STP redundancy

[beckerola]

Hi

I have a doubt with a particular scenario.Imagine we have 2 central Switches that act as STP root for several other switches in the network. These two Catalyst have 4 supervisors for redundancy matters, which connect to each other using their 4 Gig ports (1/1 with 1/1, 2/1 with 2/1 and son on). These four fibres form a port channel so we have one 4 Gig link.

There are some other Gig cards that connect with the rest of Switches in the network.

My doubt is, if we extract one of the supervisors (which is connected with the other supervisor), what will STP do? Will STP reconfigure itself creating a short outage until it recuperates?, or on the other hand and being one single link, nothing will happen (only half of the bandwidth used)?

[mark.edwards]

Hi, As you are using etherchannel there is only 1 interface as far as STP is confirmed. You should be able to remove interfaces from the bundle without affecting spanning tree.

[Francois Tallet]

It's true that STP only sees the etherchannel as a single interface. I just want to stress a little difference between CatOS and IOS in this regard: IOS adapts dynamically the stp cost of an interface based on its available bandwidth. That means that if a link is pulled from an etherchannel in IOS, its cost will increase and that could result in some changes in your stp topology. CatOS does not work this way and keeps its cost steady when a link is going down in an etherchannel.

Both approach have their pros and cons. Some prefer that the cost really reflect the actually bandwidth of the network while some other consider that an important role of the etherchannel is to keep the stp topology stable even in case of link failure.

Hopefully, you can have both behavior in IOS. Even if the default is to recalculate the cost dynamically, you can set it to a fixed value (by simply configuring an STP cost on the etherchannel interface) that will not change depending on number of active links in the bundle.

Regards,

Francois

[beckerola]

Thanks for your replies.