From: Scott M Vermillion (scott@it-ag.com)
Date: Thu Sep 13 2007 - 01:21:30 ART
OK all, I admit that I have always struggled with the "under-the-hood"
mechanics of MSTP. The concept of MSTP is perfectly digestible, but I've
always been a little murky on some details. Now that I own some 3560s, I'm
trying to iron it all out. Today I did the "Configuring MSTP" lab from
IEWB#1. Once the main lab was done, I set about changing and breaking
things. One thing that I did was to place SW1 into a domain named "SW1" and
SW2 into domain "SW2." I left SW3 and SW4 at default. Here is a passage
from the DocCD:
"The IST connects all the MSTP switches in a region. When the IST converges,
the root of the IST becomes the CIST regional root (called the IST master
before the implementation of the IEEE 802.1s standard) as shown in Figure
<http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12240se/scg/swm
stp.htm#wp1143584> 18-1. It is the switch within the region with the lowest
switch ID and path cost to the CIST root."
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12240se/scg/swms
tp.htm
What was bothering me about the above was the part about ".lowest switch ID
and path cost." I wondered how it came to be that those two things were
mutual. Just because you have the lowest switch ID, you somehow also have
the lowest path cost to the CIST root? Does path cost somehow incorporate
switch ID to ensure that's the case? My observations don't support that.
By dumb luck, my SW3 bridge ID is lower than that of SW4. From SW3's
perspective, it has two paths to the CIST root (SW1, which also by dumb luck
happens to have the lowest ID of all four switches). One path is via
another region ("SW2") and one path is via an "internal MSTP peer," for lack
of a more official term. Interestingly, it is SW4 that is the CIST regional
root (in spite of its having a higher bridge ID than SW3) and it is SW3 that
is the root for all other instances.
So am I being really thick here? Am I over-analyzing? It appears to me
that wrt the CIST regional root, lowest path cost wins out, regardless of
anything to do with bridge ID. Maybe they just meant lowest switch ID
amongst switches with an equal lowest path cost?
Also just BTW, you may or may not find the following of interest to the
discussion:
SW3#sh sp ms 0
##### MST0 vlans mapped: 10-4094
Bridge address 001c.f6ad.8b80 priority 32768 (32768 sysid 0)
Root address 001c.f6ad.7d80 priority 32768 (32768 sysid 0)
port Fa0/1 path cost 200000 <<<<<<<
Regional Root address 001c.f6b9.5000 priority 32768 (32768 sysid 0)
internal cost 200000 rem hops 19
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/1 Root FWD 200000 128.2 P2p
Fa0/2 Altn BLK 200000 128.3 P2p
Fa0/3 Altn BLK 200000 128.4 P2p
Fa0/4 Desg FWD 200000 128.5 P2p
Fa0/5 Desg FWD 200000 128.6 P2p
Fa0/6 Desg FWD 200000 128.7 P2p
SW4#(config)int ra fa0/1 - 3 <<<< (these ports connect SW3 and SW4 in my
topology, shutting them leaves SW3 with only a path via region "SW2" to the
CIST root in "SW1")
SW4(config-if-range)#shut
SW3#sh sp ms 0
##### MST0 vlans mapped: 10-4094
Bridge address 001c.f6ad.8b80 priority 32768 (32768 sysid 0)
Root address 001c.f6ad.7d80 priority 32768 (32768 sysid 0)
port Fa0/4 path cost 400000 <<<<<
Regional Root this switch
Operational hello time 2 , forward delay 15, max age 20, txholdcount 6
Configured hello time 2 , forward delay 15, max age 20, max hops 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- --------
--------------------------------
Fa0/4 Root FWD 200000 128.5 P2p Bound(RSTP)
Fa0/5 Altn BLK 200000 128.6 P2p Bound(RSTP)
Fa0/6 Altn BLK 200000 128.7 P2p Bound(RSTP)
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