Term
|
Definition
| A protocol that allows switches to calculate a loop-free subset of the physical topology. |
|
|
Term
| name 2 issues that L2 loops can cause. |
|
Definition
1. broadcast storms.
2. bridge table corruption. |
|
|
Term
| describe the stp bridge ID. |
|
Definition
8-byte field with 2 sub-fields:
1. Bridge Priority (2 bytes) - default 32768 - 4096 increments allow for VLAN ID in PVST.
2. MAC (backplane/sup). |
|
|
Term
|
Definition
Bridges use cost to evaluate how close they are to other bridges.
Originally based on BW, but now uses non linear scale due to BW increases.
Lower is better. |
|
|
Term
| what are the legacy path costs for some common link speeds? |
|
Definition
10M - 100
100M - 19
1G - 4
10G - 2 |
|
|
Term
| describe the 4 step decision sequence used to create a loop-free logical topology. |
|
Definition
1. lowest Root BID
2. Lowest Path Cost to Root Bridge
3. Lowest sender BID
4. Lowest Port ID |
|
|
Term
| describe the exchange of BPDUs. |
|
Definition
| A switch compares all BPDUs received on a port with the BPDU that would be sent on that port. The 4 step sequence is carried out for each BPDU to see if it is more attractive than the BPDU currently saved for that port. |
|
|
Term
| what are the 2 types of BPDU? |
|
Definition
configuration BPDUs
topology change notification (TCN) BPDUs |
|
|
Term
| what are the 3 steps of initial stp convergence? |
|
Definition
1. elect one root bridge
2. elect root ports
3. elect designated ports |
|
|
Term
| describe the 4 fields of a basic bpdu layout. |
|
Definition
root BID
root path cost
sender BID
port ID |
|
|
Term
| when are stp path costs incremented? |
|
Definition
| as BPDUs are received on a port. |
|
|
Term
| what is the difference between path cost and root path cost? |
|
Definition
path cost is the value assigned to a single port.
Root path cost is the cumulative path cost to the Root. |
|
|
Term
| describe the 5 stp states. |
|
Definition
forwarding - sending/receiving user data
learning - building bridging table i.e. CAM table.
listening - building active topology - 3 convergence steps
blocking - receives BPDUs only
disabled - admin down |
|
|
Term
| list the common stp timers. |
|
Definition
hello time - 2 secs default
forward delay - 15 secs (applies to listening and learning)
max age - 20 secs default - (time BPDU stored) |
|
|
Term
| under what conditions would a bridge transition to the listening state without waiting for the max-age timeout? |
|
Definition
| if the root port were to go down. |
|
|
Term
| what is the default stp convergence time? |
|
Definition
|
|
Term
| if a non-root bridge had modified timers, how would this affect a stp converged network? |
|
Definition
| It wouldn't. Local timers are not utilised unless the local switch becomes the root. |
|
|
Term
| how do TCN bpdu's differ from configuration bpdu's? |
|
Definition
configuration BPDUs originate from the Root and flow to the leaves of the tree.
TCN BPDUs flow upstream towards the Root to alert it about a topology change. |
|
|
Term
| describe the difference between the BID and source MACs within a BPDU frame. |
|
Definition
BID contains the MAC of the backplane or sup module.
The source MAC defines the sending interface. |
|
|
Term
| what destination mac is a bpdu sent to? |
|
Definition
|
|
Term
| what sap is used for stp? |
|
Definition
|
|
Term
| what bpdu field defines the type? |
|
Definition
Type field:
0x00 - configuration BPDU
0x80 - TCN BPDU |
|
|
Term
| under what conditions would a bridge originate a TCN BPDU? |
|
Definition
1. It transitions a port into the Forwarding state and it has at least one Designated Port.
2. It transitions a port from either the Forwarding or Learning states to the Blocking state |
|
|
Term
| describe the 7 step TCN process? |
|
Definition
1. bridge originates a TCN BPDU.
2. upstream bridge receives and processes a bpdu on a designated port.
3. the upstream bridge ACKs the TCN BPDU by setting the TC ACK flag in the next downstream BPDU.
4. upstream bridge forwards TCN BPDU out its root port.
5. steps 2-4 are repeated until the TCN BPDU reaches the root.
6. root sets the TC ACK and TC flags in the next configuration BPDU it sends.
7. root continues to set TC flag for Forward Delay + Max Age secs (default 35 secs). This instructs all bridges to shorten their bridge table aging process from 300 to Forward Delay secs (default 15). This ensures that MACs are re-learned quickly. |
|
|
Term
| what is the aim of the TCN process? |
|
Definition
| to reduce the failover time from 5 mins to 50 secs. |
|
|
Term
| How does a TCN affect CAM aging in legacy STP. |
|
Definition
| Switches will set the CAM aging timer to "forward delay" seconds when they receive a BPDU with the TC flag set. |
|
|
Term
| After receiving a TCN, for how long will a root switch set the TC flag in its outgoing BPDUs? |
|
Definition
| "forward delay" + "max age" seconds. |
|
|
Term
| How can you confirm that a TC event is occurring? |
|
Definition
sh spann
the CAM aging timer should have been reduced to "forward delay" seconds. |
|
|
