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New Member

Design Campus Layer 2 - do we still need the trunk?

For access vlans we used to design STP-triangles where the access-switch has two uplinks to two distribution switches. Between the DSWs was a trunk carrying the same vlan.

We use two layer 3 addresses, one on each DSW and HSRP for redundancy. One of the uplinks was blocked from the access-switch due to SPT-calculations and we designed the active HSRP-Address on the DSW with the active uplink. OK.

BUT: Now we have very fast access-switches and we have features like GLBP (Gateway Load Balancing Protocol) which allows us to split the clients between both available L3-addresses on the two DSWs. Therefore it would be wise to have both uplinks active for the same vlan at the same time.

So, is there still a need for the vlan trunk between the two DSWs?

I would prefer to configure a L3-link between them, so routing of traffic back into the access vlan would work even if an uplink is down.

I am looking forward to your comments

William Boye

iuvandis GmbH

network consultant


Re: Design Campus Layer 2 - do we still need the trunk?

Provided you are not using multiple ports on each dist. switch for the same VLAN, you can remove the L2 trunk between the two switches and go L3 instead. You could even turn off STP.

But if you are using multiple ports on each dist. switch for the same VLAN (whether they are access ports or trunk ports), then you must keep the STP because you will have a loop between the two distribution switches (via the access switches).

As long as you are keeping STP, set up the two dist. switches so they are the primary and backup root bridges for each VLAN they serve. Then, enable Spanning-Tree UplinkFast on each access switch.

If you were having a problem with one of the uplinks being disabled by STP before, you could increase the STP port cost on the L2 trunk between the two dist. switches. But I think it would be better to leave it as it is, and leverage the UplinkFast failover instead. That will be complete before HSRP times out the active router and cuts over to the standby.

If it were me, I would keep the L2 trunk and take advantage of UplinkFast. Of course, that assumes you have all Cisco switches.

Hope this helps.

P.S.-Where are the clients going to, that you want to load-balance them across two dist. switches? Multi-homed servers? Or a redundant-connected server farm switch? What's the speed of the uplink from the access switch to the distribution switches? Just asking, to try to get a feel for whether all these active links and load balancing are really necessary for performance, or if you are just looking for redundancy in the event of a link failure.

New Member

Re: Design Campus Layer 2 - do we still need the trunk?

Thank you, Larry. Well that's it. Your answer helped me clarifying my idea.

In my concept I was thinking of local vlans which are used only on ONE access-switch and the two vlan-interfaces on each of the DSWs.

In this case replacing the trunk between the two DSWs by a L3-link would give us better performance through real load sharing in one vlan.

But as you mentioned with multiple ports on the DSWs for the same vlan - and this makes sense only with multiple access-switches in the same vlan - we would get a blocked port somewhere on one uplink (ASW or DSW, it doesn't matter).

This would result in a possible return-traffic - if the traffic from the core lands on the "wrong" DSW with the blocked uplink to the ASW - like Core-DSW2-ASWx-DSW1-ASWy.

With the trunk and a root on one of the DSWs the return-traffic which lands on the "wrong" DSW would be Core-DSW2-DSW1-ASWy.

Do you agree?

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