L2 Stack Switch Uplink Traffic Flow

[PolarPanda]

Hi Gurus,

           I have a L2 stack switches and every member has a uplink connect to a L3 switch. When the inbound and outbound traffic go through between L2 and L3 switches, does it go through all the links between them or only active switch in the L2 stack?

           In my environment, I see only active switch passing traffic through uplink; both active and one member passing traffic; only one member passing traffic, not active switch.

[DanielP211]

Hello!

Do you have a etherchannel between stacks? If you have single uplinks its possible traffic dosen't pass both.

BR

[PolarPanda]

no etherchannel. If that's the case, should traffic only pass the active switch and the rest of uplink will be standby?

[DanielP211 · Accepted solution]

If you don't have etherchannel the redundant port will be blocked by spanning tree. Check "sh spanning-tree blocked-ports".

[PolarPanda]

you're right, i didn't even notice that before. I just checked one of the stack switches. It has three uplinks and two of them being blocked. The one not being blocked is NOT active switch. Does the stack switches just randomly choose two being blocked out of three?

[balaji.bandi]

then Port-channel config should applied both the side to fix the issue

[balaji.bandi]

 I have a L2 stack switches and every member has a uplink connect to a L3 switch

how many switches in the stack ? how many uplinks connected to L2 switch ?

what model of the switches both the side and what IOS code running ?

Can you post the configuration of the Port-channel config both the side

understand how the load balance works :

https://www.cisco.com/c/en/us/support/docs/lan-switching/etherchannel/12023-4.html

[PolarPanda]

we don't have etherchannel config on both sides. For the L2 stack switch connecting L3 core, does it usually use port-channel?

[Joseph W. Doherty]

Most often, I believe, yes, because it provides redundancy and often can take advantage of the additional link(s) bandwidth.  Further, an Etherchannel link dropping, doesn't impact the network as for long (ms. vs. multiple seconds to lots of seconds) as an active STP link dropping.

[DanielP211]

Yes you should use etherchannel for maximim redundancy and bandwith increase. You also will not have issues with STP.

BR

[Joseph W. Doherty · Accepted solution]

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@PolarPanda wrote:

we don't have etherchannel config on both sides. For the L2 stack switch connecting L3 core, does it usually use port-channel?

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Hmm, missed this reply.

When using Etherchannel, both sides must be configured for it.  Also, you want to insure you're using the best possible hashing algorithm, which might need to be different on the two sides, to take best advantage of the aggregate bandwidth.

What might be confusing, one side might be configured with an IP address, while the other side isn't.  Logically, not different how you might connect a L2 switch to a L3 switch using a single link.

[PolarPanda]

Thanks ya'll for the answers. What's the reason not use etherchannel? could be avoid bottle neck? If L3 switch only has 10g uplink to router, and we have 3 L2 stack switches and each has 4 members, then the total of each stack has 4g link to L3. Total 3 L2 switches would be 12g. Could that be a bottle neck to route traffic?

Also, if i leave the design like this without etherchannel, apparently, only one uplink work at a time, if this link down, will stack pick next uplink randomly or the standby switch?

[Joseph W. Doherty · Accepted solution]

"What's the reason not use etherchannel?"

Perhaps it's not possible in some use cases.

Or, perhaps, for heavy bandwidth needs, as higher bandwidth link would serve better.

For example, given a choice between 3 gig Etherchannel vs. 2.5 mGig or 6 gig Etherchannel vs. 5 mGig, which is better?

"If L3 switch only has 10g uplink to router, and we have 3 L2 stack switches and each has 4 members, then the total of each stack has 4g link to L3. Total 3 L2 switches would be 12g. Could that be a bottle neck to route traffic?"

You're assuming 4x gig Etherchannel will provide 4 gig, that's unlikely.  You would do well if you got 2 gig on the 4x gig Etherchannel.

Second, even if you had 12g, from the 3 downstream stacks, it's unlikely the 3 downstream stacks would concurrently drive their links at full capacity for substantial time, and as 10g is always available, a bottleneck would normally be of short duration

The last is much the same as any case of oversubscription.  How much bandwidth might each of those 4 member stacks generate?  Why if each stack member was only 24 FE ports, that 2.4 gig per switch time 4 switch members, is 9.6 Gbps!!!  Bottleneck!  Or, if each stack member had 48 gig ports, why then its 192 Gbps!!!!!  Is your 4x gig Etherchannel a major bottleneck?

"Also, if i leave the design like this without etherchannel, apparently, only one uplink work at a time, if this link down, will stack pick next uplink randomly or the standby switch?"

No, it's usually not random, it depends on loop prevention protocol's rules.