Multiple trunks between switches

Answered Question
Feb 23rd, 2009
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I'm relatively new to the field and need some advice. I have a request from above to physically segregate traffic on a particular vlan from the rest of the network. My question is basically whether or not this can be accomplished by having two trunk ports with different vlans on each trunk between the switches and if this could cause issues (STP?).


Example of what I'm trying to accomplish

Switch A

Port 1 Trunking vlans 1-4

Port 2 Trunking vlans 5-8


Switch B

Port 1 Trunking vlans 1-4

Port 2 Trunking vlans 5-8


Just to limit anyone's time who looks at this putting in more switches is out and etherchannel has been considered.


Thanks in advance.

Correct Answer by Jon Marshall about 8 years 2 months ago

Yes but the switching fabric is greater than one individual port so by separating the vlans over 2 separate ports you are in fact spreading the load altho it is very dependant on per vlan traffic amount.


I would also recommend Adam's suggestion of using the links as backups for each by manipulating the STP priority.


Jon

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Jon Marshall Mon, 02/23/2009 - 12:08
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Heath


This won't physcially segregate the traffic as such. You are still using the same switch infrastructure just with 2 trunks instead of one.


You can do what you suggest but it amy help to have more details. To stop one vlan communicating with others you could access-lists on the L3 vlan interfaces or you might not even need a L3 vlan interface for this isolated vlan.


Either way if you need to physcially segregate then you need separate switches etc.


Jon

heath.barnhart Mon, 02/23/2009 - 12:14
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Yeah, I realize that I'm not actually physically segregating the network. What I meant to say was to decrease the utilization on the link. Will that even be achieved though? It still goes through the same switching fabric at the switches.


The goal here isn't to stop communications between the vlans but to limit possible congestion.


Feel free to tell me if I'm being to vague.

Correct Answer
Jon Marshall Mon, 02/23/2009 - 12:16
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Yes but the switching fabric is greater than one individual port so by separating the vlans over 2 separate ports you are in fact spreading the load altho it is very dependant on per vlan traffic amount.


I would also recommend Adam's suggestion of using the links as backups for each by manipulating the STP priority.


Jon

adamclarkuk_2 Mon, 02/23/2009 - 12:12
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Hi


What you have suggested is fine and will work.


Maybe a better solution would be to trunk all 8 vlans across both trunks and use STP (802.1w or 802.1s) to load balance by changing the priority on the 2 groups so switch A is root for 1-4 and switch B is root for 5-8. This At least this way you will have a "hot standby" link in case one dies.

Marwan ALshawi Thu, 02/26/2009 - 14:44
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even if you keep one switch is the root (which is easy)


on the root switch go to trunk one for example and just increase the port priority for vlans 1-4)


do the same on trunk 2 on the root switch but for other vlans

this way you will make each trunk in forwarding state for each group of vlans

which wil loadbalnce the traffic over your trunks

this works if you have those switches only and connected directly


HTH

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