Cisco Etherchannel: PAgP and LACP

Cisco Etherchannel: PAgP and LACP

  • Can bundle up to 8 interfaces to a port-channel.
  • Maximum number of port-channels in C3560 can be 48.
  • In Layer-2 port, configuring “channel-group” automatically configures “int po x” logical interface.
  • With Layer-3 port, manually configure “int po x” and then “no switchport”. Then assign this port-channel to physical interface with “channel-group” command.

Port Aggregation Protocol (PAgP):

  • PAgP groups the ports with the same speed, duplex mode, native VLAN, VLAN range, and trunking status and type.
  • Modes:
    • Auto: Port does not initiate the negotiation. But responds to PAgP packets initiated by other end.
    • Desirable: Port initiates the negotiation and tries to form the channel.
    • Ports configured with ‘on’ mode will not send/process PAgP packets. Form etherchannel only when the other end in ‘on’ mode. Non-compatible ports are suspended.
    • So following configuration will not form the channel: auto-auto, on- auto/desirable.
    • Use “non-silent” sub-mode, when other end switch is PAgP capable. Use “silent” sub-mode, when the other end device will never send any packet. Silent option makes the link to join channel after 15 seconds wait for any PAgP and forward packet.
  • DTP/CDP packets are sent on each physical interface (channel members)
  • For trunk port, PAgp PDUs are sent on lower number VLAN ID.
  • MAC address of the port-channel = MAC address of the first member joining the channel.
  • Configure up to 8 interfaces as channel members.

Link aggregation control protocol (LACP):

  • IEEE 802.3ad standard.
  • Modes:
    • Passive: Port does not initiate the negotiation. But responds to PAgP packets initiated by other end.
    • Active: Port initiates the negotiation and tries to form the channel.
  • Can configure up to 16 members. 8 members will be active and 8 members will be hot-standby.
  • LACP and PAgp will not interop.

Load balancing:

  • src-ip, dst-ip, src-dst-ip, src-mac, dst-mac, src-dst-mac. (Supported options in C3560)
  • XOR operations are done on the selection bits. Load balance option can be configured only in global mode for all ether channels. “port-channel load-balance method
  • src-mac” is default for L2 switching and “src-des-ip” is default for L3 switching.

Default values:

  • PAgP priority: 128 for all ports. “pagp port-priority” port mode CLI to change.
  • LACP system priority: 32768. System ID = 2 byte priority + 4 byte system MAC address. “lacp system-priority” global config CLI.
  • LACP port priority: 32768 for all ports. Port ID = 2 byte priority + 2 byte port number. “lacp port-priority” port mode CLI to change.
  • Lower value is preferred. Lower system ID switch is responsible to decide which ports should be active. Lower port ID is preferred over higher port ID. I.e.; if ports 1-10 are grouped for port-channel, 1-8 ports will be active and 9, 10 ports will be in suspended state.

Configuring L2 ether channel:

  • Inside physical interface configuration mode, configure switch port, trunk/access configuration, and then channel-group command.
  • Sample:

Switch(config-if)#int ra gi 0/20-21


Switch(config-if-range)#switchport mode access

Switch(config-if-range)#switchport access vlan 4

Switch(config-if-range)#channel-group 4 mode active

Switch(config-if-range)#int po 4

Switch(config-if)#no shutdown

Configuring L3 ether channel:

  • Configure port-channel interface, no switch port, assign IP address. Then inside physical interface mode, configure no switch port, no ip address and then channel-group command.
  • Sample:

Switch(config)#int po 5

Switch(config-if)#no switchport

Switch(config-if)#ip address

Switch(config-if)#no shutdown

Switch(config)#int rang gi 0/23-24

Switch(config-if-range)#no switchport

Switch(config-if-range)#no ip address

Switch(config-if-range)#channel-group 5 mode active

Show commands:

  • Show lacp { <number> | sys-id | counters | internal | neighbor}
  • Show pagp { <number> | counters | internal | neighbor}
  • Show etherchannel { <number> | summary | port | details | load-balance | port-channel }

PAgP learning-method:

  • Aggregate-port learners:  Default – Learns address on the aggregate port (port-channel). Packets to learned address can be sent via any member links.
  • Physical-port learners: Packets are sent only on the member interface on which address was learned. Usually used when the other end device is not capable of Aggregate-port learners. Use “src-mac” load balancing. Commands: “pagp learn-method physical-port” on interface mode.

Link-state tracking (Trunk failover):

  • Certain interfaces are configured as ‘upstream’ (which are connected to distro switches) and ‘downstream’ connected to servers (with Dual NIC -primary and secondary).
  • A link-state group is configured and upstream, downstream ports are added to the group. When all the upstream interfaces went down, the switch makes all downstream ports to err-disabled (link-down) state, so that server will switch over to secondary NIC seamlessly.
  • An interface can be member of only one group and it can be either upstream or downstream.
  • There can be only 2 link-state groups configured in C3560. 1 and 2 (default:1,if no value is given)
  • Commands:
    • link state track <number>” in global config mode.
    • link state group <number> {upstream|downstream}” in interface config mode.
    • show link state group

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