Chapter 13 Spanning Tree Protocol
GS2210 Series User’s Guide
122
The Swit ch uses I EEE 802.1w RSTP ( Rapid Spanning Tr ee Pr ot ocol) t hat allow s fast er conver gence
of t he spanning t r ee t han STP ( w hile also being backwar ds com pat ible w it h STP- only awar e
br idges) . I n RSTP, t opology change infor m at ion is dir ect ly pr opagat ed t hr oughout t he net w or k fr om
t he device t hat generat es t he t opology change. I n STP, a longer delay is r equir ed as t he device t hat
causes a t opology change fir st not ifies t he root bridge t hat t hen not ifies t he net w or k. Bot h RSTP
and STP flush unwant ed lear ned addr esses fr om t he filt er ing dat abase. I n RSTP, t he por t st at es ar e
Discar ding, Lear ning, and For war ding.
Not e: I n t his user ’s guide, “ STP” r efer s t o bot h STP and RSTP.
STP Terminology
The r oot br idge is t he base of t he spanning t r ee.
Pat h cost is t he cost of t ransm it t ing a fram e ont o a LAN t hr ough t hat por t . The r ecom m ended cost
is assigned accor ding t o t he speed of t he link t o w hich a por t is at t ached. The slow er t he m edia, t he
higher t he cost .
On each br idge, t he r oot por t is t he por t t hr ough w hich t his br idge com m unicat es w it h t he r oot . I t is
t he por t on t his sw it ch w it h t he low est pat h cost t o t he r oot ( t he r oot pat h cost ) . I f t her e is no r oot
por t , t hen t his sw it ch has been accept ed as t he r oot bridge of t he spanning t r ee net w or k.
For each LAN segm ent , a designat ed br idge is select ed. This br idge has t he low est cost t o t he r oot
am ong t he br idges connect ed t o t he LAN.
How STP Works
Aft er a br idge det er m ines t he low est cost - spanning t r ee w it h STP, it enables t he r oot por t and t he
port s t hat are t he designat ed port s for connect ed LANs, and disables all ot her por t s t hat par t icipat e
in STP. Net w or k packet s ar e t her efor e only forwar ded bet w een enabled port s, elim inat ing any
possible net w or k loops.
STP- awar e sw it ches exchange Bridge Prot ocol Dat a Unit s ( BPDUs) per iodically. When t he br idged
LAN t opology changes, a new spanning t r ee is const r uct ed.
Once a st able net w or k t opology has been est ablished, all br idges list en for Hello BPDUs ( Br idge
Pr ot ocol Dat a Unit s) t ransm it t ed fr om t he r oot br idge. I f a br idge does not get a Hello BPDU aft er a
pr edefined int er val ( Max Age) , t he br idge assum es t hat t he link t o t he r oot br idge is dow n. This
br idge t hen init iat es negot iat ions w it h ot her bridges t o r econfigur e t he net w ork t o r e- est ablish a
valid net w or k t opology.
Table 46
STP Pat h Cost s
LINK SPEED
RECOMMENDED VALUE
RECOMMENDED RANGE
ALLOWED RANGE
Pat h Cost
4Mbps
250
100 t o 1000
1 t o 65535
Pat h Cost
10Mbps
100
50 t o 600
1 t o 65535
Pat h Cost
16Mbps
62
40 t o 400
1 t o 65535
Pat h Cost
100Mbps
19
10 t o 60
1 t o 65535
Pat h Cost
1Gbps
4
3 t o 10
1 t o 65535
Pat h Cost
10Gbps
2
1 t o 5
1 t o 65535
