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    CCIE R&S 350-001 Q&As – Connectivity (1-5)

    By admin | June 18, 2014

    Tagged with:

    Section 8 – Connectivity

    QUESTION 1
    Loop guard and UniDirectional Link Detection both protect against Layer 2 STP loops. In which two ways does loop guard differ from UDLD in loop detection and prevention? (Choose two.)
    A.    Loop guard can be used with root guard simultaneously on the same port on the same VLAN while UDLD cannot.
    B.    UDLD protects against STP failures caused by cabling problems that create one-way links.
    C.    Loop guard detects and protects against duplicate packets being received and transmitted on different ports.
    D.    UDLD protects against unidirectional cabling problems on copper and fiber media.
    E.    Loop guard protects against STP failures caused by problems that result in the loss of BPDUs from a designated switch port.
    Answer: BE

    Explanation:
    The Cisco-proprietary UDLD protocol allows devices connected through fiber-optic or copper (for example, Category 5 cabling) Ethernet cables connected to LAN ports to monitor the physical configuration of the cables and detect when a unidirectional link exists. When a unidirectional link is detected, UDLD shuts down the affected LAN port and alerts the user. Unidirectional links can cause a variety of problems, including spanning tree topology loops.
    UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols.
    Based on the various design considerations, you can choose either UDLD or the loop guard feature. In regards to STP, the most noticeable difference between the two features is the absence of protection in UDLD against STP failures caused by problems in software. As a result, the designated switch does not send BPDUs. However, this type of failure is (by an order of magnitude) more rare than failures caused by unidirectional links. In return, UDLD might be more flexible in the case of unidirectional links on EtherChannel. In this case, UDLD disables only failed links, and the channel should remain functional with the links that remain. In such a failure, the loop guard puts it into loop-inconsistent state in order to block the whole channel.
    Additionally, loop guard does not work on shared links or in situations where the link has been unidirectional since the link-up. In the last case, the port never receives BPDU and becomes designated. Because this behaviour could be normal, this particular case is not covered by loop guard. UDLD provides protection against such a scenario.

    QUESTION 2
    What two features in Cisco switches help prevent Layer 2 loops? (Choose two.)
    A.    UniDirectional Link Detection
    B.    Hot Standby Router Protocol
    C.    Virtual Router Redundancy Protocol
    D.    PortFast
    E.    root guard
    F.    loop guard
    Answer: AF
    Explanation:
    Loop Guard versus UDLD
    Loop guard and Unidirectional Link Detection (UDLD) functionality overlap, partly in the sense that both protect against STP failures caused by unidirectional links. However, these two features differ in functionality and how they approach the problem.
    Based on the various design considerations, you can choose either UDLD or the loop guard feature.
    In regards to STP, the most noticeable difference between the two features is the absence of protection in UDLD against STP failures caused by problems in software. As a result, the designated switch does not send BPDUs. However, this type of failure is (by an order of magnitude) more rare
    than failures caused by unidirectional links. In return, UDLD might be more flexible in the case of unidirectional links on EtherChannel. In this case, UDLD disables only failed links, and the channel should remain functional with the links that remain. In such a failure, the loop guard puts it into
    loop-inconsistent state in order to block the whole channel.
    Additionally, loop guard does not work on shared links or in situations where the link has been unidirectional since the link-up. In the last case, the port never receives BPDU and becomes designated. Because this behaviour could be normal, this particular case is not covered by loop
    guard. UDLD provides protection against such a scenario.

    QUESTION 3
    In Layer 2 topologies, spanning-tree failures can cause loops in the network. These unblocked loops can cause network failures because of excessive traffic. Which two Catalyst 6500 features can be used to limit excessive traffic during spanning-tree loop conditions? (Choose two.)
    A.    loop guard
    B.    storm control
    C.    storm suppression
    D.    broadcast suppression
    E.    BPDU guard
    Answer: BD
    Explanation:
    Understanding Traffic Storm Control
    A traffic storm occurs when packets flood the LAN, creating excessive traffic and degrading network performance. The traffic storm control feature prevents LAN ports from being disrupted by a broadcast, multicast, or unicast traffic storm on physical interfaces.
    Traffic storm control (also called traffic suppression) monitors incoming traffic levels over a 1-second traffic storm control interval and, during the interval, compares the traffic level with the traffic storm control level that you configure. The traffic storm control level is a percentage of the total available bandwidth of the port. Each port has a single traffic storm control level that is used for all types of traffic (broadcast, multicast, and unicast).
    Traffic storm control monitors the level of each traffic type for which you enable traffic storm control in 1-second traffic storm control intervals. Within an interval, when the ingress traffic for which traffic storm control is enabled reaches the traffic storm control level that is configured on the port, traffic storm control drops the traffic until the traffic storm control interval ends.
    Broadcast suppression prevents the switched ports on a LAN from being disrupted by a broadcast storm on one of the ports. A LAN broadcast storm occurs when the broadcast or multicast packets flood the LAN, creating excessive traffic and degrading the network performance. Errors in the protocol-stack implementation or in the network configuration can cause a broadcast storm.
    Broadcast suppression uses filtering that measures the broadcast activity on a LAN over a time period (15264 nsec to ~1 sec) that varies based on the type of line card and speed setting on the port, and compares the measurement with a predefined threshold. If the threshold is reached, further broadcast activity is suppressed for the duration of a specified time period. Broadcast suppression is disabled by default.

    QUESTION 4
    A network is composed of several VRFs. It is required that VRF users VRF_A and VRF_B be able to route to and from VRF_C, which hosts shared services. However, traffic must not be allowed to flow between VRF_A and VRF_B. How can this be accomplished?
    A.    route redistribution
    B.    import and export using route descriptors
    C.    import and export using route targets
    D.    Cisco MPLS Traffic Engineering
    Answer: C
    Explanation:
    An MPLS VPN implementation is very similar to a dedicated router peer-to-peer model implementation. From a CE router’s perspective, only IPv4 updates, as well as data, are forwarded to the PE router. The CE router does not need any specific configuration to enable it to be a part of a MPLS VPN domain. The only requirement on the CE router is a routing protocol (or a static/default route) that enables the router to exchange IPv4 routing information with the connected PE router.
    In the MPLS VPN implementation, the PE router performs multiple functions. The PE router must first be capable of isolating customer traffic if more than one customer is connected to the PE router. Each customer, therefore, is assigned an independent routing table similar to a dedicated PE router in the initial peer-to-peer discussion. Routing across the SP backbone is performed using a routing process in the global routing table. P routers provide label switching between provider edge routers and are unaware of VPN routes. CE routers in the customer network are not aware of the P routers and, thus, the internal topology of the SP network is transparent to the customer
    The P routers are only responsible for label switching of packets. They do not carry VPN routes and do not participate in MPLS VPN routing. The PE routers exchange IPv4 routes with connected CE routers using individual routing protocol contexts. To enable scaling the network to large number of customer VPNs, multiprotocol BGP is configured between PE routers to carry customer routes.
    Customer isolation is achieved on the PE router by the use of virtual routing tables or instances, also called virtual routing and forwarding tables/instances (VRFs). In essence, it is similar to maintaining multiple dedicated routers for customers connecting into the provider network. The function of a VRF is similar to a global routing table, except that it contains all routes pertaining to a specific VPN versus the global routing table. The VRF also contains a VRF-specific CEF forwarding table analogous to the global CEF table and defines the connectivity requirements and protocols for each customer site on a single PE router. The VRF defines routing protocol contexts that are part of a specific VPN as well as the interfaces on the local PE router that are part of a specific VPN and, hence, use the VRF. The interface that is part of the VRF must support CEF switching. The number of interfaces that can be bound to a VRF is only limited by the number of interfaces on the router, and a single interface (logical or physical) can be associated with only one VRF.
    The VRF contains an IP routing table analogous to the global IP routing table, a CEF table, list of interfaces that are part of the VRF, and a set of rules defining routing protocol exchange with attached CE routers (routing protocol contexts). In addition, the VRF also contains VPN identifiers as well as VPN membership information (RD and RT are covered in the next section).
    Route targets (RTs) are additional identifiers used in the MPLS VPN domain in the deployment of MPLS VPN that identify the VPN membership of the routes learned from that particular site. RTs are implemented by the use of extended BGP communities in which the higher order 16 bits of the BGP extended community (64 total bits) are encoded with a value corresponding to the VPN membership of the specific site. When a VPN route learned from a CE router is injected into VPNv4 BGP, a list of VPN route target extended community attributes is associated with it. The export route target is used in identification of VPN membership and is associated to each VRF. This export route target is appended to a customer prefix when it is converted to a VPNv4 prefix by the PE router and propagated in MP-BGP updates. The import route target is associated with each VRF and identifies the VPNv4 routes to be imported into the VRF for the specific customer. The format of a RT is the same as an RD value.

    QUESTION 5
    Which three of these statements about Dynamic Trunking Protocol are correct? (Choose three.)
    A.    It supports autonegotiation for both ISL and IEEE 802.1Q trunks.
    B.    It must be disabled on an interface if you do not want the interface to work as a trunk or start negotiation to become a trunk.
    C.    It is a point-to-multipoint protocol.
    D.    It is a point-to-point protocol.
    E.    It is not supported on private VLAN ports or tunneling ports.
    Answer: ABD
    Explanation:
    switchport mode access – This command puts the interface (access port) into permanent nontrunking mode. The interface will generate DTP frames, negotiating with the neighboring interface to convert the link into a nontrunk link. The interface becomes a nontrunk interface even if the neighboring interface does not agree to the change.
    switchport mode dynamic desirable – This command makes the interface actively attempt to convert the link to a trunk link. The interface becomes a trunk interface if the neighboring interface is set to trunk, desirable, or auto mode. This is the default mode for all Ethernet interfaces. If the neighboring interface is set to the access or non-negotiate mode, the link will become a non-trunking link.
    switchport mode dynamic auto – This command makes the interface willing to convert the link to a trunk link if the neighboring interface is set to trunk or desirable mode. Otherwise, the link will become a non-trunking link.
    switchport mode trunk – This command puts the interface into permanent trunking mode and negotiates to convert the link into a trunk link. The interface becomes a trunk interface even if the neighboring interface does not agree to the change.
    switchport nonegotiate – Prevents the interface from generating DTP frames. You can use this command only when the interface switchport mode is access or trunk. You must manually configure the neighboring interface as a trunk interface to establish a trunk link, otherwise the link will be a non-trunking link.
    Using these different trunking modes, an interface can be set to trunking or nontrunking or even able to negotiate trunking with the neighboring interface. To automatically negotiate trunking, the interfaces must be in the same VTP domain. Trunk negotiation is managed by the Dynamic Trunking Protocol (DTP), which is a Cisco proprietary Point-to-Point Protocol.

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