Cisco Systems DOC-7814982 Topo l ogy C h an ges

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Catalyst 2950 Desktop Switch Software Configuration Guide

78-14982-01

Chapter 12 Configuring RSTP and MSTP

Unders tan din g RSTP

The RSTP does not have a separate topol ogy chan ge notificati on (TCN) BPDU . It uses the topology

change (T C) flag to sho w the to pology cha nges. Ho wev er , f or interoper ability with 802 .1D switches, th e

RSTP switch proc esses and g enerates TCN B PDUs.

The learning and forwarding flags are set according to the state of the sending port.

If a p ort recei v es superi or root informati on (lo wer bri dge ID, lo wer pa th cost, and so fo rth) tha n currently

stored for the port, the RSTP triggers a reconfiguration. If the port is proposed and is selected as the new

root po rt, RST P f orce s a ll t he ot her po rts to sy nc hro nize.

If the BPDU received is an RSTP BPDU with the proposal flag set, the switch sends an agreement

message after all of the other ports are synchronized. If the BPDU is an 802.1D BPDU, the switch does

not s et th e pr opo sal flag and sta rts the forw ard -de lay t ime r for th e po rt . Th e new r oot por t re qu ires t wice

the forward-delay time to transition to the forwarding state.

If the superior information received on the port causes the port to become a backup or alternate port,

RSTP sets the por t to t he b l ocking stat e but do es not send t he a gree ment messa ge. Th e de signa ted por t

continues send ing BPDUs with the pro posal flag set until t he forw ard-delay timer expires, a t which time

the port transitions to the forwarding state.

If a d esign at ed por t re ceives an i nfe rior B PDU (hi gher b ridge ID, high er pa th co st, a nd so fort h tha n

currently stored for the po rt) with a desig nated port rol e, it immediately replies with its o wn information.

Topo l ogy C h an ges

This sect ion descri bes the di ffer ences bet ween the RSTP an d the 802.1D in handl ing spanning -tree

topology c hang es.

•Detection—Unl ike 802.1D in wh ich any tra nsition be tween the blocking an d the fo rwarding sta te

causes a t opology cha nge, only transitions fr om the bl ocki n g to the forwa rdi ng state cause a

topolo gy change w ith RSTP (onl y an incre ase in connect ivity is consid ered a topology ch ange ).

State c hange s on an e dge por t do no t c ause a topo lo gy cha ng e. W hen a n RSTP swi tch detec ts a

topolo gy cha ng e, it f lushe s th e learn ed inf orm atio n on a ll of i ts n one dg e por ts .

•Notification—Unli k e 8 0 2.1 D, wh ic h us es TCN B PDUs, the RSTP d oes not u se th em . However, fo r

802.1D interoperability, an RSTP switch processes and generates TCN BPDUs.

•Acknowledgement—When an R STP switc h rece ives a TCN m essa ge o n a desig nat ed po rt from a n

802.1D switch , it replies with an 802.1D conf i guration BPDU with the TCA bi t set. Ho wever , if the

TC-while timer (the same as the topology-change timer in 802.1D) is active on a root port connected

to an 8 02.1D swit ch an d a c onf igura tion BPDU with t he TCA b it set is rece i v ed, th e TC- while tim er

is reset.

This behavior i s only re quir ed to suppor t 802.1 D swit ches. The RS TP BPDU s never have the TCA

bit set.