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The Many Faces of Link Aggregation

Link Aggregation
United States of Tara (Courtesy Home Box Office)

If any technology in networking  had identity issues, it would have to be Link Aggregation.   This simple process can never seem to make up its mind what it wants to be called.

Link aggregation is the process of combining two or more connections into a larger pipe to allow for more throughput. If you have a single 1Gig uplink connection and need to add more bandwidth and don’t really care to go out and buy a 10Gig ethernet card, link aggregation is the way to go.

The Many Faces of Link Aggregation

Link aggregation goes by many names. Ethernet Network Interface bonding or NIC Teaming is commonly referred to on most servers or bladed systems. In the Cisco world link aggregation is commonly known as Ether-channel or Port-channel. Most Cisco equipment supports link aggregation and can use LACP (Link Aggregation Control Protocol) or PAgP (Port AggregationProtocol) as the aggregation protocol. NOTE: PAgP is a Cisco proprietary protocol and can only be used between similar Cisco equipment that supports PAgP.

Aggregation can occur at any of the lowest three layers of the OSI model. Either Layer 1, Layer 2, or Layer 3.   The most common is layer 2 within a switch or layer 3 within a router.

There are rules to this Link Aggregation Business

Interfaces participating in an aggregated channel can do so if they meet certain rules. Each interface typically needs to be of the same media type. Meaning fiber(multi mode or single mode) or copper. They all must also be configured for the same speed, 100M, 1000M and so on.

Each port must be using the same aggregation control protocol, either LACP or PAgP.

Both LACP and PAgP also have modes of operation.

There are only three valid combinations to run the LACP link aggregate, as follows:

SwitchSwitchComments
activeactiveRecommended.
activepassiveLink aggregation occurs if negotiation is successful.
ononLink aggregation occurs without LACP. Although this works, it is not recommended.

Note: By default, when an LACP channel is configured, the LACP channel mode is passive.

 

PAgP can be configured on a Cisco switch to operate in three different modes.

SwitchComments
autoPassive negotiation of the channel..
desirableActive negotiation of the channel.
onNo protocols are used, it assumes the other side has also enabled link aggregation.

 

How to Configure Link Aggregation

Here is an example of How To setup a aggregated link on Cisco equipment:

interface Port-channel1
ip address 10.0.1.3 255.255.255.0
no ip directed-broadcast
channel-group minimum active 2
no channel-group bandwidth control-propagation
!
interface GigabitEthernet8/0
negotiation auto
channel-group 1
!
interface GigabitEthernet8/1
negotiation auto
channel-group 1

Setting up link aggregation on JUNOS is a little different. Here is an example of how to setup link aggregation on a Juniper:

Iidentify what link aggregation groups are in use by using the show interface command.

router> show interfaces

In this example ae0 and ae1 are the two existing link agg groups.

Once the link agg group to use has been identified, the GigE interface must be assigned to the link aggregation group. This is done with the following command

router> set fe-x/x/x fastether-options 802.3ad ae0

In this example, fast ethernet ports will be assigned to link aggregation group ae0. This can be verified by performing the following show command

router> show fe-x/x/x fastether-options 802.3ad
!
802.3ad ae0;

Once the ports are assigned to the link aggregation group, the Ethernet options can be set on the aggregation group.

Enter the edit mode for the link agg group by using the commands below

router> edit

router> edit interfaces ae0

Once in the edit interface mode, the flow control, link speed and minimum number of links in the link agg group can be set.

router> set aggregated-ether-options no-flow-control

router> set aggregated-ether-options link-speed 1g

router> set aggregated-ether-options minimum-links 2

router> set mtu 1514

To set these options on GigE ports use the following steps.
Identify what link aggregation groups are in use by using the show interface command.

router> show interfaces

In this example ae0 and ae1 are the two existing link agg groups.

Once the link agg group to use has been identified, the GigE interface must be assigned to the link agg group. This is done with the following command

router> set ge-x/x/x gigether-options 802.3ad ae0

In this example, the port has been assigned to link agg group ae0. This can be verified by performing the following show command

router> show ge-x/x/x gigether-options 802.3ad
!
802.3ad ae0;

Once the ports are assigned to the link agg group, the Ethernet options can be set on the link agg group. Enter the edit mode for the link agg group by using the commands below

router> edit

router> edit interfaces ae0

Once in the edit interface mode, the flow control, link speed and minimum number of links in the link agg group can be set.

router> set aggregated-ether-options no-flow-control

router> set aggregated-ether-options link-speed 1g

router> set aggregated-ether-options minimum-links 2

Setting up a port-channel, ether-channel, aggregated link is fairly simple. If you’ve never setup one up before practice on some test lab equipment to get the hang of it.

Link aggregation can be a lot of fun and save you the expense of upgrading your switch port modules.

 

About Joe

Senior Network Engineer, technology enthusiast, guitar and bass player. Joe Wilson is the creator of RouterFreak.com as well as other niche websites that can be found around on the Internets.

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5 comments

  1. Heya i’m for the first time here. I came across this board and
    I find It truly useful & it helped me out much. I hope to give something back and help others like you
    aided me.

  2. Hi!
    Why is the LACP active-active setting recommended over the static on-on one?

    I believe, the on-on mode converges faster, conserves less bandwith (every bit saves a byte! 🙂 and with proper implementation procedure (to minimize the human factor) will be less prone to errors (steming from software bugs or malicious actions).
    Please, explain if i’m wrong at it.

    • Hi Ben,
      It’s debatable which setting is better. Like everything, people have opinions about what is best.

      You’re right, on-on mode will converge faster and there is less overhead… but you have no protection over broadcast storms or bridging loops in the event that something goes wrong within the physical links themselves.

      For example, say you have redundant port channels from an access switch up to redundant cores. These port channels are trunking vlans down to your access switch. If you have an engineer performing a change on the port channels to add a new vlan and he or she adds them to the physical interfaces rather than the port-channel interface you can find yourself in a bridging loop bringing down your switch fabric.

      With LACP active-active, LACP plays an “active” role in managing the port channel and will remove inconsistent physical interfaces out of the channel group ensure a more stable channel.

      Does this makes sense? Honestly it comes down to a personal preference and some can argue that LACP causes more problems than it fixes.

      Thanks for your comment – It was a good one!

      Joe

  3. Brocade.
    conf t
    int e 1/1/1 to 1/1/2
    link-aggregate conf key 10001
    link-aggregate active

    show link-aggregate

  4. Hello,

    How about an article on vPC?

    Kind Regards…

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