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spanning tree instance is associated with one or more VLANs, which must be explicitly identified as part of the configuration. It is good practice to associate VLANs with the same topology to a single instance of spanning tree. This reduces processor overhead on the GbESM and other switches in the network and it reduces the traffic generated by the BPDUs that the switches send to each other.
In general, MSTP configuration mirrors the configuration of multiple instances of classic 802.1D spanning tree which is described in 7.7.2, “Common Spanning Tree configuration - IEEE 802.1D and PVST” on page 84, except as follows:
When multiple 802.1D instances are used to interoperate with Cisco PVST+, there is typically only one VLAN associated with each Spanning Tree Group (STG). When MSTP is used, both the GbESM and any upstream switches from any vendor must explicitly identify the VLANs associated with each MSTP instance.
MSTP requires the configuration of a region name and revision number which must match for all switches if they are to recognize each other as part of the same MSTP region. In almost all cases, GbESMs should be in the same MSTP region as their upstream neighbors.
MSTP can recognize and interoperate with switches running 802.1D (classic spanning tree) or 802.1w (rapid spanning tree). The commands under /c/l2/mrst/cist define the parameters for this interoperability. These commands also govern the interactions of boundary switches which are neighbors but are part of different MSTP regions.
The MST functionality is enabled on the GbESM by the following commands:
/c/l2/mrst/on | Enables 802.1w or 802.1s (rapid and multiple |
| spanning tree) functionality |
/c/l2/mrst/mode mstp | Selects multiple spanning tree (802.1s) instead of |
| single rapid spanning tree (802.1w) |
/c/l2/mrst/cist | Menu of commands to configure the common |
| spanning tree defined by 802.1s, which is equivalent to |
| spanning tree group 0 (zero). |
/c/l2/stg <group number> | Menu of commands to configure additional instances |
| ofMSTP.Thisincludescommandstospecifybridge |
| parameters, port parameters, and to identify the |
| VLANs associated with this spanning tree group. |
/c/l2/stg <group number/onoff | Enables or disables a spanning tree instance. |
Key configuration commands to verify MSTP
On the GbESM, the /info/l2/stg command shows the status of the MSTP instances when MSTP is in use. It presents output as shown below. Note that the common internal spanning tree is instance 0 (zero) and is used to communicate outside of the MSTP region. Its details are obtained by the /info/l2/cist command.
Example 7-17 MSTP Information Display - GbESM
>> Main# /info/l2/stg
Spanning Tree Group 1: On (MSTP)
VLANs: 10 30
Current Root: | Port | Aging | |
1000 00:12:7f:ea:76:00 | 20000 | EXT1 | 300 |
Parameters: Priority Aging |
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102Nortel Networks L2/3 Ethernet Switch Module for IBM Eserver BladeCenter