Link Type - Cisco Catalyst 4500 series Administration Manual

About MST
To prevent a misconfiguration, the PortFast operation is turned off if the port receives a BPDU. You can
display the configured and operational status of PortFast by using the show spanning-tree mst interface
command.

Link Type

Rapid connectivity is established only on point-to-point links. You must configure ports explicitly to a
host or switch. However, cabling in most networks meets this requirement. By entering the
spanning-tree linktype command to treating all full-duplex links as point-to-point links, you can avoid
explicit configuration.
Message Age and Hop Count
IST and MST instances do not use the message age and maximum age timer settings in the BPDU. IST
and MST use a separate hop count mechanism that is very similar to the IP time-to live (TTL)
mechanism. You can configure each MST bridge with a maximum hop count. The root bridge of the
instance sends a BPDU (or M-record) with the remaining hop count that is equal to the maximum hop
count. When a bridge receives a BPDU (or M-record), it decrements the received remaining hop count
by one. The bridge discards the BPDU (M-record) and ages out the information held for the port if the
count reaches zero after decrementing. The nonroot bridges propagate the decremented count as the
remaining hop count in the BPDUs (M-records) they generate.
The message age and maximum age timer settings in the RST portion of the BPDU remain the same
throughout the region, and the same values are propagated by the region's designated ports at the
boundary.
MST-to-PVST+ Interoperability
Note these guidelines when you configure MST switches (in the same region) to interact with PVST+
switches:
•
Software Configuration Guide—Release IOS XE 3.5.0SG and IOS 15.2(2)SG
22-28
Configure the root for all VLANs inside the MST region as shown in this example:
Switch# 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 Root FWD 20000
3 Boun FWD 20000
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 renewable loop guard on that PVST+ switch.
(trunk) port guard : none
bpdu filter: disable
(PVST) bpdu guard : disable
Prio.Nbr Vlans mapped
128.1 1-2,4-2999,4000-4094
128.1 3,3000-3999
Chapter 22 Configuring STP and MST
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