CCNA-Notes/22 Rapid Spanning Tree Protocol.md

id, aliases, tags

id	aliases	tags
22 Rapid Spanning Tree Protocol		CCNA

Rapid Spanning Tree Protocol

Spanning Tree Version

Industry standards (IEEE)

Spanning Tree Protocol (802.1D)

• The original STP
• ALl VLANs share one STP instance.
• Therefore, cannot load balance.

Rapid spanning Tree Protocol (802.1w)

• Much faster at converging/adaptating to network changes than 802.1D
• All VLANs share one STP instance.
• Therefore, cannot load balance

Multiple Spanning Tree Protocol (802.1s)

• Uses modified RSTP mechanics.
• Can group multiple VLANs into different instacnces (ie. VLANs 1-5 instance 1, VLANs 6-10 in instance 2 ) to perform load balancing

CISCO Versions

Per-VLAN Spanning Tree Plus (PVST+)

• Cisco's upgrade to 802.1D
• Each VLAN has its own STP instance.
• Can load balance by blocking different ports in each VLAN.

Rapid Per-VLAN Spanning Tree Plus (Rapid PVST+)

• Cisco's upgrade to 802.1w
• Each VLAN has its own STP instance
• Can load balance by blocking different ports in each VLAN

Rapid spanning Tree Protocol

Cisco's summary: "RSTP is not a timer-based spanning tree algorithm like 802.1D. therefore, RSTP offers an improvement over the 30 seconds or more that 902.1d takes to move a link to forwarding. the heart of the protocol is new bridge-bridge handshake mechanism, which allows ports to move directly to forwarding."

Similarities between STP and RSTP:

RSTP serves the same purpoe as STP, blocking specific ports to prevent Layer2 loops.

• RSTP elect a root bridge with same rules as STP
• RSTP elects root ports with the same rules as STP.
• RSTP elects designated ports with the same rules as STP

Differences

COST

speed	STP COST	STP COST
10 Mbps	100	2,000,000
100 Mbps	19	200,000
1 Gbps	4	20,000
10 Gbps	2	2,000
100 Gbps	x	200
1 Tbps	x	20

Port State

STP Port State	Send/Receive BPDUs	Frame Forwarding	MAC Learning	Stable/Transitional
Discarding	NO/YES	NO	NO	Stable
Learing	YES/YES	NO	YES	Tranisitional
Forwarding	YES/YES	YES	YES	Stable

• if a port is administratively disabled (shutdown command) = discarding state
• if a port is enabled but blocking traffic to prevent Layer 2 loops = discarding state

Port Roles

The root port role remains unchanged in RSTP.

• The port wthat is closet to the root bridge becomes the root port for the switch.
• The root bridge is the only switch that doesn't have a root port.

The designated port role remains unchanged in RSTP.

• The port on a segment (collision domain) that sends the best BPDU is that segment's designated port (only one per segment)

• The non designated port role is split into two separate roles in RSTP:

  • The alternate port role
  • the backup port role

Alternate port Role

The RSTP alternate port role is discarding port that receives a superior BPDU from another switch.

• This is the same as what you've learned about blocking ports in classic STP.

• Functions as a backup to the root port.

• If the root port fails, the switch can immediately move its best alternate port to forwarding

  This immediate move to fowarding state functions like a classic STP optional feature called 
  UplinkFast. Because it is built into RSTP, you do not need to activate UplinkFast when using 
  RSTP/Rapid PVST+
  
  One more STP optional feature that was built into RSTP is BackboneFast.
  Backbone Fast allows SW3 to expire the made age timers on its interface and rapidly forward the superior
  BPDUs to SW2.
  This functionality is built into RSTP, so it does not need to be configured
  

Backup port role

The RSTP backup port role is a discarding port that receives a superior BPDU from another interface on the same switch.

• This only happnes when two interfaces are connected to the same collision domain (via a hub)
• Hubs are not used in modern networks, so you will probably not encounter an RSTP backup port.
• Function as backup for a designated port
• The interface with the lowest port ID will be selected as the designated port and the other will be the backup port

BPDU

In classic STP, only the root bridge originated BPDUs, and other switches just forwarded the BPDUs they received. In Rapid STP, all Switches originate and send their own BPDUs from their designated ports

All switches running Rapid STP send their own BPDUs every hello time (2 seconds).

• Switches 'age' the BPDU information much more quickly, in classic STP a switch waits 10 hello intervales (20 seconds). In rapid STP, a switch considers a neighbor lost if it misses 3 BPDUs (6 seconds) It will the 'Flush' All MAV addresses learned on that interface

RSTP Link Types

RSTP distiguishes between three different 'link types'

• Edge: a port that is connected to an end host. Moves directly to forwarding without negotiation.
• Point-to-point: a direct connection between two switches
• Shared a connection to a hub. Must operate in half-duplex mode.

Edge

Edge ports are connected to end hosts.

• Because ther is no risk of creating a loop, they can move stright to the forwarding state without the negotiation process.
• They function like a classic STP port with PortFast enabled

SW1(config-if)# spanning-tree portfast

Point-to-Point

Point-to-point ports connect directly to another switch

• They function in full-duplex
• you don't need to configure the interface as point-to-point (it should be detected)

SW1(config-if)# spanning-tree link-type point-to-point

Shared

shared Ports connect to another switch (or switches) via a hub.

• They function in half-duplex
• You don't need to configure the interface as shared (it should be detected)

SW1(config-if)# spanning-tree link-type shared

Configuration

SW3(config)# spanning-tree mode rapid-pvst

SW3(config)# do show spanning-tree

Review

Comparison of STP versions (standard vs Cisco)

Rapid PVST+