Cisco 6500 Series Software Configuration Manual page 207

Chapter 15 Configuring STP and IEEE 802.1s MST
An MST region appears as an SST or pseudobridge to STP running in the SST region. Pseudobridges
operate as follows:
• The same values for root identifiers and root path costs are sent in all BPDUs of all the pseudobridge
ports. Pseudobridges differ from a single SST bridge as follows:
–
–
Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port follows
•
a path entirely contained within the pseudobridge or MST region.
Data traffic belonging to different VLANs may follow different paths within the MST regions
•
established by MST.
Loop prevention is achieved by either of the following:
•
–
–
A pseudobridge differs from a single SST bridge because the BPDUs sent from the pseudobridge's
•
ports have different bridge identifiers. The root identifier and root cost are the same for both bridges.
These guidelines apply in a topology where you configure MST switches (all in the same region) to
interact with PVST+ switches:
Configure the root for all VLANs inside the MST region as shown in this example:
•
Router# show spanning-tree mst interface gigabitethernet 1/1
GigabitEthernet1/1 of MST00 is root forwarding
Edge port: no
Link type: point-to-point (auto)
Boundary : boundary
Bpdus sent 10, received 310
Instance Role Sts Cost
-------- ---- --- --------- -------- -------------------------------
0
3
The ports that belong to the MST switch at the boundary simulate PVST+ and send PVST+ BPDUs
for all the VLANs.
If you enable loop guard on the PVST+ switches, the ports might change to a loop-inconsistent state
when the MST switches change their configuration. To correct the loop-inconsistent state, you must
disable and reenable loop guard on that PVST+ switch.
Do not locate the root for some or all of the VLANs inside the PVST+ side of the MST switch
•
because when the MST switch at the boundary receives PVST+ BPDUs for all or some of the
VLANs on its designated ports, root guard sets the port to the blocking state. Do not designate
switches with a slower CPU running PVST+ as a switch running MST.
78-14099-04
The pseudobridge BPDUs have different bridge identifiers. This difference does not affect STP
operation in the neighboring SST regions because the root identifier and root cost are the same.
BPDUs sent from the pseudobridge ports may have significantly different message ages.
Because the message age increases by 1 second for each hop, the difference in the message age
is in the order of seconds.
Blocking the appropriate pseudobridge ports by allowing one forwarding port on the boundary
and blocking all other ports.
Setting the CST partitions to block the ports of the SST regions.
(trunk)
(PVST)
Root FWD 20000
Boun FWD 20000
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
port guard : none
bpdu filter: disable
bpdu guard : disable
Prio.Nbr Vlans mapped
128.1 1-2,4-2999,4000-4094
128.1 3,3000-3999
Understanding How IEEE 802.1s MST Works
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