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'''Spanning Tree Sudy Guide''' | By Andrew Westin | ||
='''Spanning Tree Sudy Guide'''= | |||
Evolution of STP Basics Evolution of STP What is STP? (Spanning Tree Protocol) – Eliminates loops at layer 2 topology | Evolution of STP Basics Evolution of STP What is STP? (Spanning Tree Protocol) – Eliminates loops at layer 2 topology | ||
'''There are many varieties of STP...[pg 120]''' | |||
='''There are many varieties of STP...[pg 120]'''= | |||
DEC STP | DEC STP | ||
802.1D | 802.1D | ||
| Line 11: | Line 17: | ||
PVRST+ | PVRST+ | ||
'''STP initially converges on a logically loop-free network topology''' | ='''STP initially converges on a logically loop-free network topology'''= | ||
1) Elects one root bridge | 1) Elects one root bridge | ||
2) Selects the root port on all nonroot bridges | 2) Selects the root port on all nonroot bridges | ||
3) Selects the designated port on each segment | 3) Selects the designated port on each segment | ||
'''Elects One Root Bridge''' | ='''Elects One Root Bridge'''= | ||
• The protocol uses a process to elect a root bridge | • The protocol uses a process to elect a root bridge | ||
• Only one bridge acts as the root bridge in a given network per VLAN | • Only one bridge acts as the root bridge in a given network per VLAN | ||
• On the root bridge, all ports act as designated ports | • On the root bridge, all ports act as designated ports | ||
• Switch with lowest priority parameter is elected as root | • Switch with lowest priority parameter is elected as root | ||
• STP Step 2 (cont) | • STP Step 2 (cont) | ||
'''Selects the Root Port on All Non-root Bridges''' | ='''Selects the Root Port on All Non-root Bridges'''= | ||
• The protocol establishes one root port on each nonroot bridge. | • The protocol establishes one root port on each nonroot bridge. | ||
• The root port is the lowest-cost path from the nonroot bridge to the root bridge | • The root port is the lowest-cost path from the nonroot bridge to the root bridge | ||
'''Selects Designated Port on Each Segment | ='''Selects Designated Port on Each Segment'''= | ||
''' • On each segment, STP establishes one designated port on the bridge that has the lowest path cost to the root bridge | |||
• On each segment, STP establishes one designated port on the bridge that has the lowest path cost to the root bridge | |||
• The switch primarily chooses a designated port as the least-cost path to the root bridge | • The switch primarily chooses a designated port as the least-cost path to the root bridge | ||
• In the event of a tie, the bridge ID acts as the tiebreaker | • In the event of a tie, the bridge ID acts as the tiebreaker | ||
• Port Roles | • Port Roles | ||
'''There are four roles of ports under STP | ='''There are four roles of ports under STP'''= | ||
''' 1) Root | |||
1) Root | |||
2) Designated | 2) Designated | ||
3) Non-Designated | 3) Non-Designated | ||
4) Disabled | 4) Disabled | ||
'''There are Five Port States | ='''There are Five Port States'''= | ||
''' • Blocking | |||
• Blocking | |||
• Listening | • Listening | ||
• Learning | • Learning | ||
• Forwarding | • Forwarding | ||
• Disabled | • Disabled | ||
'''RSTP | ='''RSTP'''= | ||
'''A variation of Spanning Tree Protocol (STP) designed to ensures a topology that doesn’t have loops. | |||
A variation of Spanning Tree Protocol (STP) designed to ensures a topology that doesn’t have loops. | |||
='''IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w'''= | |||
Detection of root switch failure is done in 3 hello times | |||
STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × Hello times *(default: 3 times 2 seconds) or within a few milliseconds of a physical link failure | STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × Hello times *(default: 3 times 2 seconds) or within a few milliseconds of a physical link failure | ||
RSTP maintains backup details regarding the discarding status of ports. | RSTP maintains backup details regarding the discarding status of ports. | ||
'''RSTP bridge port roles: | ='''RSTP bridge port roles:'''= | ||
''' Root - A forwarding port that is the best port from Non root-bridge to Root bridge | |||
Root - A forwarding port that is the best port from Non root-bridge to Root bridge | |||
Designated - A forwarding port for every LAN segment | Designated - A forwarding port for every LAN segment | ||
Alternate - An alternate path to the root bridge. This path is different than using the root port. | Alternate - An alternate path to the root bridge. This path is different than using the root port. | ||
Backup - A backup/redundant path to a segment where another bridge port already connects. | Backup - A backup/redundant path to a segment where another bridge port already connects. | ||
Disabled - Not strictly part of STP, a network administrator can manually disable a port | Disabled - Not strictly part of STP, a network administrator can manually disable a port | ||
'''RSTP Topology Change Mechanism | ='''RSTP Topology Change Mechanism'''= | ||
'''When an RSTP bridge detects a topology change it performs the following actions. | |||
When an RSTP bridge detects a topology change it performs the following actions. | |||
The RSTP Bridge starts the TC while time with a value equal to twice the hello time for all its non-edge designated ports and its root, port if necessary. The TC While time is the interval during which the RSTP Bridge actively informs the rest of the bridges in the network of a topology. It then flushes the MAC addresses associated with all non-edge ports. And as long as the TC While timer is running the BPDUs will be sent out of the port will have the TC bit set. | The RSTP Bridge starts the TC while time with a value equal to twice the hello time for all its non-edge designated ports and its root, port if necessary. The TC While time is the interval during which the RSTP Bridge actively informs the rest of the bridges in the network of a topology. It then flushes the MAC addresses associated with all non-edge ports. And as long as the TC While timer is running the BPDUs will be sent out of the port will have the TC bit set. | ||
'''Bridge Identifier for PVRST+ | ='''Bridge Identifier for PVRST+'''= | ||
''' Bridge Priority A 4-bit field still used to carry bridge priority | |||
Bridge Priority A 4-bit field still used to carry bridge priority | |||
Extended System ID | Extended System ID | ||
Mac Address | Mac Address | ||
='''Only four high-order bits of the 16 bit bridge priority field carry actual priority'''= | |||
Cisco STP Default Config | Cisco STP Default Config | ||
PVST+ | PVST+ | ||
PVRST+ | PVRST+ | ||
MST | MST | ||
='''Config the Basic Parameters of PVRST+'''= | |||
conf t | conf t | ||
spanning-tree mode rapid-pvst | spanning-tree mode rapid-pvst | ||
spanning-tree vlan 2 root primary | spanning-tree vlan 2 root primary | ||
spanning tree- vlan 3 root secondary | spanning tree- vlan 3 root secondary | ||
'''PortFast''' | ='''PortFast'''= | ||
Spanning Tree Portfast causes an interface configured as a layer 2 access port to enter the forwarding state immediately. Conf t int fa 0/1 spanning-tree portfast | |||
Spanning Tree Portfast causes an interface configured as a layer 2 access port to enter the forwarding state immediately. | |||
Conf t int fa 0/1 spanning-tree portfast | |||
confirm with show spanning-tree interface Fastethernet 0/1. | confirm with show spanning-tree interface Fastethernet 0/1. | ||
Config the Basic Parameters of PVRST+ conf t spanning-tree mode rapid-pvst spanning-tree vlan 2 root primary spanning tree- vlan 3 root secondary | Config the Basic Parameters of PVRST+ conf t spanning-tree mode rapid-pvst spanning-tree vlan 2 root primary spanning tree- vlan 3 root secondary | ||
'''Multiple Spanning Tree | ='''Multiple Spanning Tree'''= | ||
'''MST extends the IEEE 802.1w RST algorithm to multiple spanning trees. The main purpose of MST is to reduce the total number of spanning-tree instances. | |||
MST extends the IEEE 802.1w RST algorithm to multiple spanning trees. The main purpose of MST is to reduce the total number of spanning-tree instances. | |||
Latest revision as of 15:31, 6 February 2012
By Andrew Westin
Spanning Tree Sudy Guide
Evolution of STP Basics Evolution of STP What is STP? (Spanning Tree Protocol) – Eliminates loops at layer 2 topology
There are many varieties of STP...[pg 120]
DEC STP 802.1D Common Spanning Tree (CST) Per VLAN Spanning Tree Plus+ Rapid STP (RSTP)/IEEE 802.1w Multiple Spanning Tree(MST) PVRST+
STP initially converges on a logically loop-free network topology
1) Elects one root bridge
2) Selects the root port on all nonroot bridges
3) Selects the designated port on each segment
Elects One Root Bridge
• The protocol uses a process to elect a root bridge
• Only one bridge acts as the root bridge in a given network per VLAN
• On the root bridge, all ports act as designated ports
• Switch with lowest priority parameter is elected as root
• STP Step 2 (cont)
Selects the Root Port on All Non-root Bridges
• The protocol establishes one root port on each nonroot bridge.
• The root port is the lowest-cost path from the nonroot bridge to the root bridge
Selects Designated Port on Each Segment
• On each segment, STP establishes one designated port on the bridge that has the lowest path cost to the root bridge • The switch primarily chooses a designated port as the least-cost path to the root bridge • In the event of a tie, the bridge ID acts as the tiebreaker • Port Roles
There are four roles of ports under STP
1) Root
2) Designated
3) Non-Designated
4) Disabled
There are Five Port States
• Blocking
• Listening
• Learning
• Forwarding
• Disabled
RSTP
A variation of Spanning Tree Protocol (STP) designed to ensures a topology that doesn’t have loops.
IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w
Detection of root switch failure is done in 3 hello times STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × Hello times *(default: 3 times 2 seconds) or within a few milliseconds of a physical link failure RSTP maintains backup details regarding the discarding status of ports.
RSTP bridge port roles:
Root - A forwarding port that is the best port from Non root-bridge to Root bridge
Designated - A forwarding port for every LAN segment
Alternate - An alternate path to the root bridge. This path is different than using the root port.
Backup - A backup/redundant path to a segment where another bridge port already connects.
Disabled - Not strictly part of STP, a network administrator can manually disable a port
RSTP Topology Change Mechanism
When an RSTP bridge detects a topology change it performs the following actions. The RSTP Bridge starts the TC while time with a value equal to twice the hello time for all its non-edge designated ports and its root, port if necessary. The TC While time is the interval during which the RSTP Bridge actively informs the rest of the bridges in the network of a topology. It then flushes the MAC addresses associated with all non-edge ports. And as long as the TC While timer is running the BPDUs will be sent out of the port will have the TC bit set.
Bridge Identifier for PVRST+
Bridge Priority A 4-bit field still used to carry bridge priority
Extended System ID
Mac Address
Only four high-order bits of the 16 bit bridge priority field carry actual priority
Cisco STP Default Config
PVST+
PVRST+
MST
Config the Basic Parameters of PVRST+
conf t
spanning-tree mode rapid-pvst
spanning-tree vlan 2 root primary
spanning tree- vlan 3 root secondary
PortFast
Spanning Tree Portfast causes an interface configured as a layer 2 access port to enter the forwarding state immediately.
Conf t int fa 0/1 spanning-tree portfast
confirm with show spanning-tree interface Fastethernet 0/1.
Config the Basic Parameters of PVRST+ conf t spanning-tree mode rapid-pvst spanning-tree vlan 2 root primary spanning tree- vlan 3 root secondary
Multiple Spanning Tree
MST extends the IEEE 802.1w RST algorithm to multiple spanning trees. The main purpose of MST is to reduce the total number of spanning-tree instances.