Choose LACP or PAgP for EtherChannel ?

Choose which one?  LACP or PAgP ?

When you design EtherChannel, the first questions is which protocol you choose for etherchannel. Below is the guide line to help you make the decision. 

1. In general, first choice is using LACP since it is the  IEEE standard. it is supported by multiple vendors. you can use LACP to form the port-channel between different vendor's switches.
2. If you need form the etherchannel on stack switch, it is better to choose LACP since LACP support cross-stack Etherchannel.  The PAgP doesn't support cross-stack etherchannel on cisco3750 .  Cross-stack etherchannel means member of etherchannel physical ports can be spread on different physical switch which is member of  the whole stacked switch
3. If you has other choice, don't use static Mode  (mode on).

EtherChannel Overview

You can configure EtherChannels manually or you can use the Port Aggregation Control Protocol (Page) or the Link Aggregation Control Protocol (LACP) to form EtherChannels. 

The EtherChannel protocols allow ports with similar characteristics to form an EtherChannel through dynamic negotiation with connected network devices. PAgP is a Cisco-proprietary protocol and LACP is defined in IEEE 802.3ad.

PAgP and LACP do not interoperate with each other. Ports configured to use PAgP cannot form EtherChannels with ports configured to use LACP. Ports configured to use LACP cannot form EtherChannels with ports configured to use PAgP. Neither interoperates with ports configured manually.

You can configure an EtherChannel in one of these modes: Port Aggregation Protocol (PAgP), Link Aggregation Control Protocol (LACP), or On. Configure both ends of the EtherChannel in the same mode:

•When you configure one end of an EtherChannel in either PAgP or LACP mode, the system negotiates with the other end of the channel to determine which ports should become active. If the remote port cannot negotiate an EtherChannel, the local port is put into an independent state and continues to carry data traffic as would any other single link. The port configuration does not change, but the port does not participate in the EtherChannel.

•When you configure an EtherChannel in the on mode, no negotiations take place. The switch forces all compatible ports to become active in the EtherChannel. The other end of the channel (on the other switch) must also be configured in the on mode; otherwise, packet loss can occur.

You can create an EtherChannel on a standalone switch, on a single switch in the stack, or on multiple switches in the stack (known as cross-stack EtherChannel). 

If a link within an EtherChannel fails, traffic previously carried over that failed link moves to the remaining links within the EtherChannel. If traps are enabled on the switch, a trap is sent for a failure that identifies the switch, the EtherChannel, and the failed link. Inbound broadcast and multicast packets on one link in an EtherChannel are blocked from returning on any other link of the EtherChannel

Table 1 lists the user-configurable EtherChannel modes.

Table 2 lists the EtherChannel member port states.

Table 1 EtherChannel Modes 

Mode	Description
on	Mode that forces the LAN port to channel unconditionally. In the on mode, a usable EtherChannel exists only when a LAN port group in the on mode is connected to another LAN port group in the on mode. Because ports configured in the on mode do not negotiate, there is no negotiation traffic between the ports. You cannot configure the on mode with an EtherChannel protocol. If one end uses the on mode, the other end must also.
auto	PAgP mode that places a LAN port into a passive negotiating state, in which the port responds to PAgP packets it receives but does not initiate PAgP negotiation. (Default)
desirable	PAgP mode that places a LAN port into an active negotiating state, in which the port initiates negotiations with other LAN ports by sending PAgP packets.
passive	LACP mode that places a port into a passive negotiating state, in which the port responds to LACP packets it receives but does not initiate LACP negotiation. (Default)
active	LACP mode that places a port into an active negotiating state, in which the port initiates negotiations with other ports by sending LACP packets.

Table 2 EtherChannel Member Port States

Port States	Description
bundled	The port is part of an EtherChannel and can send and receive BPDUs and data traffic.
suspended	The port is not part of an EtherChannel. The port can receive BPDUs but cannot send them. Data traffic is blocked.
standalone	The port is not bundled in an EtherChannel. The port functions as a standalone data port. The port can send and receive BPDUs and data traffic. Note When one end of an EtherChannel has more members than the other, the unmatched ports enter the standalone state. In a topology that is not protected from Layer 2 loops by the spanning tree protocol (STP), a port in the standalone state can cause significant network errors.              For csico 65XX switch, you can enter the port-channel standalone-disable interface configuration mode command to put ports into the suspended state instead of the standalone state.  this command is no support on cisco3750

Port Aggregation Control Protocol (Page) 

Both the auto and desirable modes enable ports to negotiate with partner ports to form an EtherChannel based on criteria such as port speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers.

Ports can form an EtherChannel when they are in different PAgP modes as long as the modes are compatible. For example:

•A port in the desirable mode can form an EtherChannel with another port that is in the desirable or auto mode.

•A port in the auto mode can form an EtherChannel with another port in the desirable mode.

A port in the auto mode cannot form an EtherChannel with another port that is also in the auto mode because neither port starts PAgP negotiation.

If your switch is connected to a partner that is PAgP-capable, you can configure the switch port for nonsilent operation by using the non-silent keyword. If you do not specify non-silent with the auto or desirable mode, silent mode is assumed.

Use the silent mode when the switch is connected to a device that is not PAgP-capable and seldom, if ever, sends packets. An example of a silent partner is a file server or a packet analyzer that is not generating traffic. In this case, running PAgP on a physical port connected to a silent partner prevents that switch port from ever becoming operational. However, the silent setting allows PAgP to operate, to attach the port to a channel group, and to use the port for transmission.

Link Aggregation Control Protocol (LACP)

Both the active and passive LACP modes enable ports to negotiate with partner ports to an EtherChannel based on criteria such as port speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers.

Ports can form an EtherChannel when they are in different LACP modes as long as the modes are compatible. For example:

•A port in the active mode can form an EtherChannel with another port that is in the active or passive mode.

•A port in the passive mode cannot form an EtherChannel with another port that is also in the passive mode because neither port starts LACP negotiation.

Basic Configuration of LACP EtherChannel

This document uses the network setup shown in the diagram below. the switch is cisco3750

L2 Trunk port-channel configuration

SwitchA configuration

interface Port-channel1
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 100,200,300,301
 switchport mode trunk

interface FastEthernet1/0/1
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 100,200,300,301
 switchport mode trunk
 speed 100
 duplex full
 channel-group 1 mode active

interface FastEthernet1/0/2
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 100,200,300,301
 switchport mode trunk
 speed 100
 duplex full
 channel-group 1 mode active


SwitchB configuration

interface Port-channel1
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 100,200,300,301
 switchport mode trunk

interface FastEthernet1/0/3
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 100,200,300,301
 switchport mode trunk
 speed 100
 duplex full
 channel-group 1 mode active

interface FastEthernet1/0/4
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 100,200,300,301
 switchport mode trunk
 speed 100
 duplex full
 channel-group 1 mode active

Note:

1. Speed and duplex of physical ports in the port-channel  must be identical. otherwise it can't be bundled to the port-channel. 
2. Name of Port-channel interface on both side switches don't have to be same.
3. If you need add or remove vlan from port-channel,  you have to under port-channel interface to add or remove the vlan, the related vlan on  physical ports will be added or removed automatically.  Don't add or remove vlan under physical interface of port-channel. if you do so, vlan on this physical port will mismatch with vlan on port-channel interface, this physical port will be put into suspend stage.

L3 port-channel configuration

SwitchA configuration

interface port-channel 1
no switchport
ip address 10.1.1.1 255.255.255.252

interface FastEthernet1/0/1
 no switchport 
 speed 100
 duplex full
 channel-group 1 mode active

interface FastEthernet1/0/2
 no switchport
 speed 100
 duplex full
 channel-group 1 mode active


SwitchB configuration

interface port-channel 1
no switchport
ip address 10.1.1.2 255.255.255.252

interface FastEthernet1/0/3
 no switchport 
 speed 100
 duplex full
 channel-group 1 mode active

interface FastEthernet1/0/4
 no switchport
 speed 100
 duplex full
 channel-group 1 mode active

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