Nov
4
2008

Configuring Link Aggregation with EtherChannel

EtherChannel bundles individual Ethernet links into a single logical link that provides bandwidth up to 1600 Mbps (Fast EtherChannel, full duplex) or 16 Gbps (Gigabit EtherChannel) between two Cisco Catalyst switches. All interfaces in each EtherChannel must be the same speed and duplex, and both ends of the channel must be configured as either a Layer 2 or Layer 3 interface.

If a link within the EtherChannel bundle fails, traffic previously carried over the failed link is carried over the remaining links within the EtherChannel.

There are two protocols used for the link aggregation:

  • Cisco’s proprietary Port Aggregation Protocol (PAgP).
  • IEEE standard Link Aggregation Protocol (LACP)

 

PAgP packets are sent between Fast EtherChannel-capable ports to negotiate the forming of a channel. When PAgP identifies matched Ethernet links, it groups the links into an EtherChannel. The EtherChannel is then added to the spanning tree as a single bridge port.

Mode (PAgP) Description
Auto Places an interface into a passive negotiating state, in which the interface responds to PAgP packets it receives but does not start PAgP packet negotiation. This setting minimizes the transmission of PAgP packets and is the default.
Desirable Places an interface into an active negotiating state, in which the interface starts negotiations with other interfaces by sending PAgP packets.
On Forces the interface to channel without PAgP. With the on mode, a usable EtherChannel exists only when an interface group in the on mode is connected to another interface group in the on mode.

 

LACP is part of an IEEE specification (802.3ad) that allows several physical ports to be bundled together to form a single logical channel. LACP allows a switch to negotiate an automatic bundle by sending LACP packets to the peer. It performs a similar function as PAgP with Cisco EtherChannel. Because LACP is an IEEE standard, it can be used to facilitate EtherChannels in mixed-switch environments.

Mode (LACP) Description
Passive The switch does not initiate the channel, but does understand incoming LACP packets. The peer (in active state) initiates negotiation (by sending out an LACP packet) which we receive and reply to, eventually forming the aggregation channel with the peer. This is similar to the auto mode in PAgP.
Active We are willing to form an aggregate link, and initiate the negotiation. The link aggregate will be formed if the other end is running in LACP active or passive mode. This is similar to the desirable mode of PAgP.
On The link aggregation is forced to be formed without any LACP negotiation .In other words, the switch will neither send the LACP packet nor process any incoming LACP packet. This is similar to the on state for PAgP.

 

 

Etherchannel Negotiation Protocols

Negotiation Mode Packets Sent? Charateristics
PAgP LACP
on on No All ports channeling
auto passive Yes Waits to channel until asked
desirable active Yes Actively asks to form a channel

 

Matrix of Load Balancing Methods
Cisco EtherChannel technology is composed of several Fast Ethernet links and is capable of load balancing traffic across those links. Unicast, broadcast, and multicast traffic is evenly distributed across the links, providing higher performance and redundant parallel paths. When a link fails, traffic is redirected to the remaining links within the channel without user intervention and with minimal packet loss. To define the load-balance method, enter in the global configuration mode and use the command “port-channel load-balance ?” to see the load-distribution method availables.

This matrix consolidates the load balancing methods:

Platform Address Used in XOR Source-Based? Destination-Based? Source-Destination-Based? Load Balancing Method—Configurable/Fixed?
6500/6000 Layer 2, Layer 3 addresses, Layer 4 information, or MPLS information ² Yes Yes Yes Configurable
5500/5000 Layer 2 address only Yes Cannot change the method
4500/4000 Layer 2, Layer 3 addresses, or Layer 4 information Yes Yes Yes Configurable
2900XL/3500XL Layer 2 address only Yes Yes Configurable
3750/3560 Layer 2 or Layer 3 address only Yes Yes Yes Configurable
2950/2955/3550 Layer 2 address only ¹ Yes Yes —1 Configurable
1900/2820 These platforms use a special method of load balancing. See the Catalyst 1900/2820 section for details.
8500 Layer 3 address only Yes Cannot change the method

¹ For the 3550 series switch, when source-MAC address forwarding is used, load distribution based on the source and destination IP address is also enabled for routed IP traffic. All routed IP traffic chooses a port based on the source and destination IP address.
² For the 6500 series switches that run Cisco IOS, MPLS layer 2 information can also be used for load balancing MPLS packets.

 

Example #1: Configuring Layer2 Etherchannel

In this example, the FastEthernet0/0-0/1-0/2 (on Ciscozine_SW1 and Ciscozine_SW2) must belong to vlan 10; it is required to create a Layer2 etherchannel using LACP with desirable mode on Ciscozine_SW1 and passive mode on the Ciscozine_SW2. The configuration is:

Ciscozine_SW1 etherchannel L2 configuration

Ciscozine_SW1# configure terminal
Ciscozine_SW1(config)# interface range fastethernet0/0 -2
Ciscozine_SW1(config-if-range)# switchport mode access
Ciscozine_SW1(config-if-range)# switchport access vlan 10
Ciscozine_SW1(config-if-range)# channel-protocol lacp
Ciscozine_SW1(config-if-range)# channel-group 1 mode active
Ciscozine_SW2 etherchannel L2 configuration

 

Ciscozine_SW2 etherchannel L2 configuration

Ciscozine_SW2# configure terminal
Ciscozine_SW2(config)# interface range fastethernet0/0 -2
Ciscozine_SW2(config-if-range)# switchport mode access
Ciscozine_SW2(config-if-range)# switchport access vlan 10
Ciscozine_SW1(config-if-range)# channel-protocol lacp
Ciscozine_SW2(config-if-range)# channel-group 1 mode passive

 

 

Example #2: Configuring Layer3 Etherchannel

In this example, the FastEthernet0/0-0/1-0/2 (on Ciscozine_SW1 and Ciscozine_SW2) must be aggregated; it is required to create a Layer3 etherchannel using PAgP with desirable mode on Ciscozine_SW1 and auto mode on the Ciscozine_SW2. The configuration is:

Ciscozine_SW1 etherchannel L3 configuration

Ciscozine_SW1# configure terminal
Ciscozine_SW1(config)# interface port-channel 1
Ciscozine_SW1(config-if)# no switchport
Ciscozine_SW1(config-if)# ip address 172.16.1.11 255.255.255.0
Ciscozine_SW1(config-if)# end
Ciscozine_SW1# configure terminal
Ciscozine_SW1(config)# interface range fastethernet0/0 -2
Ciscozine_SW1(config-if-range)# no switchport
Ciscozine_SW1(config-if-range)# no ip address
Ciscozine_SW1(config-if-range)# channel-group 1 mode desirable
Ciscozine_SW1(config-if-range)# end
Ciscozine_SW2 etherchannel L3 configuration

 

Ciscozine_SW2 etherchannel L3 configuration

Ciscozine_SW2# configure terminal
Ciscozine_SW2(config)# interface port-channel 1
Ciscozine_SW2(config-if)# no switchport
Ciscozine_SW2(config-if)# ip address 172.16.1.12 255.255.255.0
Ciscozine_SW2(config-if)# end
Ciscozine_SW2# configure terminal
Ciscozine_SW2(config)# interface range fastethernet0/0 -2
Ciscozine_SW2(config-if-range)# no switchport
Ciscozine_SW2(config-if-range)# no ip address
Ciscozine_SW2(config-if-range)# channel-group 1 mode auto
Ciscozine_SW2(config-if-range)# end

 

REMEMBER: The “no switchport” command is required to change interface from layer2 to layer3 mode.

Use the “show etherchannel” command to display port-channel information after configuration and remember to save the configuration!

 

References:

Summary
Article Name
Configuring Link Aggregation with EtherChannel
Author
Description
A port channel bundles up to eight individual interfaces into a group to provide increased bandwidth and redundancy. Port channeling also load balances traffic across these physical interfaces.
  • Marc Courteaux

    The third table, comparing PAgP and LACP, should say “No” below “Packets Sent?”.

  • Ernie Gronblom

    Which? 1600 Mbps (Fast EtherChannel, full duplex) or 16 Gbps (Gigabit EtherChannel)
    1600 Mbps is 1.6 Gbps! Probably 16000 Mbps.

  • khattar

    Is it possible to have one end configured as l2 while the other end as l3 port-channel? Does this kind of configuration (mix of l2&l3) work?

    Many thanx

  • http://www.linkedin.com/in/fabiosemperboni Fabio Semperboni

    No, the interface on each sides must be the same: L2 – L2 or L3 – L3

  • m

    Nice

  • http://www.facebook.com/fabio.semperboni Fabio Semperboni

    When PagP or LACP are ‘on’, no packet will be sent to make a channel. Indeed: “The link aggregation is forced to be formed without any LACP negotiation .In other words, the switch will neither send the LACP packet nor process any incoming LACP packet. This is similar to the on state for PAgP.”

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