13-7
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 13 Configuring Optional Spanning-Tree Fea tures Understanding Optional Spanning-Tree Features
The switch sending the fast-transition request needs to do a fast transition to the forwarding state of a
port that it has chosen as the root port, and it must obtain an acknowledgement from each stack switch
before perf orm ing the fast tra nsitio n.
Each swit ch in the stack d etermines if the sen ding switch is a better choice th an itself to be th e stack root
of this spanning-tree instance by comparing the root, cost, and bridge ID. If the sending switch is the
best choi ce as the s tack r oot , e ach swi tch in the st ack ret urn s an ac kn owledgeme nt; othe rwis e, it do es
not respon d to t he s end ing switc h (drops the p acket). The send ing s wit ch th en ha s not rec eived
acknowledgements from all stack switches.
When ackn owledgements are received from all stack sw itches, the Fast Uplink Transition Protocol on
the sending switch immediately transitions its alternate stack-root port to the forwarding state. If
acknowledgements from all stack switches are not obtained by the sending switch, the normal
spannin g-tr ee t ransition s ( bloc king, li ste ning, le ar ning , an d f orward ing) ta ke pla ce, and t he
spannin g-tr ee top ology c onverges a t its norm al rate (2 * forward-d elay tim e + max-ag e ti me).
The Fast Uplink Transition Protocol is implemented on a per-VLAN basis and affects only one
spanning-tree instance at a time.

Events That Cause Fast Convergence

Depen ding on the ne twor k event or fai lure , t he CSU F fa st c onvergence migh t or mi gh t n ot oc cur.
Fast convergence (less than 1 second und er norm al network c ondition s) occu rs under th ese
circumstances:
•The stac k-roo t po rt l ink fail s.
If two switches in the stack have alternate paths to the root, only one of the switches performs the
fast transition.
•The failed li nk, w hic h co nnec ts t he sta ck r oot to the s pan ning -t ree ro ot, re covers.
•A network recon figuration ca uses a new stack-roo t switch to be selected .
•A network recon figuration ca uses a new port on the cu rrent stac k-root sw itch to be chose n as the
stack- root po rt.
Note The fast transition might not occur if multiple events occur simultaneously. For example, if a stack
member switch is powered off, and at the same time, the link connecting the stack root to the
spanning-tree root comes back up, the normal spanning-tree convergence occurs.
Normal spa nnin g-t ree c onvergence (3 0 to 40 se co nds) o cc ur s u nder th es e co ndi tions:
•The stack-root switch is powered off, or the software failed.
•The stac k-root switch, wh ich was powered off or failed, is powered on.
•A new switch, which might become the stack root, is added to the stack.
•A swit ch ot her tha n the st ack ro ot i s powered o ff or faile d.
•A link fails between stack ports on the multidrop backbone.