BW utilization and vPC

[visitor68]

The point of vPC, of course, is to leverage both uplinks from a switch (or server) and not have STP block either one of them. Split LAG. Got it. The benefit is supposed to be the full bi-sectional BW you get.

How about if I have, say, 20 VLANs configured and at a point in time each VLAN is offering 1Gbps of offered load. Moreover, I have those VLANs salt-and-peppered across [EDIT] 2 x 10G [EDIT] uplinks using PVST+. In this case, I will still be able to pass the 20Gbps of traffic offered by the 20 VLANs, albeit each VLAN will only be able t use 1 uplink. But who cares if in the end you were able to pass all your traffic. In other words, 20Gbps is being sent on those uplinks, period.

Maybe I am confusing myself, but I fail to see the benefit if PVST+ is used. What am I missing, besides a brain? :-)

Can someone help clarify things or offer a different perspective?

Thanks

[Jon Marshall]

Oh, I just realised what you meant.

You meant using PVST+ as opposed to vPC.

Jon

[visitor68]

Correct, Jon. I mean using the legacy PVST+ salt and peppered the way I described as opposed to vPC.

[Jon Marshall]

If you had 20Gbps with of uplink bandwidth and you knew for sure that 10 vlans worth of traffic on one of the uplinks never went above 10Gbps and 10 vlans on the other uplink never went above 10Gbps then yes you could use a variant of PVST+.

Two issues with this -

1) blocking an uplink may well create a suboptimal path in traffic depending on the topology ie. each Nexus switch tries to forward traffic out of it's local vPC ports and it can because all vPC ports are forwarding whereas traffic may arrive at one Nexus switch and it's link may be blocked so it has to send it to the other switch to be forwarded.

2) Realistically it's very hard to know for sure each only needs 1Gbps and that 10 vlans will never need more than 10Gbps. If you need more than 10Gbps on an uplink for the 10 vlans that are forwarding on it you can't have it.

Using a vPC gives a lot more flexibility because you just need to scale the vPC for the amount of traffic you want for all vlans and not have to worry about how it is allocated per vlan.

Jon

[visitor68]

Something funny, Jon...I thought of this right after I posted my question.

Of course given the scenario I gave, besides the issue of sub-optimal traffic patterns, it fits perfectly into the 20Gbps/all VLANs offering the same load/salt and peppered criteria. (I should have specified that my scenario offers 2 x 10G uplinks, one in a vPC and one in a legacy config). Therefore, vPC is not needed to get full bi-sectional BW. In other words, I gave a scenario that represents a perfect storm, so to speak.

But how about if, say, VLANs 1, 3, 5 and 7, which all point to the same root bridge, are offering 4 Gbps of BW? Now, in that case, there will be delay in sending the traffic because the other uplink is off limits to these VLANs. In that case, vPC would have assisted tremendously. This is just one scenario that pops up in my mind.

I think I was just mind f-cking myself this afternoon. Better stay off the cheap red wine. :-)

[Jon Marshall]

Better stay off the cheap red wine. :-)

Good idea, especially with the hangovers you get :-)

Jon

[visitor68]

Jon, fyi, one of your posts is empty - no message.

[Jon Marshall]

Well I said "Hi Victor ?" and got no response so I assumed I was either mistaken or you wanted to remain incognito so I thought I'd remove it :-)

Jon

[visitor68]

Oh, Im sorry. I didnt notice that. No, my name is Tony. Sorry for overlooking

[Jon Marshall]

No problem, my mistake.

Victor is an old friend off the site who doesn't post any more but your posting just reminded me of him.

Jon

[Jon Marshall · Accepted solution]

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