Dec 10, 2014

EtherChannel vs LACP vs PAgP

1:13 AM  pakitong  No comments

What is EtherChannel?

EtherChannel links formed when two or more links budled together for the purposes of aggregating available bandwidth and providing a measure of physical redundancy. Without EtherChannel, only one link will be available while the rest of the links will be disabled by STP, to prevent loop.
p/s# Etherchannel is a term normally used by Cisco, other vendors might calling this with a different term such as port trunking, trunking (do not confuse with cisco’s trunk port definition), bonding, teaming, aggregation etc.


What is LACP

Standards-based negotiation protocol, known as IEEE 802.1ax Link Aggregation Control Protocol, is simply a way to dynamically build an EtherChannel. Essentially, the “active” end of the LACP group sends out special frames advertising the ability and desire to form an EtherChannel. It’s possible, and quite common, that both ends are set to an “active” state (versus a passive state). Once these frames are exchanged, and if the ports on both side agree that they support the requirements, LACP will form an EtherChannel.

What is PAgP

Cisco’s proprietary negotiation protocol before LACP is introduced and endorsed by IEEE. EtherChannel technology was invented in the early 1990s. They were later acquired by Cisco Systems in 1994. In 2000 the IEEE passed 802.3ad (LACP) which is an open standard version of EtherChannel.

EtherChannel Negotiation

An EtherChannel can be established using one of three mechanisms:
  • PAgP - Cisco’s proprietary negotiation protocol
  • LACP (IEEE 802.3ad) – Standards-based negotiation protocol
  • Static Persistence (“On”) – No negotiation protocol is used

Any of these three mechanisms will suffice for most scenarios, however the choice does deserve some consideration. PAgP, while perfectly able, should probably be disqualified as a legacy proprietary protocol unless you have a specific need for it (such as ancient hardware). That leaves LACP and “on“, both of which have a specific benefit.

PAgP/LACP Advantages over Static

a) Prevent Network Error

LACP helps protect against switching loops caused by misconfiguration; when enabled, an EtherChannel will only be formed after successful negotiation between its two ends. However, this negotiation introduces an overhead and delay in initialization. Statically configuring an EtherChannel (“on”) imposes no delay yet can cause serious problems if not properly configured at both ends.

b) Hot-Standby Ports

If you add more than the supported number of ports to an LACP port channel, it has the ability to place these extra ports into a hot-standby mode. If a failure occurs on an active port, the hot-standby port can replace it.

c) Failover

If there is a dumb device sitting in between the two end points of an EtherChannel, such as a media converter, and a single link fails, LACP will adapt by no longer sending traffic down this dead link. Static doesn’t monitor this. This is not typically the case for most vSphere environments I’ve seen, but it may be of an advantage in some scenarios.

d) Configuration Confirmation

LACP won’t form if there is an issue with either end or a problem with configuration. This helps ensure things are working properly. Static will form without any verification, so you have to make sure things are good to go.

To configure an EtherChannel using LACP negotiation, each side must be set to either active or passive; only interfaces configured in active mode will attempt to negotiate an EtherChannel. Passive interfaces merely respond to LACP requests. PAgP behaves the same, but its two modes are refered to as desirable and auto.


3750X(config-if)#channel-group 1 mode ?
  active     Enable LACP unconditionally
  auto       Enable PAgP only if a PAgP device is detected
  desirable  Enable PAgP unconditionally
  on         Enable Etherchannel only
  passive    Enable LACP only if a LACP device is detected

Conclusion

Etherchannel/port trunking/link bundling/bonding/teaming is to combine multiple network interface.
PAgP/LACP is just a protocol to form the etherchannel link. You can have etherchannel without protocol, but not advisable.

Sources:

http://en.wikipedia.org/wiki/EtherChannel
http://packetlife.net/blog/2010/jan/18/etherchannel-considerations/
http://wahlnetwork.com/2012/05/09/demystifying-lacp-vs-static-etherchannel-for-vsphere/

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