STP and RSTP

STP:
The original: 802.1D spanning-tree
Switches forward brodcast packets out all ports by design
Redundand connections are necessary in buisness networks
The place of the spanning tree: drop tress on redundant links (until they are needed)

Core:
Original STP was created to prevent loops
Switches Send “probes” into the network called Bridge Protocol Data Units (BPDUS) to discover loops
The BPDU probes also help elect the core switch of the network called the root bridge
The BPDU’s are multicast
The simlistic view of STP: All switches find the best way to reach the root bridge the block all the redundant links

BPDUS are sent once every two seconds
Priority is some value between 0 to 61440 (Default is 32768); Increments of 4096 -lower is better

Three Port Types:
-Root Port: used to reach the root bridge
-Designated Port: Forwarding port, one per link
-Blocking/Non-DesignatedPort: Where tree fell

(Root bridge never blocks a port)

How STP finds the Best Path:
1) Elect the root
2) Swictches find lowest cost path to root
3) use lower bridge id on equal cost paths
4) Use lower port to break a tie

PVST(Per Vlan Spanning Tree Protocol)
All modern switches run PVST
One Instance of STP per VLAN
One root Bridge elected for each VLAN
Helps LOAD-BALANCE more effectively

The spanning-tree command
#spanning-tree Vlan x root primary
#spanning-tree vlan x root secondary
#spanning-tree vlan x priority <number>

Problems with Spanning Tree:

Listening – 15 sec of listening for BPDUs. Switch sends/recieve BPDUs
|
Learning – 15 secs of learning Mac Address. Populates switch CAM table
|
Forwarding – Port is forwarding traffic

(Blocking – Switch will wait upto 20 seconds before moving a blocked port into a listening phase.)

Problems and Solutions:
Problems with PC’s: Modern PC’s can boot faster than 30 seconds

-Solution: PortFast
PortFast disables Spanning tree on the port.

Problem with uplink ports: 50 Seconds of down time casues big problems
RSTP:
– 802.1w
– Proactive system
– Redefined port roles
– Many STP similarites

RSTP Port States:
-Discasrding (STP Blocking)
-Learning (STP Learning)
– Forwarding (STP Forwarding)

RSTP Port Roles:
– Root port (Reaching the root not the root)
– Designated port (All root bridge ports are designated)
– Alternate Port (Rememers a blocked port instead of forgetting as in STP)
– Edge Port (conncts to non switch devices similar to portfast)

Why RSTP is better:
1) Does not forget the ports.
2) Proactive nature many of the safety timers of STP are eliminated e.g. bonus,learning
3) Any change to trunk ports flood through the network to other switches

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