LTM Connection to Dual Switches

The 'problematic' configurations when specifying speed and duplex on a cisco and auto on the connecting interface is due to the fact that cisco read the specs slightly differently from others... When you specify duplex on a cisco switch port then the switch no longer advertises the duplex to the connected port. Speed is easy (That's voltage). But duplex needs advertising. However cisco reads the spec as saying that if duplex is hard-set, then you don't advertise any more.

 

 

The sad part of that is that if you hard-set the cisco switch port to full-duplex and have auto/auto on the connected port, then speed is detected by voltage, but because there's no advertising the connected switch port chooses half-duplex (because a half-duplex hub doesn't advertise).

 

 

A connection that's full-duplex at one end and full at the other then generates unexpected collisions... It'll work fine at low speeds, but if you try to push too much data through it, it will just crawl. Also some versions of cisco (Catos especially, but you can configure IOS to do the same) will disable the switch port if it's getting errors (And collisions on a full-duplex port is an error).

 

 

Most other systems advertise when you hard-set duplex... e.g. Nokia, AIX, Solaris... It's safe to say that unless you set a cisco to auto duplex, you'll probably get problems (UNless you're willing to put up with the pain of hard-setting ALL your devices. Not sure why you would, but I have seen it done).

 

 

H

 

Hamesh.... really lilked your clarification of trun & vlan and port broadcasting of duplex......for F5 & Cisco do you have any particular document which can explain this ... can you provide a link to any....

 

 

Mikand really liked ur info for the auto/auto link aggregation do you have any document stating this... i mean any further details systamatic information

Techgeeeg: You mean regarding the LACP active and LACP passive modes?

 

 

When LACP active is set the unit will send LACP packets every now and then (at least at the moment when a link goes up) to inform the other side that this unit wants to do LACP (instead of letting STP (spanning tree) disable the "looping" interface if you have STP enabled).

 

 

The other unit must be in either LACP active or LACP passive mode in order to having this LACP trunk to form.

 

 

So except from manually set up the bundling (which I would recommend because then you know where you are expected to have a bundle or not, at least by manually set dedicated interfaces into LACP active mode) the auto feature works as:

 

 

unit1: LACP active

 

unit2: LACP passive

 

= LACP trunk will form

 

 

unit1: LACP active

 

unit2: LACP active

 

= LACP trunk will form

 

 

unit1: LACP passive

 

unit2: LACP passive

 

= no LACP trunk will form (loop occurs unless you have STP enabled)

 

 

unit1: LACP active

 

unit2: no LACP

 

= no LACP trunk will form (loop occurs unless you have STP enabled)

 

 

unit1: LACP passive

 

unit2: no LACP

 

= no LACP trunk will form (loop occurs unless you have STP enabled)

Thank you Techgeeeg, Mikland, and Hamish for your time on this issue. Your feedback and guidance is truly appreciated. Just as a final note, I will explain the reason behind the speed 1000. On the Nexus 5548UP, when I installed the GBIC and issued a no shutdown on the port, it goes into an invalid state. The only way to bring it out of invalid is to place speed 1000 into the configuration. Maybe something odd or a bug.

Hi. can yu insert the diagram..unable to read your diagram link. tq.