Optimising LAN Resiliance

Unanswered Question
Aug 7th, 2007

In the standard Cisco model with Access, Ditribution and Core switches, running Layer 2 between Access and Distribution, and Layer 3 between Distribution and Core where should the HSRP Active and PVST roots be.

i.e for the attached diagram, which distribution switch should be the spanning tree root for VLAN 10,110 and should the same device have priority for the HSRP group.

I have this problem too.
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Jon Marshall Tue, 08/07/2007 - 03:37

Hi Mark

A standard way to do this is to make Distro 1 spanning-tree root for odd vlans and distro 2 spanning-tree root for even vlans assuming you are using PVST+ or RPVST+.

If you do this then yes you would match up the HSRP active gateway so:

distro1 - Spanning-tree root 10 110

HSRP active 10 110

Spanning-tree secondary 20 120

HSRP standby 20 120

distro2 - Spanning-tree root 20 120

HSRP active 20 120

Spanning-tree secondary 10 110

HSRP standby 10 110

This way you utilse both uplinks from the access-layer switch.

Other people would argue that you should just make distro1 spanning-tree root and HSRP active for all vlans and distro 2 spanning-tree secondary and HSRP standby.

It really depends on how much traffic you have from the access-layer. If one of the links fails can the other link take the additional traffic or not.



wochanda Tue, 08/07/2007 - 05:19

One thing you may want to consider in your design is swapping out your L2 link between distribution switches for an L3 link.

Historically, network designs included a L2 link between distribution switches in order to accommodate VLANs that SPAN access layer switches. Since your design limits any particular VLAN to a single access switch, killing that VLAN on the trunk between distribution switches would cause both of your uplinks to move to forwarding!

There are plenty of benefits to this, including:

-Load-sharing over both uplinks

-No STP convergence delays

-Less likely to run into L2 loop

Now since you're already doing load-sharing by alternating voice/data VLAN HSRP primaries, leaving HSRP in this design wouldn't add any bandwidth. Implementing GLBP however on these VLANs would allow both the voice and data VLANs to share both uplinks.

Here's a good SRND talking about this:


And here is a link to more campus design SRND's:


mark.j.hodge Wed, 08/08/2007 - 04:30

There are actualy multiple Access switches in each switch block, so L2 links are required. I only put one per block in the diagram for the sake of simplicity.

Thanks for the link to design docs, very useful.

cdusio Tue, 08/07/2007 - 06:51

Wherever your layer 3 connections are is where your HSRP should be cofigured. In your exxample, it would be on the distribution switches. The root of the spanning trees should be configured on the distribution switches that are the active HRSP for that vlan..



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