Fabric Port Channel

[dani_bosch]

Hello,

Once in production, when adding cables in a Fabric PortChannel between an IOM and a FI, ¿is it needed to re-ack the chassis? (so that, we'll have downtime of the whole chassis). Or rather, as it is a Port-Channel the cables are detected automatically, so that it is not needed to re-ack, and so there's no downtime at all...which one is the correct statement?

Thanks,

[padramas]

Hello,

Addition / removal of cables requires re-acknowledgement of chassis.

http://www.cisco.com/en/US/docs/unified_computing/ucs/sw/gui/config/guide/2.0/b_UCSM_GUI_Configuration_Guide_2_0_chapter_01100.html#d105053e23a1635

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Link Grouping

Once a fabric port channel is created, links can  be added or removed by changing the link group preference and  reacknowledging the chassis, or by enabling or disabling the chassis  from the port channel.

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HTH

Padma

[dani_bosch]

I don't fully understand this. Let's see:

a) "links can  be added or removed by changing the link group preference"

Why should I change the link group preference if I only want to add/remove cables from the Port Channel? I do NOT want to switch to NONE...I just want to add or remove cables from the port channel...

b) If I want to remove cables from the Port Channel why should I reacknowledge the chassis?? Isn't it the same as if a link fails?? In the case of failure nothing should happen as the LACP has sub-second re-convergence... ¿Am I wrong?

[padramas]

Hello,

Let us consider following scenario on adding or removing links to existing deployment.

Initial deployment

UCSM 2.0, FI 62XX ,2208 IOM and 2 physical links with port channel configuration ( 2 link discovery policy )

Change

1) If additional links are added and if they need to be utilized and part of port channel, we need to re-ack the chassis.

2) If a link goes down,traffic is distributed across other links in port channel and does not require re-ack. Parameters like bandwidth, VIF ports that are calculated based on initial configuration remains the same.

3) If we physically remove the additional links between IOM / FI, re-acknowledment would create port-channel with original two links.Failure to re-ack will create over subscription ( bandwidth, available VIF ports ) of the network links.Re-ack is not mandatory but it creates appropriate configuration that matches required design criteria.

Caution

Adding or removing links  from a fabric port channel is disruptive and may affect the available amount of VIF namespace.

http://www.cisco.com/en/US/docs/unified_computing/ucs/sw/gui/config/guide/2.0/b_UCSM_GUI_Configuration_Guide_2_0_chapter_0101.html#concept_5D3D88341BFB43468B62B5A77876C67B

Padma

[dani_bosch]

Thanks Padma for your reply.

Another question: is it possible to have different LINK GROUPING PREFERENCES in the same UCS instance? For example, to have chassis 1 in Fabric Port-Channel mode, and chassis 2 in NONE option...

I'm a bit confused as I do not see any place in the particular chassis where to confgigure whether Port-Channel or None mode...rather this is a GLOBAL POLICY config...so I'm trying to figure out how to play with this in case I want to change this feature for a particular chassis in leave it as it is for another one.

Can you help me understanding that?

[padramas · Accepted solution]

Hello,

Yes, it is possible to have fabric port channel per fabric ( A / B ) per chassis within a UCS instance.

Option is available under

Equipment > Chassis > Chassis X > Connectivity policy

http://www.cisco.com/en/US/docs/unified_computing/ucs/sw/gui/config/guide/2.0/b_UCSM_GUI_Configuration_Guide_2_0_chapter_01100.html#d106201e585a1635

HTH

Padma

[dani_bosch]

Great, I didn't see it because the UCS in my lab hasn't got the new 6200 FIs and new 2200 FEXs.

Thanks a lot Padma,

[dani_bosch]

Just a last question:

What's the difference when comparing Port Expander + VIC 1240 versus VIC 1280 + VIC 1240?? (forget about redundancy and pricing...I'm rather focusing on connectivity).

Do the port-channels in these cards (that one can read in the B200-M3 Tech Specs sheet) have anything to do with the Fabric Port Channels between the FIs and the FEXs??

Thanks,

[dani_bosch]

Hello Padma,

Yesterday, during Data Center Partner Virtual Team, your Cisco colleague Arnaud Bassaler said to the audience that adding/removing cables from a Fabric PortChannel between FEX and FI did NOT suppose any downtime...you told me it IS required to re-ack the chassis, that is, there's a downtime related to it...

I'd like to clearify who's wrong of you two.

Thanks a lot,

[padramas]

Dani,

Yes, adding / removing links to a chassis configured in fabric port channel mode does NOT require a re-ack of chassis.

But there are scenarios where re-ack is desired to reflect available VIFs space.

Number of member links affects the VIF space. If we are permanently removing link ( s)  from a fabric port channel, re-ack will re-calculate the VIF space.

Consider a scenario where a chassis with 2 link discovery policy and 4 links exists between FI / IOM and is configured for port-channel mode.

Now, if we disable or bring down two links, it does not affect available VIFs. However, if the ports are down and we re-ack the chassis, VIF will be re-calculated even though there still 4 links in port channel. Same behavior if we physically remove the links between IOM - FI but port channel will be created with two member links.

HTH

Padma

[dani_bosch]

Padma, so sorry but I'd need you to rephrase your last post...it was a bit difficult to understand this.

a) when you say VIF's you mean the "up to 256 PCIe" interfaces of the Cisco VIC?

b) let's consider a scenario where I have 2 links in the Fabric Port-Channel....if I add 2 links more, ¿will the final 4 links be 100% up and running?? I understood you said YES but then there's something wrong with the so called VIF Namespace which I don't exactly know what does it mean and what does this imply...

c) your colleague also mentioned in Port-Channel mode, only 1-2-4-8 links are supported by the TAC...whereas 3-5-6-7 link configs do technically work, but are NOT supported by the TAC. Is that correct? Why is this so?

Thanks a lot, really appreciate it