Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring MSTP (Rev. 03)
Overview
© 2008 Foundry Networks, Inc
Page 8 of 70
Regional Root
The root bridge of each MSTI in a region is referred to as the MSTI’s regional root. In the case of
the IST (MSTI0), it is referred to as the CIST Regional root. Therefore, the terms “IST Master”
and “CIST Regional root” are interchangeable.
Edge Ports
A port that is connected to a non-bridging 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.
MSTP 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 MSTP.
Link Type
Rapid connectivity is established only on point-to-point links. If you connect a port to another port
through a point-to-point link and the local port becomes a designated port, the RSTP negotiates a
rapid transition with the other port by using the proposal-agreement handshake to ensure a loop-
free topology. By default, the link type is automatically determined by the duplex state of the port.
If you have a half-duplex link physically connected point-to-point to a single port on a remote
switch running RSTP, you can override the default setting of the link type and enable rapid
transitions to the forwarding state.
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 non-root 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 region’s designated ports at the boundary propagate the same
values.
Port Priority
MSTP uses the port priority when selecting an interface to put into the forwarding state if a loop
occurs. To interfaces that you want to be selected first, you can assign higher priorities, and to
interfaces that you want selected last you can assign lower priorities. A higher priority is specified
by a lower numerical value and a lower priority is specified by a higher numerical value. If all
interfaces have the same priority, MSTP puts the interface with the lowest interface number in the
forwarding state and blocks the other interfaces.