From CCIE Study Wiki
Multiple Spanning Tree allows a single network to run multiple instances of the Spanning Tree Protocol.
Theory
MST defines a way to use multiple instances of STP in a network that uses 802.1Q.
- MST allows for the tuning of STP parameters, like PVST+, that allows some ports to block one VLAN while forwarding another
- It always uses 802.1w RSTP for faster convergence
- It Does not require an STP instance for each vlan, best designs will use one instance per redundant path
A group of switches together that use MST is called an MST region. You must define a name, a revision number, and map vlan's to a specific instance on each switch. A vlan can be part of only one instance at a time.
When connecting a MST region to a non-MST region or different MST region, MST makes the region appear to be a single switch, as MST can guarantee loop-free behavior inside the MST region. To accomplish this, MST participates in an STP instance called IST or Internal Spanning Tree.
There is no limit to the number of MST regions in a network, but each region can support up to 65 ST instances. Instances can be identified with any number from 1 to 4094 (0 is reserved for the IST instance). All MST instances within the same region share the same protocol timers, but each MST instance can have its own topology. Configure parameters on BPDU transmission only on the CST instance, but topology parameters can be configured on either/both the CST and MST instance(s).
IST is the only spanning tree instance in BPDU that sends and recieves BPDUs. All other spanning-tree info for instances is contained within M-records, which are then encapsulated within the IST MSTP BPDU. This reduces the number of BPDUs needed to support MST.
CIST is the collection of ISTs from each MST region in a network. Common Spanning Tree (CST) interconnects the MST regions, treating each region as if they were a single switch. The root switch of the IST instance for a region becomes the CIST regional root - it has the lowest switch ID and path cost to the CIST root. It is also the CIST root switch if there is only one region on the network. CIST regional root switches (assuming ther are multiple regions) will go through their own election to determine the overall CIST root switch. All switches in the MST region must agree on the same CIST regional root.
IST and MST instances use path cost to the root and a hop-count mechanism to compute the spanning tree topology (they do not use the message-age and max-age info in config BPDUs). The root switch of an instance sends a BPDU (or m-record) with a cost of 0 and the hop count set to the max value (configurable, what is the default setting?). When a switch relieves the BPDU/m-record, it decrements the received remaining hop count by one and propagates it. When the count reaches zero, the switch discards the BPDU and ages the info held for the port.
The concept of a boundary port is still maintained in the Cisco MST. A boundary port receives messages external to the MST region. If there is a legacy switch on the segment, messages are always considered external.
Switches cannot always detect when they are connected to a non-MST switch, so a config command can be used
Commands
- spanning-tree mode mst (global) - enables MST, automatically enables RSTP along with it.
- spanning-tree mst configuration (global) - to enter MST config mode
- instance instance-id vlan vlan-range (mst) - map vlan's to a particular instance, instance-id range 0-4094, vlan-range is 1-4094
- name name (mst)
- revision version (mst)
- spanning-tree mst instance-id root primary [diameter net-diameter [hello-time seconds]] (mst) - configures the specified instance to automatically become the root by checking the other switches in the instance and making its priority 24576 or lower (if needed). The diameter keyword is only available to instance 0 and specifies the max number of hops between any two switches. Using this hop info, the switch will set optimal timers for a network of that diameter. Use the keyword secondary instead of primary to set a backup root switch.
- spanning-tree mst instance-id priority priority (mst)- configure the switch to make it more likely to become the root
- spanning-tree mst instance-id port-priority priority (mst) - sets the priority for a port, must be in increments of 16 (128 is default). may be used when the switch needs to determine which interface to place in the forwarding state
- spanning-tree mst instance-id cost cost (interface) - Interface command used to set the STP cost of a particular interface
- spanning-tree mst max-hops hop-count (global) - can be 1 to 255, optional command
- spanning-tree mst pre-standard (interface) - specifies a neighbor device that is not MST-aware, the switch will only send pre-standard BPDUs out of this interface
- clear spanning-tree detected-protocols interface interface-id (global) - Since a switch can no longer detect if it's neighbor has changed and now can understand MST, this command must be used to reset the interface
Default Settings
- Same default timer values as RSTP
- vlans assigned to the IST instance 0 by default
- all vlan to instance must match in order to load balance over links
- default hop count for all mst instances is 20
Verification
Troubleshooting, Tips, and Tricks
- Configure the same parameters on ALL SWITCHES IN THE REGION.
Online Resources
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