Message Age And Hop Count - Cisco WS-C6506 Software Manual

Chapter 7 Configuring Spanning Tree
Edge Ports
A port that is connected to a nonbridging device (for example, a host or a router) 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 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 nonedge 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 configure a port as an edge port, you enable PortFast on that port. When you enter the show spantree
Note
portfast mod/port command, if the designation for a port is displayed as edge, that port is also a PortFast
port. For more information, see
BackboneFast, and Loop Guard."
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 spantree mst mod/port
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 set spantree mst
link-type 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 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 that is 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) that 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-8978-04
Chapter 9, "Configuring Spanning-Tree PortFast, UplinkFast,
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
Understanding How Multiple Spanning Tree Works
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