MST

Multiple STP

• Started as Cisco’s MISTP
• Originally starndard defined in IEE 802.1d
• Now standard per IEEE 802.1q- 2005

How does it Work.

• STP instance to VLAN mappings is use defined
• Topology calculation is done by RSTP

Result is higher scalability

• (Rapid) PVST uses one instance per VLAN
• As VLANs scale , control plane dies.
• PVST is inefficient because there are typically only 3 possible trees anyways

MST Defines a Region as Bridges that agree upon , 

• instance name
• Revision number
• VLAN to STP instance

Intra vs Inter Region 

Intra Region

• Details of the region are known within the region
• VLAN to STP are manually defined
• Undefined VLANS fall into CIST (MST 0 )

CIST – Common Internal Spanning Tree.

Inter Region

• Details between region are  not known
• Different regions see each other as virtual bridges
• Result  is simplified inter-Region calculation
• Intra-region MSTIs are collapsed into CIST.

MST interoperability

• MST is backward compatible with legacy CST and PVST plus
• Behaves like inter-region MST
• CST Root must be within MST Domain

MST Configuration

Define the following MST Configuration mode

• Region name
• Revision number
• VLAN to instance mappings

Enable MST globally

• Real deployment must start at root and work down.

Same election prcoess as Legacy STP

spanning-tree mst configuration
name MST1
revision 1
instance 1 vlan 10-20
instance 2 vlan 21-30
instance 3 vlan 31-40
!

Rapid STP

Rapid Spanning-Tree Protocol.

• New standard originally defined in IEEE 802.1w
•  Now incorporated as IEEE 802.1D -2004

Changes cs Legacy STP

• Simplifies port states
• additional port roles
•  Rapid convergence based on synchronization process
• Path calculation remains the same.

Legacy STP Uses

• Disabled
• Blocking
• Listening
• Learning
• Forwarding

RSTP Simplifies to…

• Discarding – Dropping frames
• Learning – Dropping Frames but building the CAM
• Forwarding – Normal Forwarding

RSTP Ports Roles 

• Port roles are decoupled from port states
• Root Port & Designated Port
• New Roles : Alternate(compared to uplinkfast ) , Backup Designated ( activates if the primary Designated port fails)  & Edge ( immediately transitions to forwarding , Do not generate TCN for state change).

Maintains edge status as long as no BPDUs are received.

• If BPDU received , remove edge status and Generate TCN.

RSTP Link Types

• Non-edge
• Point – point
• shared

Only Point to point Designated ports use the sync process for rapid convergence.

RSTP Sync Process

• Goal is for a bridge to synchronize its root port with the rest for the topology.
• When a bridge elects a root port it assumes all non-edge ports to be designated ( all no-edge ports are discarding at this moment ).
• Bridge sends proposals out all designated ports ( Proposal has port roles set to designated ports : Proposal contains root bridge info ( priority , cost , etc ).
• Downstream bridges review this information ( if they don’t have better paths to the root they agree : If they do have it they announce their information.)
• When designated port receives agreement , it is unblocked .
• If downstream bridge sends a better root information , local bridge changes root port.
• if downsteam bridge agrees to upstream proposal , then it ( elects a local root port , Blocks all non-edge designated ports , Starts sync process on all designated ports
• Port blocking is essential in preventing transient loops.

RSTP Fault Detection

• In legacy STP , BPDUs are only generated by root Bridge ( all other bridges forward them on )
• Is RSTP , each bridge generates BPDU every Hello interval.
• If 3 hellos are missed from a neighbor re-convergence begins ( 6sec vs 20 sec Max age )

Max Age is used as hop Count

• Every bridge sends BPDUs on its own
• Age incremented by every bridge
• Max Age also on shared ports for legacy STP backward compatibility.

Faults can be detected faster by means of physical signalling.

RSTP Convergence

• RSTP needs to re-converge when root ports  lost
• If there is an Alternate port , it is selected in place of old Root Port ( new root is then Synchronized with down stream bridges.)
• If there are no Alternate ports and no better info  ( declare itself as root , Synchronise adapt to better info )

RSTP Topology Change 

• Originated by switch that detected the event
• uses special BPDU bit to signal topology change
• flooded by all switches using reverse path forwading

Flushes MAC table address tables

• causes temporary excessive unicast traffic flooding
• Use Egde ports as much as possible.

spanning-tree portfast bpduguard default global configuration command. Spanning tree shuts down ports that are in a Port Fast-operational state if any BPDU is received on them. In a valid configuration, Port Fast-enabled ports do not receive BPDUs. Receiving a BPDU on a Port Fast-enabled port means an invalid configuration, such as the connection of an unauthorized device, and the BPDU guard feature puts the port in the error-disabled state. When this happens, the switch shuts down the entire port on which the violation occurred.

panning-tree portfast bpdufilter default global configuration command. This command prevents interfaces that are in a Port Fast-operational state from sending or receiving BPDUs. The interfaces still send a few BPDUs at link-up before the switch begins to filter outbound BPDUs. You should globally enable BPDU filtering on a switch so that hosts connected to these interfaces do not receive BPDUs. If a BPDU is received on a Port Fast-enabled interface, the interface loses its Port Fast-operational status, and BPDU filtering is disabled.

STP Convergence Optimizations

STP Topology Change Notifications

• In normal STP operation, a bridge keeps receiving configuration BPDUs from the root bridge on its root port. But, it never sends out a BPDU toward the root bridge. In order to achieve that, a special BPDU called the topology change notification (TCN) BPDU has been introduced. Therefore, when a bridge needs to signal a topology change, it starts to send TCNs on its root port. The designated bridge receives the TCN, acknowledges it, and generates another one for its own root port. The process continues until the TCN hits the root bridge.
• The TCN is a very simple BPDU that contains absolutely no information that a bridge sends out every hello_time seconds (this is locally configured hello_time, not the hello_time specified in configuration BPDUs). The designated bridge acknowledges the TCN by immediately sending back a normal configuration BPDU with the topology change acknowledgement (TCA) bit set. The bridge that notifies the topology change does not stop sending its TCN until the designated bridge has acknowledged it. Therefore, the designated bridge answers the TCN even though it does not receive configuration BPDU from its root
• With legacy STP 802.1D , ports that are facing away from the root bride and are connected to devices that are not participating in STP ( PCs ) 
• If a port connecting a PC by default goes down in STP , the port sends a TCN up towards the root bridge and the root bridge is then going to send a TCN Ack down to the other devices , in the case of legacy STP what does the STP TCN do ? 
• It tells the other bridges to change the MAC address aging time ( 300 sec ) to the maximum age time ( 20 sec ) , this means in a legacy Spanning Tree Design  for any of your edge ports that are not configured as Edge ports (portfast ) when PC powers on & off  , its gonna cause the entire Layer network to to age out the entire MAC address table in 20 secs.
• Basically you end up having the large spikes in broadcast traffic in unknown traffic because the MAC address get flushed out  & the network needs to re-learn them.
• In Rapid STP the issue is even worse  because when TCN is generated  it  causes the switches to immediately Flush out the MAC table as opposed to wait for the MAX Age timer ( RSTP is event driven ).

# spanning tree portfast default – enables portfast on all switch ports that are not receiving BPDUs.

# spanning tree portfast bpdufilter default – enable both portfast and BPDU filter on all ports that are not Receiving BPDUs.