Monday, February 27, 2012

Cisco Etherchannel / Port-Channel (LACP / PAgP)


On different Cisco switches it is possible to create logical connections that

are made-up off different fysical interface. It is needed that these interface

do have the same speed.

Most Cisco switches support max 64 etherchannels. These interfaces do not have to be contiguous.

or even on the same module. Each channel must be made up of min 2, max 8 interfaces.

The best is to use 2 ,4 or 8 interfaces. This will give the perfect load-balancing.

The load-balancing can be bases on layer 2/3 or 4 information.

It is not possible to have different load-balancing methods for different Etherchannels

on one switch. If the load-balancing method is change, it is applicable for all.

The load-balancing method can be:

- src-mac

- dst-mac

- src-dst-mac

- src-ip

- dst-ip

- src-dst-ip

- src-port

- dst-port

- src-dst-port

By using <port-channel load-balance {option}> it is possible to change to method.

If a single header is used to load-balance the low-order bits is used to dictate to

witch interface the frame is send. If two headers are used a XOR function is used on

the low-order bits.

It is possible to change what path with-in the etherchannel a frame takes by used the following command:

<test etherchannel load-balance interface port-channel [#] ip [src] [dst]>

to assign a physical interface towards a port-channel use the following command:

channel-group [#] mode on

There are two type of dynamic protocols to negotiate an etherchannels: PAgP and LACP 8021.AD

PAgP is Cisco proprietary

When these protocols start to communicate there is an exchange of information before the port-channel can be formed. The following items must match before a port-channel can form:

- same speed/duplex

- Access VLAN (if not trunked)

- Same trunking type, allowed VLAN and native VLAN (if trunked)

- Each port must have the same STP cost per VLAN with-in the portchannel

- No SPAN ports

channel-group [#] mode on (disables PAgP en LACP)

channel-group [#] mode off (disables PAgP en LACP and prevent the ports to form a port-channel)

channel-group [#] mode auto (use PAgP in a passive mode, it will wait until a PAgP packet will be send)

channel-group [#] mode passive (use LACP in a passive mode, it will wait until a PAgP packet will be send)

channel-group [#] mode desirable (use PAgP in an active mode, it will start to send PAgP packets)

channel-group [#] mode active (use LACP in an active mode, it will start to send LACP packets)

3750(config)#interface range gigabitethernet 1/0/2 – 4

3750(config-if-range)#channel-group 1 mode on


3750(config-if-range)#switchport trunk encapsulation dot1q

3750(config-if-range)#switchport mode trunk

First type the channel-group command. After that all interface commands will be automatically duplicated on all the interfaces group in the channel-group.

With LACP it is possible to create a port-channel on a stack (3560/3750 switches).

This is based on IOS version 12.2(25)SEE

Show command’s

· show interfaces port-channel [channel-group-number]

· show etherchannel [channel-group-number] summary

Verify command’s

  • test etherchannel load-balance interface port-channel [#] ip [src] [dst

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