STP Question

Unanswered Question
May 27th, 2008

Quick question about STP. In an environment where you have two core switches running a dozen VLANS or so with HSRP, where core1 is primary for some vlans and core2 for others, and all other switches/routers are plugged into the core with Layer3 interfaces...does it make sense to keep the stp root together with the the primary hsrp vlans (For example core2 is primary for vlan 15, stp root for vlan 15 is also core2), or just keep one core as root for all vlans? Does it even make a difference in an environment like that? Assuming both cores are the same spec hardware.

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Overall Rating: 5 (2 ratings)
Mohamed Anees M... Tue, 05/27/2008 - 13:56

If you are not going to expand this VLANs on the other switches, you don't need to worry about placing root vlans and HSRP active on the root vlan, etc..

My question is why do you need HSRP if only these two switches are going to have the vlans. Tue, 05/27/2008 - 15:18

There are servers plugged into those vlans...the idea is to have two network connections to each server, one to each switch. The reality is that there is one network connection, but they're somwhat split between the two switches. :)

Edison Ortiz Tue, 05/27/2008 - 20:19

does it make sense to keep the stp root together with the the primary hsrp vlans

Yes, having L2 and L3 flow via the same path is ideal.



wilson_1234_2 Wed, 05/28/2008 - 18:07


Can you explain this?

The layer 2 flow could take a different path than the layer 3, how would that happen?

Or, what could possibly be the flows for the different paths of layer 2 and 3?

Edison Ortiz Wed, 05/28/2008 - 18:53

Rather simple:

Dual Core implementation. Core A STP Root of Vlan 2 and Core B STP Root of Vlan 3.

From leaf switches connecting to both cores, any Layer2 forwarding will go to Core A for Vlan 2 as the other port connecting to Core B will be in blocked state (STP normal operation).

However, if Core A is the HSRP active router for Vlan 3, the Layer2 flows will go to Core B (STP operation) but Layer3 flow will go to the active HSRP router.

wilson_1234_2 Wed, 05/28/2008 - 19:08

Sorry, but,

I guess I don't see how it gets seperated,

Is there a document on the layer 2 trffic flows you are talking about


Edison Ortiz Wed, 05/28/2008 - 19:55

I never meant they were separated but they don't exactly follow the same path.

As I explained before on my example, the HSRP active router is not the STP root so to reach the VIP, you need to hop an extra device due to the STP root placement.

If you were to do the design correctly, the STP Root would match the Active HSRP router and with this design you would avoid the extra hop due to STP.


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