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New Member

Spanning Tree Help

Hello all,

I am running Rapid-PVST+ throughout my switch fabric design. I have the switches set up in a test lab right now, with two distribution switches and four access switches (meshed).

When all the links are up, the paths the traffic takes are fine. However, when I take down one of the links....the switches choose an alternate path that I do not want. I attached a JPEG of what I want to happen. Unfortunately, what I want is not occurring.

If the the link between an access switch and the primary distribution switch goes down, I want the secondary distribution switch to take over handling the traffic. Instead, traffic goes to the secondary distribution switch (which is what it is supposed to do)....but that switch forwards all the traffic over to a different access switch it is not intended for. It is almost as if this different access switch is acting as the distribution switch. My guess is because this access switch is connected to the lowest port number of the distribution switch.

You will see in the JPEG what I want to happen if the link to the primary switch goes down. What is the STP configuration parameters I need to make on the secondary distribution switch that will enable it to "take over" if a direct link link from an access switch to the primary distribution switch goes down?

Here is how I have the secondary distribution switch STP currently configured...

spanning-tree mode rapid-pvst

spanning-tree portfast bpduguard default

spanning-tree extend system-id

spanning-tree vlan 31,99 priority 28672

spanning-tree vlan 31,99 forward-time 9

spanning-tree vlan 31,99 max-age 12

And here is how the primary distribution switch STP is configured, in case you need to see....

spanning-tree mode rapid-pvst

spanning-tree portfast bpduguard default

spanning-tree extend system-id

spanning-tree vlan 31,99 priority 24576

spanning-tree vlan 31,99 forward-time 9

spanning-tree vlan 31,99 max-age 12

I used the "primary" and "secondary" vlan commands for the two switches.

7 REPLIES
Blue

Re: Spanning Tree Help

you should be able to set STP Port Costs for specific ports to allow for a predictable path selection to occur as you would like.

the path selection chooses the next path by the lowest cost to the root. if you adjust the STP Port Cost for specific ports, carefully planned and drawn out to be sure you know the impact, you should be able to manipulate the path selection process.

please see the following link for more STP Port Cost info:

http://www.cisco.com/en/US/products/hw/switches/ps663/products_configuration_guide_chapter09186a00800e47e5.html#1043106

New Member

Re: Spanning Tree Help

Thats the thing, I am not exactly sure how I need to structure the port costs.

Here is how my secondary distribution switch looks like right now...

Gi0/43 Desg BLK 4 128.43

Gi0/44 Desg BLK 4 128.44

Gi0/45 Desg BLK 4 128.45

Gi0/46 Desg BLK 4 128.46

Gi0/47 Root FWD 4 128.47

Port 47 is the distribution interconnect to the primary distribution switch (primary root bridge). Port numbers 43 to 46 are links to various access switches.

Here is what happens when I remove that interconnect link on port 47...

Gi0/43 Root FWD 4 128.43

Gi0/44 Altn BLK 4 128.44

Gi0/45 Altn BLK 4 128.45

Gi0/46 Altn BLK 4 128.46

As you can see, traffic for this VLAN is now forwarded to the access switch on port 43. I dont want the switch on port 43 having a root port, and all others blocked. I want this distribution switch to be able to forward traffic out through all of its ports...depending on the access switch it needs to go to. Exactly how I showed it in the original JPEG image.

This secondary distribution switch does take over as the root when the primary distribution switch goes down. However, when I remove a single link (such as the interconnect on port 47)...this distribution switch is sends everything out port 43.

Hall of Fame Super Bronze

Re: Spanning Tree Help

Dave,

As stated before, when the cost are equal, it will pick the interface with the lowest port number.

You can alter this selection by changing the port cost on the access switches, for instance lower G0/47 to port cost 2 and port cost 3 to the secondary distribution switch.

Re: Spanning Tree Help

Hi Dave,

Basically, you want the secondary root to become primary root as a result of the link primary-secondary failing. There is nothing in the STP that allows you to do so. However, I don't think this is a big deal in your case. Indeed, the secondary now has connectivity to the root through an access (which is generally not recommended). But your access switches still have direct connectivity to the root. Nothing changes for them. The port that used to block between the access and the secondary are now blocking on the side of the secondary instead of the access... not a big change either.

What is it exactly that is bothering you in that scenario?

Regards,

Francois

New Member

Re: Spanning Tree Help

Thanks guys, here is what is bothering me....

The secondary distribution switch has a direct connection to ALL of the access switches itself, as illustrated in the JPEG. However, it does NOT use those direct connections at all.

Instead, in the event of a link failure, the secondary distribution switch receives all frames - and forwards them to a specific access switch (which is the access switch on port 43 in my earlier posts). That access switch then forwards the frames back to the primary distribution switch (root). The primary distribution switch then forwards the frames to its destination.

It seems there are a lot of unnecessary hops there, when all the secondary distribution switch needs to do is just receive the frames from one port and forward it out to its destination. The path should be only one hop....but the way it is set up now - it can traverse a total of five hops. That is four uneccessary hops....not to mention the access switch on port 43 is going to get hit with much more overhead from taking on the additional load from the secondary distribution switch.

I take it I am forced to living with an access switch (port 43) that will get hit with distribution traffic whenever the interconnect (port 47) goes down?

Re: Spanning Tree Help

Hi again,

Again, in the intial scenario, without the link failure, the secondary switch was not using any of its links to the access switches. The blocked port was on the access switches side, but the result was the same.

You are hitting a big limitation of the spanning tree: traffic for a given vlan is using a *unique* tree. By definition, a tree is a graph where there is only a single path between any two nodes. Take your secondary root when there is this link failure to the root. Suppose there a link forwarding between the secondary and access switch A and, at the same time, there is a link forwarding between the secondary and switch B. This is what you want, the secondary is directly connecting to both A and B. The problem is that A and B also have a forwarding link to the root bridge. Because of that, there are two paths between the secondary and the root bridge: one through A and one through B. This is a bridging loop:-( So basically, STP will only allow one connection between the secondary and the root bridge through one access bridge, never two.

Only the path to the root bridge is optimal... The IEEE is currently working on a solution to this. The draft is 802.1aq (shortest path bridging). This will allow to do bridging a little bit the way routing works: always using the shortest path between two nodes. Then, your problem will have a solution. In the meantime, you need to rely on load balancing on a per-vlan basis.

Regards,

Francois

New Member

Re: Spanning Tree Help

Thanks ftallet. I was hoping for better, but oh well.

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