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New Member

layer 3 on switches

Hi all, I recently did a switching course, We had a setup where we had say 1 distribution switch root for vlan 2, this would be secondary root for vlan 3, and a dist switch root for vlan 3 and secondary root for vlan 2. My question is If these switches are the gateways for the pc's ie running vlan routing, would you need to set hsrp on them as I see you would have to due to the spanning tree design ?


Re: layer 3 on switches

If I understand you correctly, you are using PVSTP+ on 2 L3 switches, say 3550's. If so, you don't need HSRP.

An appropriate use for HSRP would be a server farm where each server is connected to 2 switches for HA.

New Member

Re: layer 3 on switches

This is what I was told on my course, so that the spanning tree root has a backup and so do the gateways, can anyone else confirm this is correct ?


Re: layer 3 on switches

HSRP and Spanning-Tree are two very different things.

Sppanning Tree within a broadcast domain prevents having multiple parallel paths that cause Broadcast Storms.

HSRP (or VRRP) is a protocol that provides multiple physical "gateways" to be represented as a single logical "gateway" (where one physical gateway is redundant to another ... not both active at the same time in the same broadcast domain).

With HSRP/VRRP when the designated primary gateway goes down, the traffic uses the secondary.

Where things *can* get complex is when you have multiple gateways, multiple vlans.

Given that you have two gateway routers, (X and Y), and two VLANS (A and B).

It is common to do some load balancing in this scenario by making Router X the primary and Router Y as the secondary for VLAN A, while Router Y is the primary and Router X is the secondary for VLAN B.

If the outbound path from Router X fails, all traffic uses Router Y, if the outbound path from Y fails, all traffic uses Router X.

If both paths are up and functional, then the traffic is split between the two gateways (according to how you have the hosts "Default Gateway" set).

Spanning-Tree is strictly for loop prevention, there is only one root bridge.

Good Luck


New Member

Re: layer 3 on switches

yes I understand they are 2 different things, but thansk anyway, So I know best practice would be to have each switch the secondary for the other vlan, would I implement hsrp as I said ?

New Member

Re: layer 3 on switches

Hi Carl, further to ur discussion with all. you can implement HSRP with Spanning Tree Mode like PVSt+/MST & can load balance across ur main siwtches. Like If u hv two Core/distribution switches & multiple vlans on these switches then u can configure odd vlans as active on one core & even vlans active on another core with the help of HSRP, here I m talking abt load balancing with HSRP. Now with MST/PVST+ u can define CORE-1 as a root bridge for vlan 1 where odd vlans r active & can define CORE-2 as a root bridge for vlan 2 where even vlans r active. So It wud provide u proper load balancing as well as protection from Loop.

I hope you understand. If not please do not hesitate to ask again.


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