Stp

Front 1

What is STP?

Back 1

prime function is to prevent rooting loops.

Front 2

Common spanning tree (CST)

Back 2

802.D

Front 3

Rapid spanning tree (RSTP)

Back 3

802.1W

- evolution of 802.1D that provides faster convergence of STP.

- Uses alternate ports instead of blocking ports in discarding state.

- RSTP defines port states as discarding, learning or forwarding.

- preferred protocol for preventing layer 2 loops.

- protocol info can be immediately aged on a port if hellos are not received for 3 consecutive hello times (6 seconds default), or if max age timer expires

- Because BPDUs are used as a keepalive mechanism, three consecutively missed BPDUs indicate lost connectivity between a bridge and its neighboring root or designated bridge. this allows failures to be detected quickly.

Front 4

Multiple spanning tree (MSTP)

Back 4

802.1S

-main purpose is to reduce total number of spanning tree instances to match the physical topology of the network which will reduce the CPU loading of a switch.

- allows you to build multiple spanning trees over trunks by grouping VLANS and associating them with spanning tree instances.

- To be a part of a common MSTP region, a group of switches must share the same config attributes.
(config name (32 bytes), config revision number (two bytes), a 4096-element table that associates each of the potential 4096 VLANS supported on the chassis with a given instance)

- creates the STP instances and then maps VLANS to those instances

- To reduce the number if required STP instances, MSTP maps multiple VLANS that have the same traffic flow requirements into the same spanning-tree instance.

Front 5

PVST+

Per VLAN spanning tree +

Back 5

- CIsco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network.

- the seperate instance supports enhancement such as PortFast, BPDU guard, BPDU filter, root guard and loop guard.

- Creating an instance for each VLAN increases CPU and memory requirements but allows for per-VLAN root bridges.

-convergence is per VLAN

-default in Cisco switches.

- more than one trunk can block for a VLAN and load sharing can be implemented.

-BID = priority + VLAN ID + MAC address
---- eg. BID = 32768 + 10 + 000A.0033.33
---- BID = 32778 000A.0033.333

Front 6

PVRST+

Back 6

- Cisco enhancement of RSTP, that is similar to PVST+.

- Provides a seperate instance of 802.1w per VLAN.

- The seperate instance supports; PortFast, BPDU guard, BPDU filter, root guard and loop guard.

- This version addressed both the convergence issues and the suboptimal traffic flow issues.

- This version has the largest CPU and memory requirements.

Front 7

Bridge protocol data unit (BPDU)

Back 7

- Bridges pass spanning tree info between themselves using special frames known as a BPDU

- a bridge uses a four-step decision sequence to save a copy of the "best" BPDU seen on every port

- When making this evaluation, it considers all the BPDUs received on the port as well as the BPDU that would be sent on that port

- As every BPDU arrives, it is checked to see if it is more attractive (lower in value) than the existing BPDU saved for that port.

- If the new BPDU is more attractive, the old value is replaced.

Front 8

BPDU's contain info that allow switches to perform specific actions:

Back 8

- Select a single switch that will act as the root of the spanning-tree

- calculate the shortest path from itself to the root switch

- designate one of the switches as the closest one to the root, for each LAN segment. This switch is called the designated switch.

- the designated switch handles all communication from that LAN segment towards the root bride

- Each non-root switch chooses one of its ports as its root port - the interface that gives the best path to the root switch.

- select ports that are part of the spanning-tree. these ports are called designated ports. non designated ports are blocked.

Front 9

Bridge ID (BID)

Back 9

- the BID consists of a bridge priority (default 32768) and the switch MAC address.

- The lower the priority the more chance the switch has of becoming the root.

Front 10

when the spanning tree is creating a loop-free topology, it always uses the same 4 step decision sequence.

Back 10

- Lowest root bridge ID (BID)

- lowest path cost to root bridge

- lowest sender bridge ID

- Lowest port ID

Front 11

Root bridge election process

Back 11

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Front 12

Spanning tree protection

Back 12

Portfast - rapid transition to forwarding state for access ports.

BPDU guard - protects portfast ports from creating loops

Root guard - controls which ports are eligible to participate in root election

Unidirectional link detection (UDLD) - prevents links transitioning to forwarding state under unidirectional fault conditions

loopguard - prevents links transitioning to forwarding under unidirectional fault conditions if designated port still operational.