STP basics & CST Flashcards
Define the BPDU.
Bridge protocol data unit. Used to build STP topologies and elect the root switch.
Switching loops are also known as ___.
Bridging loops.
What three issues can switching loops cause?
Frames can’t reach their destination, unnecessary strain is put on the CPU, more bandwidth than needed is used.
Briefly define a configuration BPDU.
BPDUs created by the root bridge. Used by all switches during an election process. Also carry STP timer info.
Define a BID.
Bridge ID Priority value. Carries a switch’s Priority value along with its MAC address. Lowest value becomes the root bridge.
How often is a config BPDU sent?
Every two seconds.
When does the root bridge election end?
Never. If a better BID is introduced into the network, the switch originating it will become the root bridge.
How many root ports can the root bridge have?
None. Root ports lead to the root bridge, so the root bridge cannot have root ports.
All ports on the root bridge are in ___.
Forwarding/designated mode.
In STP, a switch’s path cost to the root bridge is a ___ value.
Local value. It is not advertised to other switches.
Briefly define the root path cost.
The cumulative value of all the ports on the way to the root bridge. Faster the port, the lower the value. The root path cost is carried by the configuration BPDU, and is incremented as it passes through the network.
The root path cost increments as the configuration BPDU is ___.
Received.
Explain the steps for choosing the root port.
- Choose the port receiving the superior BPDU. 2. Choose the port with the lower root path cost. 3. If multiple ports have the same cost, choose the one with the lower sender BID. 4. If one port has not been chosen, choose the port with the lower sender port ID.
When running the “show spanning vlan x” command, under interface type, what does P2p Peer(STP) indicate?
The connected switch is running CST.
How can a switch tell what version of STP another switch is running?
Via the version number in the BPDU.
Briefly define the root path cost.
The cumulative value of all the ports on the way to the root bridge. Faster the port, the lower the value. The root path cost is carried by the configuration BPDU, and is incremented as it passes through the network.
Ports on the root bridge are ___.
Designated ports. They will always be forwarding.
For each segment, you need ___.
One designated port. Both ports in a segment will never be simultaneously blocking.
On a segment, which switch will have its port in designated mode?
The switch with the lower root path cost. If it’s a tie, the switch with the lower BID wins.
What are the root path costs for 10 Gbps, 1 Gbps, and 100 Mbps, respectively?
2, 4, and 19.
Is the physically shortest path always the logically shortest path?
No. Two 10 Gbps links are much logically shorter than one 100 Mbps link.
How could we create link per-VLAN load balancing?
By setting manual path costs per-VLAN, through the interface configuration mode.