MSTP instance 0 and 1

yoram12345 · ‎03-07-2009

Hi

I have a question regarding a topology of 3 switches :

1. When i configure cisco switch 3750 IOS(12.2) to work with MSTP instance 0 , is it mean that i work RSTP ? or MSTP ?

2.Why i needed to define portfast for fast convergence to the Host ports for Instance 0 (otherwise convegence time was 30 sec)

but when i configured the switch instance 1 i did not have to do so.

The convegence time was less than 1 sec ?

Giuseppe Larosa · ‎03-08-2009

Hello Yoram,

1) MST 802.1s supports multiple RSTP 802.1W instances

2) In RSTP it is very important to define the edge ports (with spanning-tree portfast) in order to improve convergence time: each edge port is excluded from the handshake and sync process it is not an optional part.

see

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#edge

3)

Actually MST doesn't send out distinct BPDUs for each instance: a single set of BPDUs is sent out with sections about other instances.

see

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_44_se/configuration/guide/swmstp.html#wp1039828

Hope to help

Giuseppe

yoram12345 · ‎03-08-2009

hi Giuseppe,

Thanks for the info.

Actually i would like to test interop between cisco MSTP to other switch RSTP.

So teh question is if i should define the vlan on CISCO on instance 0 or different than 0 ?

Or it does not matter.

Yoram

Giuseppe Larosa · ‎03-08-2009

Hello Yoram,

I report here a link to a thread with an answer from Francois Tallet one of greatest experts in STP in the forums :

see

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40^1%40%40.1dde86cf/5#selected_message

It should be fine with instance 0, and probably also with instance 1

Hope to help

Giuseppe