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802.3ad on 3750


I have a server with 2 nics plugged into my 3750.  both nics can be teamed and I have the option on the server for

  1. Smart Load Balancing and Failover
  2. Generic Trunking (FEC/GEC)/802.3ad-Draft Static
  3. Link Aggregration (802.3ad)

I just wnat the server to LB and failover so I normally use the first option, but a nortel/cisco engineer (we had onsite for another issue) said 802.3ad is much better, does anyone know what this is and do I need to make any changes to the interfaces the servers are plugged into?



Re: 802.3ad on 3750


802.3ad is the LACP protocol. All it does is bind multiple physical ports into one logical one and it allows you to pass traffic over those physical links using different load sharing methods.

It is oftentimes used in a L2 switched environment to augment bandwidth and 'fool' STP into thinking there's only one connection between switches, instead of mutiple ones, which would cause STP to block all but one to avoid bridging loops.

In a L3 application, it is used to aggregate multiple links to augment bandwidth without having to deploy a subnet for each connection. So, 4 ports can be bundled into one L3 LACP channel, and that logical interface that results has the one /30 IP address applied to it.



Community Member

Re: 802.3ad on 3750

Thanks, what I have noticed with option 1 is sometimes only one nic will work, so is it possible this option is causing a loop and being shut down?  Maybe 802.3ad would be better as the server users 2 nics but has only one IP address?


Re: 802.3ad on 3750


Smart Load Balancing will not cause a L2 loop. Each NIC card will use a separate MAC address. Moreover, STP would not be run on edge ports connected to [fixed typo] servers. The loops we are concerned with involve trunk ports that carry traffic for multiple subnets/vlans across the switch fabric.

I'm not a server engineer, so I can't say I'm very well versed on the nuanced differences between NIC teaming methods and their implications in different topologies.

Perhaps it would be beterr if you asked this engineer to give the rationale behind his suggestion.I can't get into his head, of course.


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