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    CCIE R&S 350-001 Q&As – Implement Spanning Tree Protocol (STP)(1-5)

    By admin | June 18, 2014

    Tagged with:

    Section 1 – Implement Spanning Tree Protocol (STP)

    QUESTION 1
    Spanning Tree Protocol IEEE 802.1s defines the ability to deploy which of these?
    A.    one global STP instance for all VLANs
    B.    one STP instance for each VLAN
    C.    one STP instance per set of VLANs
    D.    one STP instance per set of bridges
    Answer: C

    Explanation:
    The IEEE 802.1s standard is the Multiple Spanning Tree (MST). With MST, you can group VLANs and run one instance of Spanning Tree for a group of VLANs.
    Other STP types:
    + Common Spanning Tree (CST), which is defined with IEEE 802.1Q, defines one spanning tree instance for all VLANs.
    + Rapid Spanning Tree (RSTP), which is defined with 802.1w,  is used to speed up STP convergence. Switch ports exchange an explicit handshake when they transition to forwarding.

    QUESTION 2
    Which two of these are used in the selection of a root bridge in a network utilizing Spanning Tree Protocol IEEE 802.1D? (Choose two.)
    A.    Designated Root Cost
    B.    bridge ID priority
    C.    max age
    D.    bridge ID MAC address
    E.    Designated Root Priority
    F.    forward delay
    Answer: BD
    Explanation:
    Select a root bridge. The root bridge of the spanning tree is the bridge with the smallest (lowest) bridge ID. Each bridge has a unique identifier (ID) and a configurable priority number; the bridge ID contains both numbers. To compare two bridge IDs, the priority is compared first. If two bridges have equal priority, then the MAC addresses are compared. For example, if switches A (MAC=0200.0000.1111) and B (MAC=0200.0000.2222) both have a priority of 10, then switch A will be selected as the root bridge. If the network administrators would like switch B to become the root bridge, they must set its priority to be less than 10.

    QUESTION 3
    If a port configured with STP loop guard stops receiving BPDUs, the port will be put into which state?
    A.    learning state
    B.    listening state
    C.    forwarding state
    D.    root-inconsistent state
    Answer: D
    Explanation:
    Loop Guard protects a switch trunk port from causing loops. It prevents switch ports from wrongly moving from a blocking to a forwarding state when a unidirectional link exists in the network.
    Unidirectional links are simply links for which one of the two transmission paths on the link has failed, but not both. This can happen as a result of miscabling, cutting one ber cable, unplugging one ber or other reasons. Let’s consider an example.
    The network consists of 3 switches without Loop Guard feature. Switch 1 is the root switch. A port on Switch 3 is in blocking state, other ports are forwarding normally.

    clip_image001

    Suppose that Switch 3 does not receive BPDUs (Hellos) from Switch 2 due to unidirectional link failure on the link between switch 2 and switch 3.

    Switch 3 then transitions to forwarding state, and now all trunks on all switches are forwarding. Well, we have a loop!

    clip_image002

    QUESTION 4
    What is the purpose of the STP PortFast BPDU guard feature?
    A.    enforce the placement of the root bridge in the network
    B.    ensure that a port is transitioned to a forwarding state quickly if a BPDU is received
    C.    enforce the borders of an STP domain
    D.    ensure that any BPDUs received are forwarded into the STP domain
    Answer: C
    Explanation:
    By default, STP runs on all ports on a switch but most of these ports are connected to an end-user device (printers, PCs, servers).
    Suppose that someone turns off the PC and then turns on, it will take up to 50 seconds before the port transits to the forwarding state and can be usable (15 seconds for Listening to Learning, and 15 seconds for Learning to Forwarding and if that port is running Port Aggregation Protocol (PAgP) to negotiate EtherChannel con?guration, an additional 20-second delay can occur).
    Therefore the STP PortFast feature is used to allow immediate transition of the port into forwarding state. Notice that PortFast is for access (user) ports only.
    It causes the port to bypass the STP listening and learning states and transition directly to forwarding. However, Spanning-tree loop detection is still in operation and the port moves into the Blocking state if a loop is ever detected on the port.
    But there is an issue with PortFast feature. For example, if we connect a switch to a PortFast port, the loop can occur or this new switch can make the STP block important ports if it takes over the root bridge function.
    This situation can be prevented with the BPDU guard feature. This feature disables (shuts down) the port as soon as the switch receives the STP BPDU from the port which has been configured with BPDU guard, placing it in the errdisable state.
    The STP PortFast BPDU guard enhancement allows network designers to enforce the STP domain borders and keep the active topology predictable. The devices behind the ports that have STP PortFast enabled are not able to influence the STP topology.

    QUESTION 5
    When STP UplinkFast is enabled on a switch utilizing the default bridge priority, what will the new bridge priority be changed to?
    A.    8192
    B.    16384
    C.    49152
    D.    65535
    Answer: C
    Explanation:
    When you enable spanning-tree UplinkFast on the switch, UplinkFast processing is enabled and the spanning-tree bridge priority for all VLANs is set to 49152, making it unlikely that the switch will become the root switch. In addition, the spanning-tree port cost and port-VLAN cost of all ports on the switch is increased by 3000.
    The station_update_rate value in the UplinkFast command represents the number of dummy multicast packets transmitted per 100 milliseconds (the default is 15 packets per 100 milliseconds) in the event of a direct link failure.
    Use the all-protocols on keywords on switches that have UplinkFast enabled but do not have protocol filtering enabled, and that are connected to upstream switches in the network that have protocol filtering enabled. The all-protocols on keywords cause the switch to generate multicasts for each protocol-filtering group.
    On switches with both UplinkFast and protocol filtering enabled, or if no other switches have protocol filtering enabled, you do not need to use the all-protocols on keywords.

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