CCNA-Notes/21. Protocole Spanning Tree (part2).md

id, aliases, tags

id	aliases	tags
1778913634-WEMK	Protocole Spanning Tree (part2)

Protocole Spanning Tree (part2)

Spanning Tree Port State

STP Port State	Stable/Transitional
Blocking	Statble
Listening	Transitional
Learning	Transitional
Forwarding	Stable
Disabled	Stable

• Root/Designated ports remain stab;e in Forwarding state.
• Non-designated ports remain stabl in a Blocking state.
• Listening and Learning are transitional states which are passed through when an interface is activated, or when a Blocking port must transition to a Forwarding state due to a change in the network topology

Blocking state

• Non designated ports are in a Blocking state
• Interfaces in a Blocking state are effectively disabed to prevent loops.
• Interfaces in a Blocking stae do not send/receive regular naetwork traffic.
• Interfaces in a Blocking state receive STP BPDUs.
• Interfaces in a Blocing state do NOT forward STP BPDUs.
• Interfaces in a Blocking state do NOT learn MAC addresses.

Listening state

• After the Blocking state interfaces with the Designated or Root role enter Listening state.
• Only Designated or Root ports enter the Listening state (Non-designated ports are always Blocking)
• The Listening state is 15 seconds long by default. This is determined by the orward delay timer.
• An interface in the listening state ONLY forwards/receives STP BPDUs.
• An interface in the Listening state does NOT send/receive regular traffic
• An interface in the Listening state does NOT Learn MAC addresses from regular traffic that arrives on the interface

Learning state

• After the Listeing state, a Designated or ROOT port will enter Learning state.
• The Learning state is 15 seconds long by default. This is determined by the Forward delay timer ( the same timer is used for both the Listening and Learning states).
• An interface in the Learning state ONLY sends/receives STP BPDUs.
• An interface in the Learning state learns MAC addresses from regular traffic that arrives on the interface.

Forwarding state

• Root and designated ports are in a Forwarding state.
• A port in the Forwarding state operate as normal
• A port in the Forwarding state sends/receives BPDUs.
• A port in the Forwarding state sends/receives normal traffic.
• A port in the Forwarding state learns MAC addresses.

summary

STP Port State	Send/Receive BPDUs	Frame forwarding	Mac address learning	Stable/Transitional
Blocking	No/Yes	NO	NO	Statble
Listening	Yes/Yes	NO	NO	Transitional
Learning	Yes/Yes	NO	Yes	Transitional
Forwarding	Yes/Yes	Yes	Yes	Stable
Disabled	NO/NO	NO	NO	Stable

note: Switches do not forward the BPDUs out of their root ports and non-designated ports,
only their designated ports

STP timers

Hello

How often the root bridge sends hello BPDUs duration: 2 sec

Forward delay

How long the switch will stay in the Listening and Learning states (each stae is 15 seconds = total 30 seconds) duration: 15sec

Max Age

How long an interface will wait after ceasing to receive Hello BDPUs to change the STP topology duration: 20 sec (10*hello)

• If another BPDU i received before the max age timer counts down to 0, the time will reset to 20 seconds and no changes will ocur.

• If another BPDU is not received the max age timer counts down to 0 and the switch will reevaluate its STP choices, including root bridge, and local root, designated, and non-designated ports

• if a non-designated port is selected to become a designated or root port, it will transition from the blocking state to the listening state (15 seconds), learning state (15 seconds), and then finally the forwarding state. So it can take a total of 50 seconds for a blocking interface to transition to forwarding

• These timers and transitional states are to make sure that loops aren't accidentally created by interface moving to forwarding state too soon.

  note: A forwarding interface can move directly to a blocking state (there is no worry
  about creating a loop by blocking an interface).
  A blocking interface cannot move directly to forwarding state, it must go through the
  listening and learning states.
  

Dst: PVST+ (01:00:0c:cc:cc:cd)

PVST = Only ISL trunk enquapsulation PVST+ = Supports 802.1Q

Regular STP (not Cisco's PVST+) uses a destination MAC address of 0180.c200.0000

The STP timers on the root bridge determine the STP timers for th entire network.

Spanning Tree Optional Features (STP Toolkit)

Portfast

Portfast allows a port to move imediately to the Forwarding state, bypassing Listening and Learning.

If used, it must be enabled only on ports connected to end hosts, if enabled on a port connected to another switch it could cause a Layer 2 loop.

SW1(config)#interface g0/2
SW1(config-if)#spanning-tree portfast

You can also enable portfast with the command:

SW1(config)#spanning-tree portfast default

This enables portfast on all access ports (not trunk ports).

BPDU Guard

if an interface with BPDU Guard enabled receives a BPDU from another switch, the interface will be shut down to prevent a loop from forming.

SW1(config)#interface g0/2
SW1(config-if)#spanning-tree bpduguard enable

You can also enable portfast with the command:

SW1(config)#spanning-tree portfast bpduguard default

Root Guard

If you enable root guard on an interface, even if it receives a superior BPDU (lower bridge ID) on that interface , the switch will not accept the new switch as the root bridge. the interface will be disabled.

Loop Guard

If you enable loop guard on an interface, even if the interface stops receiving BPDUs, it`will not start forwarding. The interface will be disabled.

Spanning Tree Configuration

Spanning Tree mode

SW1(config)#spanning-tree mode ?
  mst   multiple spanning tree mode
  pvst  Per-Vlan spanning tree mode
  rapid-pvst Per-Vlan rapid spanning tree mode //default

SW1(config)#spanning-tree mode pvst

Primary Root Bridge

SW1(config)#spanning-tree vlan 1 root primary

SW1(config)#do show spanning-tree

The spanning-tree vlan (vlan-number) root primary command sets the STP priority to 24576. if another switch already has a priority lower than 24576, it sets this switch's priority to 4096 less than the other switch's priority.

Secondary Root Bridge

SW1(config)#spanning-tree vlan 1 root secondary

SW1(config)#do show spanning-tree

The spanning-tree vlan (vlan-number) root secondary command sets the STP priority to 28672.

STP Port Setting

SW1(config)#spanning-tree vlan 1 cost 200
SW1(config)#spanning-tree vlan 1 port-priority 32

Review

• STP state/timers
• STP BPDU
• STP optional features
• STP configuration