CCNA Lan Chapter #5

The first step in the process of convergence in a spanning tree topology is the election of the root bridge. The root bridge is the most important bridge in the network and serves as the reference point for all other bridges. It is elected based on a predetermined criteria such as the bridge with the lowest bridge ID. Once the root bridge is elected, the other bridges in the network will determine their roles and port states based on their distance to the root bridge. This process ensures that the network has a loop-free topology and allows for efficient forwarding of traffic.

A switch uses two criteria to select the root bridge: bridge priority and base MAC address. The bridge priority is a numerical value that determines the likelihood of a switch becoming the root bridge. The switch with the lowest bridge priority will become the root bridge. The base MAC address is used as a tiebreaker if multiple switches have the same bridge priority. The switch with the lowest base MAC address will become the root bridge.

If STP (Spanning Tree Protocol) has been disabled on the switches in the network, switch SW1 will forward the broadcast frame out all switch ports, except the originating port. This will create an endless loop in the network because the broadcast frame will keep circulating through all the switches, continuously being forwarded out of all ports except the one it originated from. This can lead to network congestion and performance issues.

The BIDs (Bridge IDs) used in a spanning tree topology consist of a bridge priority and MAC address. They are used by the switches in a spanning tree topology to elect the root bridge. The bridge priority determines which switch becomes the root bridge, with the lowest priority being preferred. The MAC address is used as a tiebreaker if multiple switches have the same priority. Therefore, the BIDs play a crucial role in determining the root bridge and the overall topology of the spanning tree network.

To influence which STP switch becomes the root bridge, the network administrator can set the switch priority to a smaller value than that of the other switches in the network. The switch with the lowest priority value will become the root bridge. By assigning a lower priority to a specific switch, the administrator can ensure that it becomes the root bridge and takes control of the STP topology. This allows for better control and management of the network.

When PVST+ was developed, the Bridge ID was modified to include the VLAN ID information. The Bridge ID is a unique identifier used in Spanning Tree Protocol (STP) to determine the root bridge and the path cost to reach it. By including the VLAN ID in the Bridge ID, PVST+ allows for separate spanning trees to be maintained for each VLAN in a network. This ensures that each VLAN has its own root bridge and path cost calculation, improving network efficiency and reducing the risk of loops.

In the learning state of STP (Spanning Tree Protocol), a port records MAC addresses but does not forward user data. This state allows the switch to learn the MAC addresses of devices connected to the port by receiving and processing incoming frames. However, it does not yet participate in forwarding user data until it transitions to the forwarding state.

The output shown indicates that the priority was statically configured to identify the root. This means that the switch has been manually configured to have a higher priority value, making it more likely to become the root switch in the spanning tree. This can be determined because the output does not mention any other factors that could affect the root selection process, such as STP being disabled or timers being altered.

In a Layer 2 switched environment with redundant connections between switches, two true statements about the default operation of STP are: decisions on which port to block when two ports have equal cost depend on the port priority and identity, and non-root switches each have only one root port.

If STP (Spanning Tree Protocol) is not enabled, the result of the ARP request will be that Switch_A and Switch_B will continuously flood the message onto the network. This means that the ARP request will be broadcasted to all devices on the network, causing unnecessary traffic and potentially impacting network performance. Without STP to prevent loops in the network, the message will continue to circulate indefinitely until its time-to-live (TTL) value is exceeded.

In a converged network with one spanning tree, there will be one root bridge per network. The root bridge is the central bridge that is responsible for making forwarding decisions in the network. Additionally, there will be one root port per non-root bridge. The root port is the port on a non-root bridge that has the best path to the root bridge. This port is responsible for forwarding traffic towards the root bridge.

BPDUs (Bridge Protocol Data Units) are used by switches in two ways. First, they are used to identify the shortest path to the root bridge. By exchanging BPDUs, switches can determine the path with the lowest cost to reach the root bridge, which helps in building the spanning tree topology. Second, BPDUs are used to determine which ports will forward frames as part of the spanning tree. By analyzing the BPDUs received from neighboring switches, a switch can determine the designated and non-designated ports, and only forward frames through the designated ports to prevent loops in the network.

The Designated role is assigned to the forwarding port elected for every switched Ethernet LAN segment in Rapid Spanning Tree Protocol (RSTP). The Designated port is responsible for forwarding data on its respective LAN segment and is selected based on the lowest path cost to the root bridge. It is the port that provides the best path to reach the root bridge and ensures efficient and reliable communication within the LAN.

PortFast is a Cisco proprietary feature that allows access ports to immediately transition from a blocking to a forwarding state, improving convergence. Enabling PortFast on trunks that connect to other switches does not improve convergence. PortFast can negatively affect DHCP services by allowing a host to bypass the DHCP process and obtain an IP address from a previous lease.

The command "spanning-tree vlan 1 root primary" on S4 will cause Gi 0/2 on S3 to transition to a root port. This command is used to configure a switch as the root bridge for a specific VLAN. By setting S4 as the root bridge for VLAN 1, it will have the lowest bridge ID and therefore all other switches in the network will adjust their port roles accordingly. In this case, S3's Gi 0/2 will become the root port, as it is the port that offers the lowest cost path to the root bridge (S4).

The three link types that have been 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 among multiple devices. The edge-type link refers to a link that is connected to an end device. The point-to-point link refers to a link that is directly connected between two devices.

The max-age timer and the forward delay are two features of the Spanning-Tree Protocol that contribute to the time it takes for a switched network to converge after a topology change occurs. The max-age timer is the maximum time a switch will wait to receive a hello message from its root bridge before considering the root bridge unavailable. This timer affects the convergence time as it determines how long a switch will wait before recalculating the spanning-tree topology. The forward delay is the time a switch waits before transitioning a port from the blocking state to the forwarding state. This delay allows for stability in the network by ensuring that all switches have updated their forwarding tables before allowing traffic to flow through the newly designated forwarding ports.

STP and RSTP are both spanning tree protocols used to prevent loops in network topologies. The correct answer explains that one key difference between the two is their behavior when it comes to placing ports into forwarding state. STP waits for the network to converge before allowing ports to forward traffic, while RSTP immediately places alternate ports into forwarding state. This means that RSTP can provide faster convergence and reduce network downtime compared to STP.

When an RSTP edge port receives a BPDU, it immediately loses its edge status. This is because edge ports are designed to be connected to end devices that do not participate in spanning tree protocol. Receiving a BPDU indicates the presence of another bridge in the network, so the port can no longer be considered an edge port. Additionally, the edge port becomes a normal spanning-tree port, meaning it will start participating in the spanning tree protocol and forwarding BPDUs to other ports.

RSTP and STP both use the portfast command to allow ports to immediately transition to the forwarding state. This means that when a port is enabled with portfast, it will skip the listening and learning states and go directly to forwarding, reducing the convergence time. The configuration commands to establish primary and secondary root bridges are also the same for both STP and RSTP. This allows for a seamless transition between the two protocols. Additionally, RSTP is backward compatible with STP due to the format of the BPDU packet, meaning that RSTP can coexist with STP in the same network without causing any compatibility issues.