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    CCIE R&S 350-001 Q&As – Implement Spanning Tree Protocol (STP)(21-25)

    By admin | June 18, 2014

    Tagged with:

    Section 1 – Implement Spanning Tree Protocol (STP)

    QUESTION 21
    When two bridges are competing for the root bridge of an IEEE 802.1D spanning tree and both have the same bridge priority configured, which parameter determines the winner?
    A.    highest-numbered IP interface
    B.    MAC address
    C.    device uptime
    D.    root port cost
    Answer: B

    QUESTION 22
    For the following ports, which port is on every bridge in a Spanning Tree Protocol IEEE 802.1w network except the root bridge?
    A.    root port
    B.    backup port
    C.    designated port
    D.    alternate port
    Answer: A
    Explanation:
    Root Port Roles
    The port that receives the best BPDU on a bridge is the root port. This is the port that is the closest to the root bridge in terms of path cost. The STA elects a single root bridge in the whole bridged network (per-VLAN). The root bridge sends BPDUs that are more useful than the ones any other
    bridge sends. The root bridge is the only bridge in the network that does not have a root port. All other bridges receive BPDUs on at least one port.

    clip_image001

    QUESTION 23
    IEEE 802.1w is a Rapid Spanning Tree Protocol (RSTP) that can be seen as an evolution of the 802.1 standard. What are the port roles described by 802.1w?
    A.    root port, designated port, alternate port, backup port, and disabled
    B.    standby port, alternate port, root port, and disabled
    C.    standby port, designated port, backup port, and disabled
    D.    root port, designated port, alternate port, and standby port
    Answer: A
    Explanation:
    Port Roles
    The role is now a variable assigned to a given port. The root port and designated port roles remain, while the blocking port role is split into the backup and alternate port roles. The Spanning Tree Algorithm (STA) determines the role of a port based on Bridge Protocol Data Units (BPDUs). In order to simplify matters, the thing to remember about a BPDU is there is always a method to compare any two of them and decide whether one is more useful than the other. This is based on the value stored in the BPDU and occasionally on the port on which they are received.
    Root Port Roles
    The port that receives the best BPDU on a bridge is the root port. This is the port that is the closest to the root bridge in terms of path cost. The STA elects a single root bridge in the whole bridged network (per-VLAN). The root bridge sends BPDUs that are more useful than the ones any other bridge sends. The root bridge is the only bridge in the network that does not have a root port. All other bridges receive BPDUs on at least one port.

    clip_image001[1]

    Designated Port
    A port is designated if it can send the best BPDU on the segment to which it is connected. 802.1D bridges link together different segments, such as Ethernet segments, to create a bridged domain.
    On a given segment, there can only be one path toward the root bridge. If there are two, there is a bridging loop in the network. All bridges connected to a given segment listen to the BPDUs of each and agree on the bridge that sends the best BPDU as the designated bridge for the segment. The port on that bridge that corresponds is the designated port for that segment.

    clip_image002\

    Alternate and Backup Port Roles
    These two port roles correspond to the blocking state of 802.1D. A blocked port is defined as not being the designated or root port. A blocked port receives a more useful BPDU than the one it sends out on its segment. Remember that a port absolutely needs to receive BPDUs in order to
    stay blocked. RSTP introduces these two roles for this purpose.
    An alternate port receives more useful BPDUs from another bridge and is a port blocked. This is shown in this diagram:
    clip_image003

    A backup port receives more useful BPDUs from the same bridge it is on and is a port blocked. This is shown in this diagram

    clip_image004
    This distinction is already made internally within 802.1D. This is essentially how Cisco UplinkFast functions. The rationale is that an alternate port provides an alternate path to the root bridge and therefore can replace the root port if it fails. Of course, a backup port provides redundant connectivity to the same segment and cannot guarantee an alternate connectivity to the root bridge. Therefore, it is excluded from the uplink group.
    As a result, RSTP calculates the final topology for the spanning tree that uses the same criteria as 802.1D. There is absolutely no change in the way the different bridge and port priorities are used. The name blocking is used for the discarding state in Cisco implementation. CatOS releases 7.1 and later still display the listening and learning states. This gives even more information about a port than the IEEE standard requires. However, the new feature is now there is a difference between the role the protocol determines for a port and its current state. For example, it is now perfectly valid for a port to be designated and blocking at the same time. While this typically occurs for very short periods of time, it simply means that this port is in a transitory state towards the designated forwarding state.

    QUESTION 24
    This question is about the Spanning Tree Protocol (STP) root guard feature. What is the STP root guard feature designed to prevent?
    A.    a root port being transitioned to the blocking state
    B.    a port being assigned as a root port
    C.    a port being assigned as an alternate port
    D.    a root port being transitioned to the forwarding state
    Answer: B
    Explanation:
    The standard STP does not provide any means for the network administrator to securely enforce the topology of the switched Layer 2 (L2) network. A means to enforce topology can be especially important in networks with shared administrative control, where different administrative entities or companies control one switched network.
    The forwarding topology of the switched network is calculated. The calculation is based on the root bridge position, among other parameters. Any switch can be the root bridge in a network. But a more optimal forwarding topology places the root bridge at a specific predetermined location. With the standard STP, any bridge in the network with a lower bridge ID takes the role of the root bridge. The administrator cannot
    enforce the position of the root bridge.
    Note: The administrator can set the root bridge priority to 0 in an effort to secure the root bridge position. But there is no guarantee against a bridge with a priority of 0 and a lower MAC address.
    The root guard feature provides a way to enforce the root bridge placement in the network.
    The root guard ensures that the port on which root guard is enabled is the designated port. Normally, root bridge ports are all designated ports, unless two or more ports of the root bridge are connected together. If the bridge receives superior STP Bridge Protocol Data Units (BPDUs) on a root guard?enabled port, root guard moves this port to a root?inconsistent STP state. This root?inconsistent state is effectively equal to a listening
    state. No traffic is forwarded across this port. In this way, the root guard enforces the position of the root bridge.
    The example in this section demonstrates how a rogue root bridge can cause problems on the network and how root guard can help.
    In Figure 1, Switches A and B comprise the core of the network, and A is the root bridge for a VLAN. Switch C is an access layer switch. The link between B and C is blocking on the C side. The arrows show the flow of STP BPDUs.
    Figure 1

    clip_image005

    In Figure 2, device D begins to participate in STP. For example, software?based bridge applications are launched on PCs or other switches that a customer connects to a service?provider network. If the priority of bridge D is 0 or any value lower than the priority of the root bridge, device D is elected as a root bridge for this VLAN. If the link between device A and B is 1 gigabit and links between A and C as well as B and C are 100 Mbps, the election of D as root causes the Gigabit Ethernet link that connects the two core switches to block. This block causes all the data in that VLAN to flow via a 100?Mbps link across the access layer. If more data flow via the core in that VLAN than this link can accommodate, the drop of some frames occurs.
    The frame drop leads to a performance loss or a connectivity outage.
    Figure 2

    clip_image006

    The root guard feature protects the network against such issues.
    The configuration of root guard is on a per?port basis. Root guard does not allow the port to become an STP root port, so the port is always STP?designated. If a better BPDU arrives on this port, root guard does not take the BPDU into account and elect a new STP root. Instead, root guard puts the port into the root?inconsistent STP state. You must enable root guard on all ports where the root bridge should not appear. In a way, you can configure a perimeter around the part of the network where the STP root is able to be located.
    In Figure 2, enable root guard on the Switch C port that connects to Switch D. Switch C in Figure 2 blocks the port that connects to Switch D, after the switch receives a superior BPDU.
    Root guard puts the port in the root?inconsistent STP state. No traffic passes through the port in this state.
    After device D ceases to send superior BPDUs, the port is unblocked again. Via STP, the port goes from the listening state to the learning state, and eventually transitions to the forwarding state. Recovery is automatic; no human intervention is necessary.
    This message appears after root guard blocks a port:
    %SPANTREE?2?ROOTGUARDBLOCK: Port 1/1 tried to become non?designated in VLAN 77.
    Moved to root?inconsistent state

    QUESTION 25
    Refer to the exhibit. Catalyst R is the root bridge for both VLAN 1 and VLAN 2. What is the easiest way to load-share traffic across both trunks and maintain redundancy in case a link fails, without using any type of EtherChannel link-bundling?

    clip_image007
    A.    Increase the root bridge priority (increasing the numerical priority number) for VLAN 2 on Catalyst D so that port D2 becomes the root port on Catalyst D for VLAN 2.
    B.    Decrease the port priority on R2 for VLAN 2 on Catalyst R so that port D1 will be blocked for VLAN 2 and port D2 will remain blocked for VLAN 1.
    C.    Decrease the path cost on R2 on Catalyst R for VLAN 2 so that port D1 will be blocked for VLAN 2 and port D2 will remain blocked for VLAN 1.
    D.    Increase the root bridge priority (decreasing the numerical priority number) for VLAN 2 on Catalyst R so that R2 becomes the root port on Catalyst D for VLAN 2.
    Answer: B
    Explanation:
    First we should understand what will happen if nothing is configured (use default values). Because R is the root bridge so all of its ports will forward. D will need to block one of its ports to avoid a bridging loop between the two switches. But how does D select its blocked port? Well, the answer is based on the BPDUs it receives from R. A BPDU is superior than another if it has:
    1. A lower Root Bridge ID
    2. A lower path cost to the Root
    3. A lower Sending Bridge ID
    4. A lower Sending Port ID
    These four parameters are examined in order. In this case, all the BPDUs sent by R have the same Root Bridge ID, same path cost to the Root and same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). If using default values, the default port priority’s value is 32 or 128 (128 is much more popular today), so D will compare port index values, which are unique to each port on the switch, and because port R2 is inferior to port R1 (the port’s number of R2 is higher than that of R1, for example port Fa0/2 is inferior to port Fa0/1), D will select the port connected with port R1 as its root port and block the other port.
    The problem here is port D2 is blocked for both VLAN 1 & 2 and that means we can’t use the underneath link for load-sharing. The underneath link is just used in the case the above link fails.
    Now as you can guess, the easiest way to load-share traffic across both trunks is decreasing the port priority on R2 for VLAN 2 on Catalyst R so that port D1 will be blocked for VLAN 2. Notice that “decreasing” here means make that port ID superior to the other port.

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