MST runs in instances whose existence is not directly related to any particular VLAN. Instances are created by configuration, and VLANs are subsequently mapped onto
them. It uses 802.1w RSTP for rapid convergence in each instance, inheriting all its rapid convergence properties including BPDU expiry in a 3x Hello interval, acceptances of inferior BPDUs from designated switches, port roles/states/types, link types, Proposal/Agreement, and so on.
MST is the only standards-based and interoperable version of STP supporting VLANs and suitable in multivendor switched environments. 802.1D STP supports only 1 Vlan.
MST organizes the network into one or more regions. An MST region is a group of switches that together use MST in a consistent way-they run the same number of MST instances and map the same sets of VLANs onto these instances, among other things. MST standard allows for at most 65 active MST instances (instance 0 plus at most 64 user-definable instances). In MST, a port sends BPDUs if it is Designated for at least one MST instance. As MST uses a single BPDU for all instances, it is possible to see both switches on a point-to-point link to send BPDUs to each other if each of these switches is Designated in a different MST instance.
The instance 0 has a special meaning and it's also called the Internal Spanning Tree, or IST. This instance always exists even if no other MST instances are created. Without any additional configuration, all VLANs are mapped onto the IST. Second, the IST is the only instance that interacts with STP run on switches outside the MST region. If the network consists of several MST regions, each of them can be visualized as a single switch.
Common Spanning Tree(CST) is created for interaction of individual ISTs on region boundaries, and constitutes a spanning tree between individual regions, consisting purely of links between MST regions. If there was a non-MST (either STP or RSTP) part of the network, it would become an integral part of the CST. This CST has no per-VLAN semantics—it is a spanning tree interconnecting MST region boundaries and optionally spanning non-MST regions, shared by all VLANs. CST has two main purposes: (1)it determines loop-free paths between regions. (2)CST is the only spanning tree that can be understood and participated in by non-MST (that is, STP and RSTP)switches. Costs in CST reflect only the costs of links between regions and in non-MST parts of the network. These costs are called external costs by MST.
In each MST region, the CST on the region’s boundary merges with the IST inside the region. The resulting tree consists of a loop-free interconnection between MST regions “glued together” with loop-free interconnection inside each MST region, and is called the Common and Internal Spanning Tree, or CIST.
As each MST region has its own IST root, CIST—consisting of ISTs inside regions and CST between regions—can have multiple root switches as a result. These switches are recognized as the CIST Root Switch (exactly one for the entire CIST) and CIST Regional Root Switches (exactly one for the IST inside each region). The CIST Root Switch is elected by the lowest Bridge ID from all switches that participate in CIST, that is, from all MST switches across all regions according to their IST Bridge IDs (composed of IST priority, instance number 0, and their base MAC address), and from all STP/RSTP switches, if present, according to the only Bridge IDs they have.
In other MST regions that do not contain the CIST Root Switch, IST root switch is elected from boundary switches with lowest external root path cost(sum of costs of inter-region links to reach the region with CIST Root switch) to CIST Root switch. If tie,lowest IST bridge is elected.
A CIST Regional Root Switch has a particular importance for a region: Its own CIST Root Port, that is, the Root Port to reach the CIST Root Switch outside the region, is called the Master port (this is an added port role in MST), and provides connectivity from the region toward the CIST Root for all MST instances inside the region.
Interoperability Between MST and Other STP Versions
The MST instance 0 has a key role here—it speaks to non-MST neighbors and it processes BPDUs received from them.If it is necessary to operate a mixed MST and PVST+ network, it is recommended to make sure that the MST region appears as a root switch to all PVST+ instances by lowering its IST root’s priority below the priorities of all PVST+ switches in all VLANs.
If a Cisco MST switch faces a pure 802.1D STP or 802.1w RSTP switch, it will revert to the appropriate STP version on the interconnecting port, that is, STP or RSTP, according to the neighbor type. However, if a Cisco MST switch is connected to a PVST+ or RPVST+ switch, it will always revert to PVST+.
PVST Simulation consistency criteria require that for an MST Boundary port toward a PVST+ region to be Forwarding, one of the following conditions must be met:
-Either the boundary port’s own IST BPDUs are superior to all received PVST+ BPDUs regardless of their VLAN (in this case, the port becomes Designated; “if be
Designated Port for VLAN 1, then be Designated Port for all VLANs”)
-Or the boundary port’s own IST BPDUs are inferior to received PVST+ BPDUs for VLAN 1, and they are in turn identical or inferior to received PVST+ BPDUs for other
VLANs (in this case, the port becomes Root Port; “if be Root Port for VLAN 1, then be Root Port for all VLANs”)
MST Configuration
The name, revision number, and VLAN-to-instance mappings are three mandatory elements of MST configuration and must match on all switches of a single region.
As changes to VLAN-to-instance mappings are most common, it is recommended to premap VLANs into instances even before the VLANs are created.
(config)#spanning-tree mst configuration
(config-mst)#name CCIE
(config-mst)#revision 1
(config-mst)#instance 1 vlan 1-500
(config-mst)#instance 2 vlan 501-1000
(config-mst)#instance 3 vlan 1001-2047
(config-mst)#instance 4 vlan 2048-4094
(config-mst)#show current //show current MST config
(config-mst)#show pending //show modified but still unapplied config
(config-mst)#exit
(config)#spanning-tree mode mst
//note there's no vlan keyword to modify the properties
(config)#spanning-tree cost mst
(config)#spanning-tree port-priority mst
(config)#spanning-tree mst 0 priority 0
(config)#spanning-tree mst 1 priority 4096
show spanning-tree mst configuration
show spanning-tree summary
No comments:
Post a Comment