Review - Chapter 4 Flashcards
STP
- Layer 2 protocol
* Provides network link redundancy while eliminating potential problems
STP Limitation
Convergence delay after a topology change
BPDUs (STP)
○ Used to determine current topology information and how to react if any devices are added/removed/changed in the topology
○ Sent every 2 seconds by default on all switch ports
STP Rules
○ One root bridge
○ One root port on each non-root switch
○ One designated port per LAN segment
○ Non designated ports are blocked
STP Timers
○ Hello time (2s)
○ Forward delay (15s)
○ Max age (20s)
Key Types of STP
○ STP (original IEEE 802.1D version) ○ Common Spanning Tree (CST) ○ PVST and PVST+ (Cisco proprietary) ○ Multiple Spanning Tree (MST) ○ Rapid Spanning Tree (RSTP) ○ RPVST+ (Cisco implementation of RSTP based on PVST+)
Root Bridge
Selected based on lowest Bridge ID (BID)
Bridge ID (BID)
Two parts:
○ Priority
○ Mac address
Root Port Factors
○ Path Cost
○ Lowest Sender BID
○ Lowest Port ID
Bridge Protocol Data Units (BPDUs)
- Used to exchange STP information, specifically for root bridge election and loop identification
- Sent every 2 seconds by default
Types of BPDUs
○ Configuration BPDUs
○ TCN (topology change notification) BPDUs
STP Port States
○ Blocking ○ Listening (15s) ○ Learning (15s) ○ Forwarding ○ Disabled
Per VLAN STP+ (PVST+)
• Runs one spanning-tree instance for each VLAN
• The original 16-bit bridge priority is split into two fields, resulting in the following components in the BID:
○ Bridge priority
○ Extended system ID
○ MAC address
Bridge priority
○ 4-bit field carrying bridge priority
○ Default is 32,768 (midrange)
○ Conveyed in discrete values in increments of 4096
Extended system ID
12-bit field carrying VLAN ID
MAC address
6-byte field with MAC address of the switch
Types of Topology Changes
- Direct topology change
- Indirect topology change
- Insignificant topology change
Direct Topology Change
Can be detected on an interface
Indirect topology change
Detected because no BPDUs are received from the root bridge
Insignificant topology change
Occurs if an event causes a switch to send out TCNs (no actual topology change occurs)
Rapid Spanning Tree Protocol (RSTP)
- 802.1W
* Speeds up the recalculations after topology change
RSTP Port Roles
- Root
- Designated
- Alternate
- Disabled
- Backup
RSTP Port States
- Discarding
- Learning
- Forwarding
RSTP Rapid Transition
Rapid transitions to forward state on:
• Edge ports
• Point-to-point links
RSTP Timers
Designated ports wait 2x forwarding delay before forwarding
UplinkFast
Fast uplink failover on access switch
BackboneFast
Fast convergence in distribution/core layer when STP change occurs
PortFast
Configures access port to transition directly to forwarding state
BPDU Guard
Disables PortFast-enabled port if a BPDU is received
BPDU Filter
Suppresses BPDUs on ports
Root Guard
Prevents external switches from becoming roots
Loop Guard
Prevents an alternate port from becoming the designated port if no BPDUs are received
Unidirectional Link Detection (UDLD)
Detects when a unidirectional link exists and shuts down the affected interface
UDLD Packets
Sent to an STP neighbor every 15s
Flex Links
Layer 2 availability feature which provides link redundancy with a convergence time of less than 50ms
Multiple Spanning Tree Protocol (MST)
• Main purpose is to reduce the total number of spanning-tree instances to match the physical topology of the network (Reduces the CPU cycles of a switch)
MST Configuration Attributes
○ An alphanumeric name (32 bytes)
○ A configuration revision number (2 bytes)
○ A 4096-element table that associates each VLAN to an instance
Extend System ID
12-bit field used in MST and carries the MST instance number
MST Regions
A group of switches must share the same configuration attributes to be part of a common MST region
STP Instances with MST
MST supports a number of instances but doesn’t send BPDUs for every active STP instance separately
Instance 0 (STP with MST)
The internal spanning tree (IST)
○ Designed to carry all STP-related information
○ MSTI information is carried in the IST’s BPDU in so-called M-Record fields (one for every active MSTI)
Potential STP Problems
- Duplex mismatch
- Unidirectional link failure
- Frame corruption
- Resource errors
- PortFast configuration error
