Implementing Spanning Tree Describing the STP
Transparent Bridging
A switch has the same characteristics as a transparent bridge.
What Is a Bridge Loop?
Bridge loops can occur any time there is a redundant path or loop in the bridge network.
Preventing Bridge Loops
Bridge loops can be prevented by disabling the redundant path.
802.1D STP
?Configured root switch ?Redundant switch links ?Optimal path selection
Bridge Protocol Data Unit
BPDUs provide for the exchange of information between switches.
The STP Root Bridge
?Reference point
?One root per VLAN ?Maintains topology ?Propagates timers
Root Bridge Selection Criteria
Extended System ID in Bridge ID Field Bridge ID
Without the
Extended
System ID
Bridge ID with
the Extended
System ID
802.1D 16-bit Bridge Priority Field Using the Extended System ID
?Only four high-order
bits of the 16-bit Bridge
Priority field carry
actual priority.
?Therefore, priority can
be incremented only in steps of 4096, onto which will be added the VLAN number.?Example:
For VLAN 11: If the priority is left at default, the 16-bit Priority field will hold 32768 + 11 = 32779. Priority Values (Hex)Priority Values (Dec) 00
14096
28192
..
..
8 (default)32768
..
..
F61440
4 bits12 bits
Priority VLAN Number
20 215
Configuring the Root Bridge
Switch(config)#spanning-tree vlan 1 root primary
?This command forces this switch to be the root.
Switch(config)#spanning-tree vlan 1 root secondary
?This command configures this switch to be the secondary root. Or
Switch(config)#spanning-tree vlan 1 priority priority ?This command statically configures the priority (in increments of 4096).
Root Bridge Selection
Which switch has the lowest bridge ID?
Spanning Tree Operation ?One root bridge
per network
?One root port per
nonroot bridge
?One designated
port per segment ?Nondesignated
ports are blocking
Spanning Tree Port States
Spanning tree transitions each port through several different states.
Local Switch Root Port Election
Spanning Tree Path Cost
Link Speed Cost (Revised IEEE Spec)Cost (Previous IEEE Spec) 10 Gbps21
1 Gbps41
100 Mbps1910
10 Mbps100100
Spanning Tree Protocol
Root Port Selection
?SW X is the root bridge.
?SW Y needs to elect a root port.
?Which port is the root port on SW Y?
?Fast Ethernet total cost = 0 + 19.
?Ethernet total cost = 0 + 100.
STP Designated Port Selection
?Switch X is the root bridge.
?All ports on the root bridge are designated ports because they have a path cost of 0.
?Because the Ethernet segment has a path cost of 100, switch Y will block on that port.
?Do all segments have a designated port?
Example: Layer 2 Topology Negotiation
Enhancements to STP
?PortFast
?Per VLAN Spanning Tree+ (PVST+)
?Rapid Spanning Tree Protocol (RSTP)
?Multiple Spanning Tree Protocol (MSTP)
–MSTP is also known as Multi-Instance Spanning Tree
Protocol (MISTP) on Cisco Catalyst 6500 switches and above
?Per VLAN Rapid Spanning Tree (PVRST)