel.
• Slow Hello: PAgP sends PDUs every 30 seconds in slow mode. During negotiation, it switches to fast hellos (1 second).
• Consistent State: the port’s settings are stable. A port that is still changing would clear this flag.
• Local Device ID and Partner Device ID: SW2 identifies itself (aa:bb:cc:80:6a:00) and the switch it heard on this port (aa:bb:cc:80:69:00, SW1). Seeing the partner’s ID means the PDUs are reaching both sides.
• Group Capability 0x00010001: ports can only bundle with ports that have the same group capability. PAgP derives it from the port’s configuration.
• Local Group ifindex 6 and Partner Group ifindex 0: SW2 has put this port in its port-channel (ifindex 6), but in this PDU it doesn’t list a group for SW1 yet.
• Device Name and Physical Port Name TLVs: SW2, Et0/1. Handy when you capture on a trunk with several neighbors.

Info PDU from SW1, desirable mode (packet 2)

Port Aggregation Protocol
    Version: Info PDU (0x01)
    Flags: 0x05, Slow Hello, Consistent State
        .... ...1 = Slow Hello: Yes
        .... ..0. = Auto Mode: Desirable Mode
        .... .1.. = Consistent State: True
    Local Device ID: aa:bb:cc:80:69:00 (aa:bb:cc:80:69:00)
    Local Learn Capability: Arbitrary Distribution (0x02)
    Local Port Hot Standby Priority: 128
    Local Sent Port ifindex: 2
    Local Group Capability: 0x00010001
    Local Group ifindex: 6
    Partner Device ID: aa:bb:cc:80:6a:00 (aa:bb:cc:80:6a:00)
    Partner Learn Capability: Arbitrary Distribution (0x02)
    Partner Port Hot Standby Priority: 128
    Partner Sent Port ifindex: 2
    Partner Group Capability: 0x00010001
    Partner Group ifindex: 6
    Partner Count: 1
    Number of TLVs: 2
    TLV Entry #1
        Type: Device Name TLV (1)
        Length: 7
        Device Name: SW1
    TLV Entry #2
        Type: Physical Port Name TLV (2)
        Length: 9
        Physical Port Name: Et0/1

What do we see here?

• Auto Mode bit 0, “Desirable Mode”: SW1 actively starts the negotiation. Desirable with auto, or desirable with desirable, forms a channel.
• Flags 0x05 instead of 0x07: the only difference with SW2’s flags is that one bit.
• Partner Group ifindex 6: SW1 reports both its own and SW2’s port-channel interface. The bundle is up on both sides.