Message Age And Hop Count - Cisco 4500M Software Manual

Chapter 16 Understanding and Configuring Multiple Spanning Trees
IST Master
The IST master of an MST region is the bridge with the lowest bridge identifier and the least path cost
to the CST root. If an MST bridge is the root bridge for CST, then it is the IST master of that MST region.
If the CST root is outside the MST region, then one of the MST bridges at the boundary is selected as
the IST master. Other bridges on the boundary that belong to the same region eventually block the
boundary ports that lead to the root.
If two or more bridges at the boundary of a region have an identical path to the root, you can set a slightly
lower bridge priority to make a specific bridge the IST master.
The root path cost and message age inside a region stay constant, but the IST path cost is incremented
and the IST remaining hops are decremented at each hop. Enter the show spanning-tree mst command
to display the information about the IST master, path cost, and remaining hops for the bridge.
Edge Ports
A port that is connected to a nonbridging device (for example, a host or a switch) is an edge port. A port
that connects to a hub is also an edge port if the hub or any LAN that is connected to it does not have a
bridge. An edge port can start forwarding as soon as the link is up.
MST requires that you configure each port connected to a host. To establish rapid connectivity after a
failure, you need to block the non-edge designated ports of an intermediate bridge. If the port connects
to another bridge that can send back an agreement, then the port starts forwarding immediately.
Otherwise, the port needs twice the forward delay time to start forwarding again. You must explicitly
configure the ports that are connected to the hosts and switches as edge ports while using 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, and you can avoid explicit
configuration by treating all full-duplex links as point-to-point links by entering the spanning-tree
linktype command.

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.
OL-6696-01
Software Configuration Guide—Release 12.2(25)EW
Overview of MST
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