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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How Multiple Spanning Tree Works
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.
Note
To configure a port as an edge port, you enable PortFast on that port. When you enter the
show spantree
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
Chapter 9, “Configuring Spanning-Tree PortFast, UplinkFast,
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.
