LAN Switching And Wireless - Chapter 5 Exam

In a spanning tree topology, the first step in the process of convergence is the election of the root bridge. The root bridge is the central point of the spanning tree and is responsible for making decisions about the forwarding path of data. By electing a root bridge, the network can establish a hierarchical structure and determine the designated ports for each segment, which helps in preventing loops and ensuring efficient data transmission.

A switch uses bridge priority and base MAC address to select the root bridge. Bridge priority is a value assigned to each switch, and the switch with the lowest bridge priority becomes the root bridge. The base MAC address is also used in the selection process, with the switch having the lowest base MAC address being chosen as the root bridge. These two criteria help determine the hierarchy and path selection in a spanning tree network.

When PVST+ was developed, the Bridge ID was modified to include the VLAN ID information. The Bridge ID is a unique identifier for each bridge in a spanning tree network. By including the VLAN ID in the Bridge ID, PVST+ allows for the creation of separate spanning tree instances for each VLAN. This ensures that each VLAN has its own independent spanning tree, improving network efficiency and reducing the risk of broadcast storms.

STP and RSTP differ in their behavior regarding the convergence of the network and the forwarding of designated ports. STP waits for the network to converge, ensuring that all bridges in the network have exchanged BPDU messages and determined the root bridge, before it places ports into the forwarding state. On the other hand, RSTP places designated ports into the forwarding state immediately, without waiting for the network to converge. This allows RSTP to provide faster convergence and reduce the time it takes for a port to become operational.

If STP (Spanning Tree Protocol) is not enabled, the ARP request sent by the server will not be able to reach the default gateway. Without STP, there will be no loop prevention mechanism in the network. As a result, both Switch_A and Switch_B will continuously flood the ARP request message onto the network, causing it to circulate indefinitely.

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By setting the switch priority to a smaller value than that of the other switches in the network, the network administrator can influence which STP switch becomes the root bridge. The switch with the lowest priority value will become the root bridge.

In the learning state of STP (Spanning Tree Protocol), a port records MAC addresses but does not forward user data. This is because in the learning state, the port is still in the process of learning the network topology and building its MAC address table. Once the learning process is complete, the port transitions to the forwarding state, where it starts forwarding user data.

BIDs (Bridge IDs) in a spanning tree topology consist of a bridge priority and MAC address. They are used by the switches in the topology to elect the root bridge. The bridge with the lowest BID becomes the root bridge. The BIDs are not sent out by the root bridge only after the inferior BPDUs (Bridge Protocol Data Units) are sent, and it is not true that only the root bridge sends out a BID. The BID is not determined by the processor speed of the switch.

In the Rapid Spanning Tree Protocol (RSTP), the role assigned to the forwarding port elected for every Ethernet LAN segment that links two switches is "designated". This means that the port has been selected as the primary port for forwarding data packets on that particular segment. It is responsible for transmitting and receiving network traffic efficiently, ensuring a stable and reliable network connection between the switches.

In a converged network with one spanning tree, there will be one root bridge per network. The root bridge is the central point of the network and all other bridges or switches in the network will have a designated role in relation to the root bridge. Additionally, there will be one root port per non-root bridge. The root port is the port on a non-root bridge that provides the shortest path to the root bridge. These two elements are essential in ensuring a stable and efficient network topology.

The max-age timer is a feature of the Spanning-Tree Protocol that contributes to the time it takes for a switched network to converge after a topology change. This timer determines the maximum age of a BPDU (Bridge Protocol Data Unit) before it is considered invalid. When a topology change occurs, the max-age timer ensures that the network waits for the specified time before considering the old BPDU information as invalid and initiating the convergence process.

The forward delay is another feature that contributes to the convergence time. This delay is the amount of time a switch waits before transitioning a port from blocking to forwarding state. During this delay, the switch listens for BPDUs to ensure that no loops are present in the network. The forward delay allows the network to stabilize and ensure that all switches have updated information before allowing traffic to flow through the newly enabled port.

Both the max-age timer and the forward delay help in preventing loops and ensuring a stable network topology, which ultimately contribute to the time it takes for a switched network to converge after a topology change.

PortFast is a Cisco proprietary feature that allows an access port to immediately transition from a blocking state to a forwarding state, bypassing the usual listening and learning states. This helps to prevent and eliminate bridging loops more quickly, improving network convergence. However, enabling PortFast on trunks that connect to other switches can cause issues as it can lead to loops and negatively affect DHCP services.

The three link types defined for Rapid Spanning-Tree Protocol are shared, edge-type, and point-to-point. The shared link type refers to a link that is shared by multiple VLANs. The edge-type link is a link that connects directly to an end device, such as a host or a server. The point-to-point link is a link that connects two switches directly without any other devices in between.

The command "spanning-tree vlan 1 root primary" on S4 will cause Gi0/2 on S3 to transition to a root port. This command sets the priority of the root bridge for VLAN 1 to the lowest value, making S4 the root bridge. As a result, S3 will compare the priorities of its own interfaces, and since Gi0/2 has the lowest priority, it will become the root port.

The given output suggests that the priority was statically configured to identify the root. This means that a specific switch has been designated as the root bridge in the spanning tree protocol (STP) network. By configuring the priority, the network administrator ensures that this switch will always be elected as the root bridge, which helps in determining the best path for forwarding traffic in the network.

BPDUs (Bridge Protocol Data Units) are used by switches in two ways. Firstly, they are used to identify the shortest path to the root bridge. BPDUs contain information about the bridge ID, path cost, and port ID, which allows switches to determine the most efficient path to the root bridge. Secondly, BPDUs are used to determine which ports will forward frames as part of the spanning tree. By exchanging BPDUs, switches can establish which ports should be in forwarding mode and which should be in blocking mode, preventing loops in the network.

When an RSTP edge port receives a BPDU, it immediately loses its edge status and becomes a normal spanning-tree port. This is because the purpose of an edge port is to connect to end devices that do not participate in spanning tree protocol. When a BPDU is received, it indicates the presence of another bridge in the network, so the edge port can no longer be considered as an edge port and must transition to a normal spanning-tree port.

In a Layer 2 switched environment with redundant connections between switches, two statements are true about the default operation of STP. Firstly, decisions on which port to block when two ports have equal cost depend on the port priority and identity. This means that the switch will choose which port to block based on the priority assigned to each port. Secondly, non-root switches each have only one root port. This means that non-root switches will have one designated port that is connected to the root switch.

RSTP and STP both use the portfast command to allow ports to immediately transition to the forwarding state. The configuration commands to establish primary and secondary root bridges are also identical for both protocols. Additionally, RSTP is backward compatible with STP due to the format of the BPDU packet.