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Question about single core and multiple spanned vlans

In this scenario I have a stacked 3750 doing the core routing for the  building.  Less than a 50 users, 6 racks of equipment on the 2nd floor  and the core switch-router on the 3rd floor.  Certain vlans span the 2nd  and 3rd floor.  The question has been raised as to whether or not  having traffic from the 2nd floor come up the 3rd floor and back to the  2nd floor can become an issue. 

Currently there is a 2 port gig  etherchannel (can add 1 more port) between the core switch-router and  the core 2nd floor switch.  All switches in the racks (on 2nd floor) are  connected to the core 2nd floor switch.  Therefore, if data from a user  computer on the 2nd  floor wants to reach a computer on the test vlan on the 2nd floor it  needs to go up/down the pipe to get there. 

Is that necessarily  bad or considered poor configuration?

11 REPLIES
Hall of Fame Super Blue

Re: Question about single core and multiple spanned vlans

dpatten78 wrote:

In this scenario I have a stacked 3750 doing the core routing for the  building.  Less than a 50 users, 6 racks of equipment on the 2nd floor  and the core switch-router on the 3rd floor.  Certain vlans span the 2nd  and 3rd floor.  The question has been raised as to whether or not  having traffic from the 2nd floor come up the 3rd floor and back to the  2nd floor can become an issue. 

Currently there is a 2 port gig  etherchannel (can add 1 more port) between the core switch-router and  the core 2nd floor switch.  All switches in the racks (on 2nd floor) are  connected to the core 2nd floor switch.  Therefore, if data from a user  computer on the 2nd  floor wants to reach a computer on the test vlan on the 2nd floor it  needs to go up/down the pipe to get there. 

Is that necessarily  bad or considered poor configuration?

No not really. In fact if you have a building setup where you have multiple floors and each floor has access-layer switches that connect to a central L3 distro/core setup then it is common for inter-vlan traffic to have go up and down to get to another vlan. Where i worked last this was our standard setup ie. on one floor L3 core/distro and on each other floor we had multiple vlans connected to L2 switches. So any inter-vlan traffic had to go to the floor where the L3 distro/core switches were.

It is certainly not bad design or poor configuration and unless you are moving huge amounts of data any performance issues are unlikely to be because of this.

Jon

Hall of Fame Super Bronze

Re: Question about single core and multiple spanned vlans

If the switch in the 2nd floor is a stack, it will contain all the local MAC-Addresses locally and it will not go to the 3rd floor core switch in order to perform local L2 switching.

It will only go to the 3rd floor switch, if the user in the 2nd floor is going to another vlan allocated to the 2nd floor or if the destination is located in another switch that is not part of the stack in the second floor.

Regards

Edison.

New Member

Re: Question about single core and multiple spanned vlans

Thanks guys.

You confirmed my beliefs.


Edison, 2nd floor is 2 stacked 3750's.  I understand local traffic will stay local on the switch, however, the stack is only workstations and phones, where as the swtiches in the racks are servers and other network equipment.  So if a user wants to reach any of the equipment in the rack from their workstation they need to go up/down to get to it.

To expand on what you said Jon, "It is certainly not bad design or poor configuration and unless you are  moving huge amounts of data any performance issues are unlikely to be  because of this."  So an issue could arise if someone was pushing large amounts of data from 1 vlan to another up/down the pipe, if so what would be an ideal solution?  Enable routing on the 2nd floor stack and migrate vlans to dedicated floors?  IE separate them all and only span what absolutely needs to be spanned?

Hall of Fame Super Blue

Re: Question about single core and multiple spanned vlans

To expand on what you said Jon, "It is certainly not bad design or poor configuration and unless you are  moving huge amounts of data any performance issues are unlikely to be  because of this."  So an issue could arise if someone was pushing large amounts of data from 1 vlan to another up/down the pipe, if so what would be an ideal solution?  Enable routing on the 2nd floor stack and migrate vlans to dedicated floors?  IE separate them all and only span what absolutely needs to be spanned?

Routing in the access-layer is one solution but it's not something to do without a lot of planning. Within a campus/building environment L3 from the access-layer works well and i have used it before but it is not something i would use in a DC. It really comes down to whether or not you really do need to span vlans across floors.

If you reall do need to move very large amounts of data between certain clients/servers on the same floor then ideally you want them on the same switch. Even being on 2 separate switches interconnected via etherchannel may not be enough although with 3750 stacks this does not necessarily apply.

Jon

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New Member

Re: Question about single core and multiple spanned vlans

Not really large amounts of data across the board.  Maybe 1 user moving large files every so often.  Not enough to warrant changing the configuration in my mind.

Thanks for your help.

Hall of Fame Super Blue

Re: Question about single core and multiple spanned vlans

No problem, glad to have helped.

Jon

New Member

Re: Question about single core and multiple spanned vlans

Hi, Why dont you create vlan based on floors. Like vlan 300 for 3rd floor and make 3rd floor core switch as root bridge for this vlan similarly create vlan 200 for 2nd floor and make this switch root bridge for this vlan. if possible and let floor 3 switch do the routing.

I dont know the situation but generaly we do like that. but we usually have two core switches having one core switch as root bridge for odd vlan and another core as root bridge for even vlans.

I hope this help.

New Member

Re: Question about single core and multiple spanned vlans

Hi, Why dont you create vlan based  on floors. Like vlan 300 for 3rd floor and make 3rd floor core switch  as root bridge for this vlan similarly create vlan 200 for 2nd floor and  make this switch root bridge for this vlan. if possible and let floor 3  switch do the routing.

I dont know the situation but generaly we do like that.  but we usually have two core switches having one core switch as root  bridge for odd vlan and another core as root bridge for even vlans.

I hope this  help.

Jain, I didn't do it that way because at the time this was implemented it was a relatively small office and 1 floor, IE 25 users and very little equipment. If I were to do that then I would need to create 2 vlans for each floor, 1 DATA and 1 VOICE. At the time I didn't see the need for separate vlans for each floor.  It has outgrown that by a mile and I decided that it would be easier for me to extend them down then to add another switch/router to the mix. 

Currently the 3rd floor core-switch/router is the root bridge for all vlans and is doing the routing for all as well.  Still if I have separate vlans for each floor there is still going to be a separate vlan for TESTING, QA, DMZ, etc.. that the traffic will still need to go up/down the pipe to reach. 

Hall of Fame Super Blue

Re: Question about single core and multiple spanned vlans

Darren

Even with multiple vlans or one vlan per floor it still makes sense to have your root/secondary bridges be the distro/core switches that are responsible for inter-vlan routing.

Jon

New Member

Re: Question about single core and multiple spanned vlans

Darren

Even  with multiple vlans or one vlan per floor it still makes sense to have  your root/secondary bridges be the distro/core switches that are  responsible for inter-vlan routing.

I aggree Jon.  Not sure who Darren is though, is that jain.nitin?

Hall of Fame Super Blue

Re: Question about single core and multiple spanned vlans

I aggree Jon.  Not sure who Darren is though, is that jain.nitin?

Sorry, i got a bit distracted while posting

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