PVST+, PVST, CST Questions

Unanswered Question
Jun 6th, 2007

correct me if im wrong PVST+ cisco proprietary creates multiple ST(spanning tree), does it means im going to trunk each and every vlan i had created to the router to have a routing,, in a router on a stick mode, CST(Common spanning tree), does it only creates one ST on only a single trunk, how im going to use this CST for redundancy -, Kindly help me on this, im lookinh forward on applying this on our new site, how it is being applied, in terms of redundancy, specially on a router on a stick mode here is my setup .. router >> switch(with 3 vlans)>> departments(users).

I have this problem too.
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ivillagomez Wed, 06/06/2007 - 09:49

In router on a stick, in your router you will have to create as many subinterfaces as vlan you have , each vlan will have in this case a diff subnet. And to respond your question, yes, each subinterface need to be in trunk mode.

Another approach is to create vlan interfaces on your L3 Switch (The switch will do the routing) and on the router only assign and ip address to the fastethernet interface, this way the default gateway of your LAN will be the MLS and the default gateway of your MLS will be the Router.

About PVST, PVST+, MST and CST each of them determine (among other things) the number of STP instances running in your network, ie PVST will create an instance for each vlan. I think you are confusin STP with trunking, vlans and routing.

Hope this be of help.

cool_mhar04 Wed, 06/06/2007 - 17:00

Yha, im gettin confused on STP, and VTP. Correct me again if im wrong again STP does to avoid loops on redundant trunk, VTP-does the trunking, carries vlan identity to others and to make ease of vlans creations and port membership. My case right now, I only have one routing entity (2621) to my 3 switches(2950 catalyst sw), trunked together and it works fine, and it was also trunked going to the router. On a router it is connected thru its fastethernet0/0(encapsulated with dot1q), i also created 2 subinterface fa0/0.2, fa0/0.3, fa0/0.4 it was also encapsulated, and given an IP address with different Network, that will be associated with my vlan created, my vlans and subinterfaces are in the same network accordingly. But i was not able to ping each other?

Amit Singh Wed, 06/06/2007 - 09:56

Hello,

In both the cases i.e for PVST/PVST+ or CST, you have to create sub-interfaces for all the vlans on your router. With PVST/PVST+ each vlan will have its own roout bridge at layer 2 and we can confiure the STP load-balancing in this case. So if you have a 2 layer 3 switches in your core or distribution layer, we can use HSRP and PVST to do load-balancing both at Layer2 and Layer3.

On the other hand, CST will not give your much benefit. Your over all topology will be a flat network in terma of STP as all the vlans will have a single STP instance only. You cannot achieve load-balancing as in ealrier case. Moreover STP calculation in one vlan will efect thw whole CST domain.

You have to carry all the vlans on the trunk ports for which you want the intervlan routing on your router.

HTH,Please rate if it does.

-amit singh

cool_mhar04 Wed, 06/06/2007 - 17:13

Hi amit sigh;

on my case above that i have only one routing entity, and got 3 switches trunk together and they works fine, and to the router too, can i be able to use redundancy on this set-up? am i going to create one physical trunk going to the router? Do i have to define routing on those subnets? because right now, they still dont ping each other without the redundant path?

devang_etcom Wed, 06/06/2007 - 11:58

hi,

In short and sweet terms PVST, PVST+ or multiple ST used to utilize your network resources properly and you can achieve the loadbalancing kind of stuff in your network.

As in HSRP you creat the multiple group for different VLAN/SUBNET to make your one router active for the perticular VLAN and other router will be active for any other VLAN. Same thing is there in PVST or PVST+ is you can have different RootBridge, Root Port, Designated Port... etc for each and every VLAN. where as in CST you are having the single Switch act as RootBridge for all vlan.

regards

Devang

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