Mstp Field; Mst Field Operation - Accton Technology ES4710BD User Manual

spanning tree instances (MSTI). It applies the fast converging properties, enabling multiple VLAN
of the same topology to map to one spanning tree instance, while that spanning tree topology is
independent of the other spanning tree instances. This mechanism provides an independent
transmitting path for VLAN dataflow mapping to multiple spanning tree instances. On the other
hand, several VLAN sharing one topology instance (MSTI) have substantial fewer spanning tree
instances maintained by each bridge compared to the one-VLAN-one-spanning-tree implementation,
therefore saving CPU resources and reducing non-communication bandwidth usage.

10.1.1 MSTP field

As Multiple VLANs can be mapped to a single Spanning Tree instance, the IEEE 802.1s
counsel proposed the concept of MST field to workaround the determination of the
VLAN-Spanning Tree Instance mapping issue.
A MSTP field consists of one or more bridges with identical MCIDs (MST Configuration
Identification) and a LAN connecting all these bridges, where one bridge is a specified bridge of
that LAN, with the bridges the LAN connects not running STP. All the bridges in the field maintain
the same MSTIs.
Bridges in each field have the following properties:
Configuration Name, consisting of alphanumeric characters.
Configuration revision level.
Configuration Digest of the VLAN in the bridges mapping to spanning tree instance.
The above three properties comprise the field MCID. Bridges are considered to belong to the
same MST field only if they are identical in these three properties.
In the CIST of the whole Bridged-LAN, MSTP regard the MST as a bridge, as shown in the
figure below:
As in the network in Fig 1-1, if the bridges in the network run STP or RSTP, then one of the
ports between bridge M and bridge B should be blocked. However, if the bridges in the yellow part
of the figure run MSTP and are allocated in a MST field, then MSTP will treat that field as a single
bridge and block the port between bridge B and Root; similarly, MSTP will block a port in network
D.

10.1.1.1 MST field operation

All bridges in a field are connected via IST. When IST is running, CIST Regional Root will be
elected as its root bridge, which has the lowest route cost to the CIST Root and smallest BridgeID. If
only one field is present in the network, then that field becomes the CIST Root of the entire network;
if the CIST Root falls outside the field, then a bridge at the field's edge will be elected to be the
EES4710BD 10 Slots L2/L3/L4 Chassis Switch
Fig 00-1 CIST and MST Field
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