21. SPANNING TREE PROTOCOL (STP) : PART 2

STP STATES

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• ROOT / DESIGNATED PORTS remain STABLE in a FORWARDING state
• NON-DESIGNATED PORTS remain STABLE 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 network topology.

1) BLOCKING / STABLE

• NON-DESIGNATED PORTS are in a BLOCKING state
• Interfaces in a BLOCKING state are effectively disabled to prevent loops
• Interfaces in a BLOCKING state do NOT Send/Receive regular network traffic
• Interfaces in a BLOCKING state do NOT forward STP BPDUs
• Interfaces in a BLOCKING state do NOT learn MAC ADDRESSES

2) LISTENING / TRANSITIONAL

• After the BLOCKING state, interfaces with the DESIGNATED or ROOT role enter the 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 FORWARD DELAY TIMER
• Interfaces in a LISTENING state do NOT Send / Receive regular network traffic
• Interfaces in a LISTENING state ONLY Forward/Receive STP BPDUs
• Interfaces in a LISTENING state does NOT learn MAC ADDRESSES from regular traffic that arrives on the interface

3) LEARNING / TRANSITIONAL

• After the LISTENING state, a DESIGNATED or ROOT port will enter the LEARNING state
• The LEARNING state is 15 seconds long by Default. This is determined by the FORWARD DELAY TIMER (same one used for both LISTENING and LEARNING states)
• Interfaces in a LEARNING state do NOT Send / Receive regular network traffic
• Interfaces in a LEARNING state ONLY Sends/Receives STP BPDUs
• Interfaces in a LEARNING state learns MAC ADDRESSES from regular traffic that arrives on the interface

1. FORWARDING / STABLE

• 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 regular network traffic
• A PORT in the FORWARDING state Sends/Receives STP BPDUs
• A PORT in the FORWARDING state learns MAC ADDRESSES

SUMMARY :

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STP TIMERS

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💡 SWITCHES do NOT forward the BPDUs out of their ROOT PORTS and NON-DESIGNATED PORTS - ONLY their DESIGNATED PORTS !!!

MAX AGE TIMER:

• If another BPDU is received BEFORE MAX AGE TIMER counts down to 0, the TIME will RESET to 20 Seconds and no changes will occur.
• If another BPDU is not received, the MAX AGE TIMER counts down to 0 and the SWITCH will re-evaluate it’s STP choices, including ROOT BRIDGE, 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 50 Seconds for a BLOCKING interface to transition to FORWARDING! (MAX AGE TIMER + (LISTENING + LEARNING 15 Second timers))
• These TIMERS and TRANSITIONAL STATES are to make sure that LOOPS are not accidentally created by an INTERFACE moving to FORWARDING STATE too soon

HOWEVER …

💡 A FORWARDING interface can move DIRECTLY to a BLOCKING state (there is no worry about creating a loop)

💡 A BLOCKING interface can NOT move DIRECTLY to a FORWARDING state. It MUST go through the LISTENING and LEARNING states first!

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STP BPDU (BRIDGE PROTOCOL DATA UNIT)

Ethernet Header of a BPDU

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💡 PVST+ uses the MAC ADDRESS :

01 : 00 : 0c : cc : cc : cd

PVST = ONLY ISL Trunk Encapsulation

PVST+ = Supports 802.1Q

💡 Regular STP (not Cisco’s PVST+) uses the MAC ADDRESS :

01 : 80 : c2 : 00 : 00 : 00

💡 The STP TIMERS on the ROOT BRIDGE determine ALL STP TIMERS for the entire network!

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STP OPTIONAL FEATURES (STP TOOLKIT)

PORTFAST:

• Can be Enabled on INTERFACES which are connected to END HOSTS

💡 PORTFAST allows a PORT to move immediately 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

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You can also ENABLE PORTFAST with the following 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 loops from forming.

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You can also ENABLE BPDU GUARD with the following command:

💡 SW1(config)# spanning-tree portfast bpduguard default

This ENABLES BPDU GUARD on all PORTFAST-enabled INTERFACES

ROOT GUARD / LOOP GUARD:

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You probably do NOT have to know these STP optional features (or others such as UplinkFast, Backbone Fast, etcetera) for the CCNA.

BUT…

💡 Make sure you know PORTFAST and BPDU GUARD.

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STP CONFIGURATION

Command to CONFIGURE Spanning-Tree mode on a SWITCH

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Modern Cisco SWITCHES run rapid-pvst, by default

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CONFIGURE THE PRIMARY ROOT BRIDGE

Command to CONFIGURE Spanning-Tree PRIMARY ROOT BRIDGE on a SWITCH

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Confirm with “(do) show spanning-tree”

Can see in the above example, SW3 has become the “root”

• The “spanning-tree vlan <vlan-number> root primary” command sets the STP PRIORITY to 24576. If another SWITCH already has a priority number lower than 24576, it sets this SWITCH’s priority to 4096 LESS THAN the other SWITCH’s Priority (remember STP PART 1 lecture)

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SECONDARY ROOT BRIGE (backup ROOT BRIDGE)

Command to CONFIGURE Spanning-Tree SECONDARY ROOT BRIDGE on a SWITCH

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• The “spanning-tree vlan <vlan-number> root secondary” command sets the STP PRIORITY to 28672 (exactly 4096 higher than 24576).

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VLAN 1 TOPOLOGY running PVST+

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SW1 WAS the PRIMARY ROOT BRIDGE but :

• We have configured SW3 to be the PRIMARY
• We have configured SW2 to be the SECONDARY

The TOPOLOGY for VLAN 2, however, won’t be the same. It will be the OLD Topology.

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WHY? Because we made changes ONLY to the TOPOLOGY found in VLAN 1 (see the commands we used)

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CONFIGURE STP PORT SETTINGS

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“cost” = “ROOT COST”

“port-priority” = “PORT PRIORITY”