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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 12 Configuring RSTP and MSTP Interoperability with 802.1D STP

Interoperability with 802.1D STP

A switch running both MS TP and RSTP suppor ts a built-in p rot ocol migration mechanism that e nables
it to interoperate with legacy 802.1D switches. If this switch receives a legacy 802.1D configuration
BPDU (a BPD U with the protocol version set to 0 ), it sends only 802. 1D BPDUs on t hat port. An MST
switch can also d etect that a po rt is at the b oundar y of a r eg ion wh en i t rece i v es a l eg ac y BP DU, a n MST
BPDU (version 3) as soc iated with a di ffer ent region, or an RST BPDU (ve rsion 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives 802.1D
BPDUs becau se it cannot determi ne whether th e legacy switch has been removed from the link unless
the legacy switch is the designated switch. Also, a switch might continue to assign a boundary role to a
port when the switch to which this switch is connected has joined the region. To restart the protocol
migrati on proc e ss (for ce th e renegoti ation w it h nei gh bori ng sw itch es), y ou can use the clear
spann ing- tree de tected -p rotocol s privileged EX E C com ma nd .
If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they are
RSTP BPDUs. Th eref ore, MS TP switches send eithe r a version 0 configurat ion and TCN BPDUs or
version 3 MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the designated switch
of whic h is ei the r a sing le s pa nning -tre e sw it ch or a sw it ch w ith a different MST con figurati on.

Configuring RSTP and MSTP Fe atures

These sec tions de scri be how t o con figure b asic R STP an d MS TP fea ture s:
•Default RSTP and MSTP Configuratio n, page 12 -12
•RSTP and MST P C onfigu ratio n Gui d eline s, pa ge 1 2-1 2
•Specifyi ng the MST Region Con figuration and Enabling M STP, page 12 -13 (required)
•Configuring t he R oot Swit ch, page 1 2-14 (optional)
•Configuring a Se cond ar y R oot Swit ch, page 1 2-1 6 (optional)
•Configuring the Port Priority, page 12-17 (optional)
•Configuring the Path Cost, page 12-18 (optional)
•Configuring the Switch Priority, page 12-19 (optional)
•Configuring the Hello Time, page 12-19 (optional)
•Configuring the Forwar ding -Del ay Time, pag e 12-20 (o ptiona l)
•Configuring t he M axi mum- Ag ing Time, page 12-21 ( op tio nal)
•Configuring t he M axi mu m-Ho p C ount, pag e 12-21 (o ptiona l)
•Specifyi ng the Link Type to Ensure Rapi d Tr ansition s, page 12-22 (opt iona l)
•Restartin g the Protoc ol Mig ratio n Pr ocess, pag e 12-22 (o pti onal )