CCNP Switch Chapter 7

So Cisco doesn't recommend that you change the timer values... but if you want to improve convergence, what value(s) can you change? Where would you change it/them?

The default STP timer values are based on some assumptions about the size of the network and the length of the Hello Time. A reference model of a network having a diameter of seven switches derives these values. You can change the diameter if you need to to improve convergence by more accurately reflecting the true size of the physical network.

What is the designated port?

The designated port is the port on a network segment that is used to forward traffic to and from that segment

Is it the ingress port or the egress port of a switch that modifies the BPDU root path cost?

The ingress port modifies the BPDU root path cost

What is an Indirect Topology Change?

The link status at each switch stays up, but something between them has failed or is filtering traffic. This could be another device, such as a service provider's switch, a firewall, and so on. As a result, no data (including BPDUs) can pass between those switches.

What is the diameter of a network?

if you draw the STP topology, the diameter is the number of switches connected in series from the root bridge out to the end of any branch in the tree.

Which path cost(s) is/are included in the BPDU?

only the root path cost

Generally, what is the relationship between the bandwidth of a link and the link's cost?

the higher the bandwidth of a link, the lower the link's cost

What is the value cost of a switch link?

the path cost

What happens if two or more links have identical root path costs? What criteria are used to choose the root port?

1. Lowest root bridge ID 2. Lowest root path cost to root bridge 3. Lowest sender bridge ID 4. Lowest sender port ID

So are active redundant paths in Layer 2 environments allowed or desirable?

No you don't want them and they're not possible to make (unless you use VLANs to load balance)

Does a TCN BPDU carry data about a network topology change?

Nope, it just informs recipients that a change has occurred.

What is PVST?

Per-VLAN Spanning Tree operates a separate instance of STP for each individual VLAN. Because of its Cisco proprietary nature, PVST requires the use of Cisco Inter-Switch Link (ISL) trunking encapsulation between switches.

How do you describe the "Disabled" STP port state?

Ports that are administratively shut down by the network administrator, or by the system because of a fault condition, are in the Disabled state. This state is special and is not part of the normal STP progression for a port.

Why are STP timers important?

STP operates as switches send BPDUs to each other in an effort to form a loop-free topology. The BPDUs take a finite amount of time to travel from switch to switch. In addition, news of a topology change (such as a link or root bridge failure) can suffer from propagation delays as the announcement travels from one side of a network to the other. Because of the possibility of these delays, keeping the spanning-tree topology from settling out or converging until all switches have had time to receive accurate information is important.

If you have 2 switches in a network and Switch A has a port with a higher MAC address and a lower priority than Switch B, which will become the root bridge?

Switch A becomes the root bridge because the priority is lower and has more weight than the MAC address

What is a command you can use to determine what STP mode that a port (e.g., Fa0/1) on your switch is in?

Switch# show spanning interface fastethernet 0/1

How is a BID broken up?

- Bridge Priority (2 bytes): The priority or weight of a switch in relation to all other switches. - MAC Address (6 bytes)

What are the three traditional types of STP that are encountered in switched networks?

- CST - PVST - PVST+

What are the two kinds of BPDUs used with STP?

- Configuration BPDUs - TCN BPDUs

What was the difference in convergence time for a Direct Topology Change and an Indirect Topology Change?

- Direct Topology Change: 30 seconds... The total time that users on Catalyst C lost connectivity was roughly the time that port 1/2 spent in the Listening and Learning states. With the default STP timers, this amounts to about two times the Forward Delay period (15 seconds), or 30 seconds total. - Indirect Topology Change: 52 seconds... the time until the Max Age timer expired (20 seconds), plus the time until the next Configura- tion BPDU was received (2 seconds) on port 1/2, plus the time that port 1/2 spent in the Listening (15 seconds) and Learning (15 seconds) states.

What are the five STP states that a switch port goes through?

- Disabled - Blocking - Listening - Learning - Forwarding

What are the three different types of STP timers?

- Hello Time - Forward Delay - Max Age

How does a transparent bridge operate when it has not seen frames come in yet?

- It doesn't initial know each end device's location; so it listens to frames coming into each of its ports to figure out where each is located. - The bridge updates its bridging table when it receives a new MAC address - If it sees a broadcast address in the destination field, it floods the frame out all available ports (except the recipient port) - It floods unknown unicast frames

What are the positives and negatives about using PVST?

- Positives: A separate instance of STP for each individual VLAN allows the STP on each VLAN to be configured independently, offering better performance and tuning for specific conditions. Multiple spanning trees also make load balancing possible over redundant links when the links are assigned to different VLANs - Negative: Can't work with 802.1Q and can't interoperate with CST.

What are the positives and negatives about using CST?

- Positives: Having a single STP simplifies switch configuration and reduces switch CPU load during STP calculations - Negatives: Redundant links between switches will be blocked with no capability for load balancing. Conditions also can occur that would cause CST to mistakenly enable forwarding on a link that does not carry a specific VLAN, whereas other links would be blocked.

What value can a BID priority field have and what is its default?

- Range: 0 to 65,535 - Default: 32,768

Explain what happens to create a bridging loop using the example network from the previous notecard (assume empty bridging tables and that a frame is being sent from PC-1 to PC-4)?

- Step 1: Both switches receive PC-1s frame. Each switch records PC-1's MAC address and its port - Step 2: Because it's an unknown unicast, both switches flood - Step 3: PC-4 receives two copies of the frame. Both switches hear the floods one another. - Step 4: The switches have to relearn PC-1's location. Both flood... then the process repeats infinitely

How many root ports do you have on a root switch?

0

What is the root path cost of a BPDU coming from the root bridge?

0, because its ports sit directly on the root bridge

What is the multicast address of STP?

01-80-c2-00-00-00

How many root ports do you have on a non-root switch?

1

How would you explain the sequence of events during the Direct Topology Change shown in the previous notecard (Part 1)?

1. Catalyst C detects a link down on its port 1/1; Catalyst A detects a link down on its port 1/2. 2. Catalyst C removes the previous "best" BPDU it had received from the root over port 1/1. Port 1/1 is now down so that BPDU is no longer valid. 3. The root bridge, Catalyst A, sends a Configuration BPDU with the TCN bit set out its port 1/1. This is received and relayed by each switch along the way, informing each one of the topology change.

How would you explain the sequence of events during the Indirect Topology Change shown in the previous notecard (Part 1)?

1. Catalysts A and C both show a link up condition; data begins to be filtered elsewhere on the link. 2. No link failure is detected, so no TCN messages are sent. 3. Catalyst C already has stored the "best" BPDU it had received from the root over port 1/1. No further BPDUs are received from the root over that port. After the Max Age timer expires, no other BPDU is available to refresh the "best" entry, so it is flushed. Catalyst C now must wait to hear from the Root again on any of its ports.

How is the root path cost value determined?

1. The root bridge sends out a BPDU with a root path cost value of 0. 2. When the next-closest neighbor receives the BPDU, it adds the path cost of its own port where the BPDU arrived. 3. The neighbor sends out BPDUs with this new cumulative value as the root path cost. 4. The root path cost is incremented by the ingress port path cost as the BPDU is received at each switch down the line.

How would you explain the sequence of events during the Direct Topology Change shown in the previous notecard (Part 2)?

4. Catalysts B and C receive the TCN message. The only reaction these switches take is to shorten their bridging table aging times to the Forward Delay time 5. Catalyst C basically just sits and waits to hear from the root bridge again. The Config BPDU TCN message is received on port 1/2, which was previously in the Blocking state. This BPDU becomes the "best" one received from the root, so port 1/2 becomes the new root port. Catalyst C now can progress port 1/2 from Blocking through the Listening, Learning, and Forwarding states.

How would you explain the sequence of events during the Indirect Topology Change shown in the previous notecard (Part 2)?

4. The next Configuration BPDU from the root is heard on Catalyst C port 1/2. This BPDU becomes the new "best" entry, and port 1/2 becomes the root port. Now the port is progressed from Blocking through the Listening, Learning, and finally Forwarding states.

What IEEE standard is STP?

802.1D

What is a Direct Topology Change?

A direct topology change is one that can be detected on a switch interface. For example, if a trunk link suddenly goes down, the switch on each end of the link can immediately detect a link failure. The absence of that link changes the bridging topology, so other switches should be notified.

What's an STP BID?

A bridge ID

What is a disastrous layer 2 situation that can saturate network segments and bring hosts on those segments to a halt?

A bridging loop

What is an unknown unicast frame?

A frame that arrives at a switch whose destination is unknown by the switch

How do you describe the "Listening" STP port state?

A port is moved from Blocking to Listening if the switch thinks that the port can be selected as a root port or designated port. In other words, the port is on its way to begin forwarding traffic. In the Listening state, the port still cannot send or receive data frames. However, the port is allowed to receive and send BPDUs so that it can actively participate in the Spanning Tree topology process. Here, the port finally is allowed to become a root port or designated port because the switch can advertise the port by sending BPDUs to other switches. If the port loses its root port or designated port status, it returns to the Blocking state.

When does STP determine that a topology change has occurred?

A topology change occurs when a switch either moves a port into the Forwarding state or moves a port from the Forwarding or Learning states into the Blocking state. In other words, a port on an active switch comes up or goes down.

What is the maximum number of root ports that a Catalyst switch can have? a. 1 b. 2 c. Unlimited d. None

A: 1

How do you describe the "Learning" STP port state?

After a period of time called the Forward Delay in the Listening state, the port is allowed to move into the Learning state. The port still sends and receives BPDUs as before. In addition, the switch now can learn new MAC addresses to add to its address table. This gives the port an extra period of silent participation and allows the switch to assemble at least some address information. The port cannot yet send any data frames, however.

How do you describe the "Blocking" STP port state?

After a port initializes, it begins in the Blocking state so that no bridging loops can form. In the Blocking state, a port cannot receive or transmit data and cannot add MAC addresses to its address table. Instead, a port is allowed to receive only BPDUs so that the switch can hear from other neighboring switches. In addition, ports that are put into standby mode to remove a bridging loop enter the Blocking state.

How do you describe the "Forwarding" STP port state?

After another Forward Delay period of time in the Learning state, the port is allowed to move into the Forwarding state. The port now can send and receive data frames, collect MAC addresses in its address table, and send and receive BPDUs. The port is now a fully functioning switch port within the spanning-tree topology.

What is the default "hello" time for IEEE 802.1D? a. 1 second b. 2 seconds c. 30 seconds d. 60 seconds

B: 2 seconds

Which of the following is the Spanning Tree Protocol defined in the IEEE 802.1Q standard? a. PVST b. CST c. EST d. MST

B: CST

What mechanism is used to set STP timer values for all switches in a network? a. Configuring the timers on every switch in the network. b. Configuring the timers on the root bridge switch. c. Configuring the timers on both primary and secondary root bridge switches. d. The timers can't be adjusted.

B: Configuring the timers on the root bridge switch.

Configuration BPDUs are originated by which of the following? a. All switches in the STP domain b. Only the root bridge switch c. Only the switch that detects a topology change d. Only the secondary root bridge when it takes over

B: Only the root bridge switch

If all switches in a network are left at their default STP values, which one of the following is not true? a. The root bridge will be the switch with the lowest MAC address. b. The root bridge will be the switch with the highest MAC address. c. One or more switches will have a bridge priority of 32,768. d. A secondary root bridge will be present on the network.

B: The root bridge will be the switch with the highest MAC address.

What are BPDUs?

Bridge Protocol Data Units (BPDUs) are frames that exchange STP data messages. The point of BPDUs is to work toward the goal of electing reference points as a foundation for a stable spanning-tree topology.

If a switch has 10 VLANs defined and active, how many instances of STP will run using PVST+ versus CST? a. 1 for PVST+, 1 for CST b. 1 for PVST+, 10 for CST c. 10 for PVST+, 1 for CST d. 10 for PVST+, 10 for CST

C: 10 for PVST+, 1 for CST

How is a bridging loop best described? a. A loop formed between switches for redundancy b. A loop formed by the Spanning Tree Protocol c. A loop formed between switches where frames circulate endlessly d. The round-trip path a frame takes from source to destination

C: A loop formed between switches where frames circulate endlessly

Which of these is one of the parameters used to elect a root bridge? a. Root path cost b. Path cost c. Bridge priority d. BPDU revision number

C: Bridge priority

What happens to a port that is neither a root port nor a designated port? a. It is available for normal use. b. It can be used for load balancing. c. It is put into the Blocking state. d. It is disabled.

C: It is put into the Blocking state.

Which of these is the single most important design decision to be made in a network running STP? a. Removing any redundant links b. Making sure all switches run the same version of IEEE 802.1D c. Root bridge placement d. Making sure all switches have redundant links

C: Root bridge placement

What is CST?

Common Spanning Tree uses a single instance of STP that encompasses all VLANs. All CST BPDUs are transmitted over trunk links using the native VLAN with untagged frames.

MAC addresses can be placed into the CAM table, but no data can be sent or received if a switch port is in which of the following STP states? a. Blocking b. Forwarding c. Listening d. Learning

D: Learning

By default, how often are BPDUs sent out all switch ports?

Every 2 seconds

What benefits does PVST+ provide?

It allows devices to interoperate with both PVST and CST. PVST+ can communicate directly with PVST by using ISL trunks. To com- municate with CST, however, PVST+ exchanges BPDUs with CST as untagged frames over the native VLAN. So it supports the following types of switches: - Catalyst switches running PVST - Catalyst switches running PVST+ - Switches running CST over 802.1Q

What switch does a switch assume is the root bridge when it first boots up?

It assumes itself to be the root bridge. It has to look at the BPDUs it receives to see if there is a better root bridge on the network.

What is the purpose of Spanning Tree Protocol (STP)?

It is designed to monitor and control the Layer 2 network so that a loop-free topology is maintained. It provides network link redundancy so that a Layer 2 switched network can recover from failures without intervention in a timely manner

What criteria does a switch use to determine if there's a better root bridge on its network?

It looks at the Configuration BPDUs it receives and checks their Root BID field to see if there is a BID lower than the one that it already has. If it does, it starts to send Configuration BPDUs with this lower BID as its Root BID.

If a host receives two copies of the same frame, is it bad?

It's not a good sign that the network is working well, but it's not disastrous either

What is the root path cost?

It's the cumulative cost of all the links leading to the root bridge

You have the following three switches with their BIDs: - Joe's Switch: 32768.00-13-01-2b-c0-0a - Rob's Switch: 32768.9e-4c-b4-9e-00-0b - Tom's Switch: 32768.10-12-f0-b9-54-0c Which will become the root bridge?

Joe's Switch

Why are TCN BPDUs sent?

TCN BPDUs are sent to signal the topology change and cause all other bridges to shorten their bridge table aging times from the default (300 seconds) to the Forward Delay value (default 15 seconds). This condition causes the learned locations of MAC addresses to be flushed out much sooner than they normally would, easing the bridge table corruption that might occur because of the change in topology.

What is the purpose of the Forward Delay timer?

The Forward Delay is the time interval that a switch port spends in both the Listening and Learning states. The default value is 15 seconds.

What is the purpose of the STP Hello Time timer?

The Hello Time is the time interval between Configuration BPDUs sent by the root bridge. The Hello Time value configured in the root bridge switch determines the Hello Time for all nonroot switches because they just relay the Configuration BPDUs as they are received from the root. However, all switches have a locally configured Hello Time that is used to time TCN BPDUs when they are retransmitted. The IEEE 802.1D standard specifies a default Hello Time value of 2 seconds.

Where does the MAC address of a switch port come from? Can a user change it to whatever they want?

The MAC address can come from one of the following places: - the Supervisor module - the backplane - a pool of 1,024 addresses that are assigned to every supervisor or backplane In any event, this address is hard-coded and unique, and the user cannot change it.

What is the purpose of the Max Age timer?

The Max Age timer is the time interval that a switch stores a BPDU before discarding it. While executing the STP, each switch port keeps a copy of the "best" BPDU that it has heard. If the switch port loses contact with the BPDU's source (no more BPDUs are received from it), the switch assumes that a topology change must have occurred after the Max Age time elapsed and so the BPDU is aged out. The default Max Age value is 20 seconds.

What is the frame of reference used with STP to agree on a loop-free layer 2 topology?

The ROOT BRIDGE

How does STP deal with redundant layer 2 paths, generally?

The algorithm chooses a reference point in the network and calculates all the redundant paths to that reference point. When redundant paths are found, the spanning-tree algorithm picks one path by which to forward frames and disables, or blocks, forwarding on the other redundant paths.

What is the assumption that switches operate on when they attempt to determine where hosts are located?

The assumption switches make is that the host is in the network segment that is attached to the switch port the frames from that device have come from

What is a bridging loop?

The process of forwarding a single frame around and around between two switches is known as a bridging loop. Neither switch is aware of the other, so each happily forwards the same frame back and forth between its segments.

How do switches handle TCN BPDUs?

The switch continues sending TCN BPDUs every Hello Time interval until it gets an acknowledgment from its upstream neighbor. As the upstream neighbors receive the TCN BPDU, they propagate it on toward the root bridge and send their own acknowledgments. When the root bridge receives the TCN BPDU, it also sends out an acknowledgment. However, the root bridge sets the Topology Change flag in its Configuration BPDU, which is relayed to every other bridge in the network.

Why are some switches considered "transparent" bridges?

These switches don't modify the frames that go through them

How are unknown unicast frames handled by a switch?

They are flooded just like a broadcast frame

Can the root path cost in a BPDU be modified as it is sent through other switches?

Yes, as the BPDU with the root path cost travels toward the root bridge, the switches that forward it along the way update the root path cost in the frame to make it cumulative

What is the only way to end a bridging loop?

You have to physically break the loop by either disconnecting switch or shutting it down

What is a TCN BPDU?

a Topology Change Notification (TCN) BPDU is an STP frame used to announce changes in the network topology

What is the fundamental reason why a switch port would be allowed into the Forwarding state?

a switch port is allowed into the Forwarding state only if no redundant links (or loops) are detected and if the port has the best path to the root bridge as the root port or designated port.

What is an STP configuration BPDU?

an STP frame used for spanning-tree computation