Message Age And Hop Count - Cisco 7604 Configuration Manual

Understanding How Prestandard IEEE 802.1s MST Works
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. To display the information about the IST
master, path cost, and remaining hops for the bridge, enter the show spanning-tree mst command.
Edge Ports
An edge port is a port that is a port that is connected to a nonbridging device (for example, a host or a
router). A port that connects to a hub is also an edge port if the hub or any LAN that is connected by it
does not have a bridge. An edge port can start forwarding as soon as the link is up.
MST requires that you configure all ports for each host or router. 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 routers as edge ports while using MST.
To prevent a misconfiguration, the PortFast operation is turned off if the port receives a BPDU. To
display the configured and operational status of PortFast, enter 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 router. 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 process that is very similar to the IP TTL process. 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.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
20-20
Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST
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