Chap 2 - Spanning Tree Protocol Flashcards
What happens on a port in Disabled state?
It is admin’ed down.
What happens on a port in Blocking state?
- BPDUs: Receive only
- Modify MAC table: No
What happens on a port in Listening state?
- BPDUs: Sends and receives
- Modify MAC table: No
What happens on a port in Learning state?
- BPDUs: Send and receive
- Modify MAC table: Yes
What happens on a port in Forwarding state?
- BPDUs: Sends and receives
- Modify MAC table: Yes
- Forwarding: Yes
What are the 3 802.1D port types?
- Root Port (RP)
- Designated Port (DP)
- Blocking Port
How many RPs are there on a switch?
No more than 1 RP per VLAN
What is the Destination MAC of a BPDU?
01:80:c2:00:00:00
What are the 2 types of BPDUs?
- Configuration BPDU
- Topology Change Notification (TCN) BPDU
What 4 things does a Configuration BPDU do?
- Identifies the Root Bridge
- Identifies RP
- Identifies DP
- Identifies Blocking Ports
What are the 7 fields in a Configuration BPDU?
- STP Type
- Root Path Cost
- Root Bridge Identifier
- Local Bridge Identifier
- Hello Timer
- Forward Delay Timer
- Max Age
What does a Topology Change Notification (TCN) BPDU signify?
Communicates changes in the Layer 2 topology to other switches.
What is the Max Age time?
If a switch loses communication with the generating switch of a BPDU the Max Age time is the length of time a switch considers that BPDU to be valid.
What is the default Max Age time?
20 seconds
How is the Root Bridge determined?
- The bridge with the lowest Priority
- if there is a tie then the bridge with the lowest MAC address
What is the default Priority?
32768 + sys-id-ext
What is the sys-id-ext?
A number equal to the VLAN number.
What is the root path cost?
The cumulative outbound interface cost to get to the root bridge.
How does a local switch calculate its root path cost?
It takes the root path cost advertised by its neighbor in a BPDU and then adds the interface cost of the local interface that BPDU was received on.
How does a switch initially identify its Root Port (RP) along with 4 tie-breakers?
- Port with lowest path cost to the Root
- Port who’s neighbor has the lowest System Priority
- Port whose neighbor has the lowest base MAC address
- Port whose neighbor has the lowest port priority
- Port whose neighbor has the lowest interface index number
What can a Designated port NOT be and how is the DP initially determined along with 2 tie-breakers?
- Cannot be a RP
- Switch with lowest root path cost gets the DP
- Switch with lowest System Priority wins
- Switch with lowest MAC address wins
What is the difference between a point-to-point port and a point-to-point edge port?
Point-to-point Edge has Portfast
What happens with a switch receives a BPDU with the TCN bit flipped on?
- All switches change their MAC address timer to the forward delay time (def 15 sec)
- This causes all MAC entries that aren’t heard during that 15 seconds to be flushed.
What is the difference between a Direct Link Failure and an Indirect Link Failure?
- When a direct link failure occurs the links interfaces go down.
- When an Indirect Link Failure occurs the interfaces are still up but data is impaired or corrupted.
By default how long is a MAC address in the MAC table before it times out?
300 seconds
What do non-root switches do when they receive a Configuration BPDU with the TCN flag set?
They change their MAC address timeout from the default 300 seconds to the Forward Delay time (15 seconds) to flush the MAC table.
What are the 3 port states in RSTP?
- Discarding
- Learning
- Forwarding
In RSTP what is the Discarding State equivalent to in 802.1D STP?
Discarding combines the 802.1D port states of Disabled, Blocking, and Listening
If a switch running RSTP detects a device connecting to it what does it do?
- Attempts a RSTP handshake.
- If the device does not respond the switch enables 802.1D on that port causing the port to go through Listening and Learning states.
What are the 4 RSTP Port Roles?
- Root Port (RP)
- Designated Port (DP)
- Alternate Port
- Backup Port
What does an Alternate Port do?
Is normally in blocking state but if the switch’s RP goes down then it provides alternate connectivity toward the Root Bridge via a different switch
What does a Backup Port do and when would you see one?
- Exists only when multiple links connect between same switches but a different port on the same switch was elected as the DP.
- Normally in blocking state but if the DP goes down the backup port takes over as DP
What are the 3 RSTP Port Types and how are they defined?
- Edge Port - port running portfast
- Root Port - port with lowest cost to the root
- Point-to-point Port - port running full duplex
What is the difference between 802.1D and CST
- No difference. They are the same.
- Both provide for only one STP instance. The opposite of PVST+.
Which ‘show’ command identifies the Root Bridge?
show spanning-tree root
What ‘show’ command will display the STP state (RP, DP, Blocking)
show spanning-tree vlan (vlan_id)
Display spanning-tree detail about a specific port.
show spanning-tree int (interface_ID) detail
Display information about TCNs.
show spanning-tree vlan (vlan_id) detail
What are the short mode port costs for 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps, 20 Gbps, 100 Gbps, 1 Tbps, 10 Tbps?
- 10 Mbps - 100
- 100 Mbps - 19
- 1 Gbps - 4
- 10 Gbps - 2
- 20 Gbps and higher - 1
What are the long mode port costs for 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps, 20 Gbps, 100 Gbps, 1 Tbps, 10 Tbps?
- 10 Mbps - 2,000,000
- 100 Mbps - 200,000
- 1 Gbps - 20,000
- 10 Gbps - 2,000
- 20 Gbps - 1,000 (half of 10 Gbps)
- 100 Gbps - 200
- 1 Tbps - 20
- 10 Tbps - 2
What are the 5 factors when choosing the root port?
- Interface with the lowest root path cost
- Interface with the neighbor that has the lowest system priority
- Interface with the neighbor that has the lowest MAC address
- Multiple links to same switch - neighbor’s interface with lowest port priority
- Multiple links to same switch - neighbor’s interface that has lowest index
How is the Designated Port chosen?
- Must not be a root port
- Switch with the lowest root path cost has the DP, if there is a tie then:
- Switch with lowest system priority has the DP, if there is a tie then:
- Switch with lowest MAC address has the DP
- If multiple connections to same switch then neighbor interface with lowest priority has the DP
- If multiple connections to same switch then neighbor interface with lowest interface index has the DP
When choosing the RP or DP what are the 2 tie-breakers when there are multiple connections to the same switch?
- choose the interface with the lowest port priority, if there’s a tie then
- choose the interface with the lowest interface index
What is the RSTP negotiation process?
- sw1 sends bpdu with proposal bit set
- sw2
- moves all its non-edge ports to blocking
- sends proposal bpdu’s to non-edge switches
- replies to sw1 with bpdu with proposal bit cleared and agreement bit set
- moves its port to sw1 to forwarding state
- sw1 upon receiving agreement bpdu from sw2 moves its port to forwarding
What are the 6 802.1D port states?
- Disabled
- Blocking
- Listening
- Learning
- Forwarding
- Broken
