Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring MSTP (Rev. 03)
Overview
© 2008 Foundry Networks, Inc
Page 9 of 70
Path Cost
MSTP uses path cost when selecting an interface to put in the forwarding state if a loop occurs.
The MSTP path cost default value for single-link ports is derived from the link speed of an
interface. For link aggregations (LACP and Static AGs), the default path-cost is derived as half the
default path-cost of any of its members. When calculating the default path-cost for link
aggregations, the count of the members that the aggregation has is irrelevant. You can assign lower
cost values that you want selected first and higher cost values to interfaces that you want selected
last. If all interfaces have the same cost value, MSTP puts the interface with the lowest interface
number in the forwarding state and blocks the other interfaces.
Interoperability with 802.1D STP
A switch running both MSTP and RSTP supports a built-in protocol migration mechanism that
enables it to interoperate with legacy 802.1D switches. If this switch receives a legacy 802.1D
configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on
that port. An MST switch can also detect that a port is at the boundary of a region when it receives
a legacy BPDU, an MST BPDU (version 3) associated with a different region, or an RST BPDU
(version 2). However, the switch does not automatically revert to the MSTP mode if it no longer
receives 802.1D BPDUs because it cannot determine whether the legacy switch has been removed
from the link unless the legacy switch is the designated switch. Also, a switch might continue to
assign a boundary role to a port when the switch to which it is connected has joined the region.
If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they
are RSTP BPDUs. Therefore, MSTP switches send either a version 0 configuration and TCN
BPDUs or version 3 MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the
designated switch of which is either a single spanning-tree switch or a switch with a different MST
configuration.
Fast Ring Mode
The MSTP fast ring mode shortens the MSTP convergence time in case of a disconnection when a
ring topology is used. Using this mode of operation for any other topology is strongly disapproved.
Foundry Networks offers two Fast Ring solutions, MSTP Fast Ring and Interoperability Fast Ring:
1.
Fast Ring – Can be used when all the devices in the ring are NetIron M2404F switches and all
of them can be configured with MSTP enable.
2.
Interoperability Fast ring – Can be used when the router of the ring does not support xSTP.
NOTE
It is possible to use standard MSTP as a ring solution, if your network demands a
topology different from the one offered here.
Fast Ring
Fast Ring should be used when all of the devices in the ring are NetIron M2404F switches. Using
Fast Ring decreases the convergence time in case a disconnection in the ring occurs. To use Fast
Ring you must select one bridge to be the root bridge (by setting its bridge priority to the lowest
value - 8192).