Switches that are running STP use BPDUs to do what tasks?

- Elect a root bridge

 

- Determine the location of redundant paths

 

- Block certain ports to prevent loops

 

- Notify the network of topology changes

 

- Monitor the state of the spanning tree 

What are the 2 different types of BPDUs?

Configuration BPDUs and Topology Change Notification (TCN) BPDUs

Describe the use of a configuraion BPDU.

This type of BPSU is sent at periodic intervals by the root bridge on all its ports. These messages are generated only by the root bridge, which helps guarantee that there are no mismatch in the timers. The configuration BPDU is used to elect the root bridge and to keep the topology stable.

Describe the use of a Topology Change Notification (TCN) BPDU

A Topology Change Notification (TCN) BPDU is generated by the switch when it detects a topology change.

In a normal STP operation, what is the process flow of configuration BPDUs?

A bridge continuously receives configuration BPDUs from the root bridge on its root port, but it never sends out a configuration BPDU towards the root bridge. Note that the designated bridges or ports do send configuration BPDUs with the root bridge information, but never out of a port that is directly connected to the root bridge.

What are the 5 different states that a spanning tree port can operate in?

- Blocking - The interface does not participate in frame forwarding but listens to incoming BPDUs. The port does not learn MAC addresses of received frames

 

- Listening - The switch resolves the root and selects the root port, the designated port, and the nondesignated ports. The port does not learn the unicast address of any received frames.

 

- Learning - The L2 interface prepares to participate in frame forwarding. The port learns the MAC addresses of incoming frames in this state but does not forward the frames

 

- Forwarding - The L2 interface forwards frames. The port learns the source MAC address of received frames and forwards them appropriately based onthe destination MAC address

 

- Disabled - The L2 interface does not participate in spanning tree and does not forward frames.  

What are the 3 different timers carried in a BPDU frame?

- Hello timer -The time between each BPDU that is sent on a port by the root bridge and forwarded by other designated bridges. Default value is 2 seconds, but can be between 1 and 10

 

- Forward delay - The time that is spent in the listening and learning states. The default is 15 seconds, but is configurable between 4 and 30

 

-  Max Age - Controls the maximum length of time a bridge port saves its configuration BPDU information. The default is 20 seconds, but is configurable between 6 and 40

How many root bridges can there be on a single network?

Only one bridge acts as the root bridge in a given network per VLAN

What are the factors used by a switch to select its root port?

The root port is the lowest-cost path from the nonroot bridge to the root bridge. If 2 or more paths have equal costs, then the nonroot bridge selects the port that connects to the lowest bridge ID of the bridges that sent the BPDUs for the path. If also equal, then the nonroot bridge selects the port with the lowest port ID as the forwarding path.

What two components make up the bridge ID on a switch?

- Two-byte priority - By default, each switch defaults to 0x8000 as the priority

 

- Six-byte MAC address - This is the MAC address of the switch.

 

Remember that at the beginning, the bridge ID is equal to the root ID 

What are the 5 criteria used by a switch to determine the root ports and designated ports?

1. Lowest root bridge ID

 

2. Lowest path cost to the root bridge

 

3. Lowest sender bridge ID

 

4. Lowest port priority

 

5. Lowest port ID 

What events can occur that will trigger a switch to send out a TCN BPDU?

- A port in forwarding or listening state transitions to blocking (The case of a link failure)

 

- A port moves to forwarding state, and the bridge already has a designated port.

 

- The nonroot bridge receives a TCN on it designated port (propagation TCN is sent) 

Explain the use of Per VLAN Spanning Tree Plus.

With PVST+, the switched network maintains a separate spanning-tree instance for each VLAN. This provides load balancing on a per-VLAN basis, where a single switch can play different roles in each PVST+ instance.

How is a root bridge determined with the use of PVST+?

Each instance of PVST+ on a VLAN has a single root bridge. The root bridge propagates the spanning-tree information associated with that VLAN to all other switches in the network.

How are MAC addresses allocated when a switch is working with multiple VLANs?

A switch allocates MAC addresses sequentially, with the 1st MAC address in the range assigned to VLAN 1, the 2nd MAC address in the range to VLAN 2, and so on. The switch assigns the Supervisor Engine in-band (sc0) management interface the highest-order (last) MAC address in its range of allocated MAC addresses. 

How does MAC address allocation and reduction work with Catalyst switches?

Upon enabling MAC address reduction, the bridge ID that is stores in the spanning-tree BPDU contains an additional field called the system ID extension. The ID extension with the bridge priority functions as the unique identifier for a VLAN. The system ID extension is always the number of the VLAN or the MST instance.

What happens when you connect a Cisco switch to a non-Cisco switch through a 802.1Q trunk?

Because non-Cisco 802.1Q switches maintain only one instance of spanning-tree for all VLANs, the Cisco switch combines the spanning-tree instance of the 802.1Q native VLAN of the trunk with the spanning-tree instance of the non-Cisco 802.1Q switch.

What is the default mode for STP on Catalyst switches?

PVST+ is the default mode for STP on Catalyst switches.

What is the suggested root priority value to designate a root bridge?

The suggested root priority when designating a new root bridge is 4096

What is the recommended location for the root switch in regards to the Enterprise Composite Network model?

The root switch should be a switch locates in the building distribution submodule for network design using layer 3 in the distribution layer and layer 2 in the building access layer. Do not configure a building access switch as the spanning-tree primary root unless you are using layer 3 rouing in the building access submodule.

What are the ports states in regards to Rapid Spanning Tree Protocol(RSTP)?

- Discarding - Which is equivilant to blocking, disabled, and listening in regular STP

 

- Learning - Which is equivilant to learning in regular STP

 

- Forwarding - Which is equivilant to forwarding in regular STP 

What are the different port roles in RSTP?

- Root port - The root port is the closest port to the root bridge in terms of path cost. The root bridge is the only bridge in the network that does not have a root port.

 

- Designated port - The designated port is the port on a LAN segment with the best BPDU's to the root bridge

 

- Alternate port -  An alternate port is a port blocked from receiving root BPDUs from another bridge. The alternate port becomes the root port if the active root port fails.

 

- Backup port -  A backup port is a port blocked from receiving root BPDUs from the designated port on a shared LAN segment from the same bridge on which the port is located. 

 

- Disabled port - A disabled port has no role within the operation of spanning tree. 

How often does 802.1w send out its current configuration in BPDU messages?

With 802.1w, a bridge sends a BPDU with its current configuration every hello-time period, 2 seconds by default.

In RSTP, what do switches use as keep-alive messages?

In RSTP, transmissions of BPDUs act as keep-alive mechanisms between bridges. A bridge believes that it has lost connectivity to its neighboring root or designated bridge if it misses 3 BPDUs in a row.

What 2 new variables does 802.1w rely upon to achieve fast convergence on a port that 802.1D does not?

- Edge port - Ports that are connected directly to end workstations, which don't participate in spanning tree

 

- Link type -  Switches automatically derive the link type from the duplex mode of a port. A port in full duplex mode are point-to-point, which participate in spanning tree

What happens to an edge port when it receives a BPDU?

If an edge port receives a BPDU, it immediately loses its edge port status and becomes a normal spanning-tree port.

What happens to a switches bridging table when it receives a BPDU with the TC flag bit set?

A switch reduces its bridging-table aging time to forward-delay seconds, ensuring a relatively quick flushing of stale information.

With RSTP, what is the only thing that causes a switch to send out a topology change message?

Only non-edge ports that are moving to the forwarding state cause a topology change. Loss of connectivity does no generate a topology change, unlike 802.1D

When a RSTP bridge detects a topology change, how often does it sent out its configuration BPDUs?

The RSTP starts the TC While timer with a value equal to twice the hello time as it actively informs all non-edge designated ports and its root port of the change.

What does a RSTP switch do with its bridging table when it detects a topology change?

The RSTP switch flushes the MAC addresses associated with all non-edge ports

Describe the basic use of Multiple Spanning Tree (MST).

The main purpose of MST is to reduce the total number of spanning-tree instances to match the physical topology of the network. It allows for the building of multiple spanning trees over trunks by grouping and associating VLANs to spanning-tree instances.

What are the 3 attibutes that make up a single MST configuration, which all must match for a switch to be part of a common region?

- An alphanumeric configuration name (32 bytes)

 

- A configuration revision number (2 bytes)

 

- A 4096-element table that associates each of the potential 4096 VLANs supported on the chassis to a given instance. 

Explain the use of an Internal Spanning Tree instance (802.1s)

IST enables interopertability between 802.1 Q and 802.1s. The IST instance is capable of representing an entire MST region as a CST virtual bridge to everyone else. 

Describe the BackboneFast feature in STP.

BackboneFast reduces the default convergence time in situations where the root port is lost and the backup link leads through a different switch.

Describe the use of BPDU filtering.

BPDU filtering allows a Catalyst switch from sending BPDU on portfast-enabled interfaces.