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    CCIE R&S 350-001 Q&As – Implement IPv4 Border Gateway Protocol (BGP) (6-10)

    By admin | June 18, 2014

    Tagged with:

    QUESTION 6
    Two BGP peers connected through a routed firewall are unable to establish a peering relationship. What could be the most likely cause?
    A.    BGP peers must be Layer 2-adjacent.
    B.    EBGP multihop is not configured.
    C.    The firewall is not configured to allow IP protocol 89.
    D.    The firewall is not configured to allow UDP 179.
    Answer: B

    Explanation:
    It seems the key piece of information in the question is “routed firewall”.  Since the firewall is in routed mode and not in transparent mode and the fact that BGP packets are sent with a TTL of 1 by default, BGP multihop is needed to increase the TTL of the packets so they can successfully traverse the firewall.
    Still not a fan of this kind of ‘interpretive’ question from Cisco

    QUESTION 7
    You have a router running BGP for the MPLS network and OSPF for the local LAN network at the sales office. A route is being learned from the MPLS network that also exists on the OSPF local network. It is important that the router chooses the local LAN route being learned from the downstream switch running OSPF rather than the upstream BGP neighbor. Also, if the local OSPF route goes away, the BGP route needs to be used. What should be configured to make sure that the router will choose the LAN network as the preferred path?
    A.    static route needs to be added
    B.    floating static route needs to be added
    C.    bgp backdoor command
    D.    ospf backdoor command
    Answer: C
    Explanation:
    We often run into situations where we have two different routes to a network and we know one is faster than the other. However, a router will only keep one route in the routing table and that will be the one with the shortest administrative distance. Now if two routers (R1 and R2) in an AS are directly connected to each other via a 10meg ethernet running EIGRP (Admin distance 90) but they also have a T1 connection to a service provider running EBGP (Admin distance 20). Now, as human beings, we are smart enough to see that if we are R1 and have to reach a network located behind R2, we should use the EIGRP route via 10meg connection. Now think of this from a router’s perspective, R1 sees two routes to that network, via EIGRP and EBGP. R1 is going to take the slower path via EBGP because EBGP’s admin distance is 20. Changing the default administrative distances is not recommended because that may lead to routing loops. So this is where we can use the network backdoor command so router will prefer EIGRP route over EBGP route. BGP treats the network specified by the network backdoor command as a locally assigned network but it doesn’t advertise the network in BGP updates. In short, BGP network backdoor command changes the admin distance of that network to 200 thus making it a worst route compared to EIGRP (90).

    QUESTION 8
    In BGP routing, what does the rule of synchronization mean?
    A.    A BGP router can only advertise an EBGP learned route, provided that the route is an IGP route in the routing table.
    B.    A BGP router can only advertise an IBGP learned route, provided that the route is an IGP route in the routing table.
    C.    A BGP router can only advertise an IBGP learned route, provided that the route is an IGP route that is not in the routing table.
    D.    A BGP router can only advertise an EBGP learned route, provided that the route is a metric of 0 in the BGP table.
    Answer: B
    Explanation:
    When an AS provides transit service to other ASs and if there are non-BGP routers in the AS, transit traffic might be dropped if the intermediate non-BGP routers have not learned routes for that traffic via an IGP. The BGP synchronization rule states that if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP. The topology shown in demonstrates the synchronization rule.

    QUESTION 9
    Router 1 is configured for BGP as dual-homed on the Cisco network. Which three BGP attributes are carried in every BGP update on this router (both IBGP and EBGP)? (Choose three.)
    A.    origin
    B.    router-ID
    C.    AS-path
    D.    local-preference
    E.    next-hop
    Answer: ACE
    Explanation:
    There are basically two major types of attribute:
    i) Well Known.
    ii) Optional.
    i)Well Known :
    Well known attributes are must be recognized by each compliant of BGP implementations. Well known attributes are propagated to other neighbors also.
    Further divided into :
    1) Mandatory : It is BGP well known attributes. Mandatoryattributes are must be present in all update message passedbetween BGP peers. It is present in route description. Must be supported and propagated.
    2) Discretionary: It is BGP well known attributes. Discretionary attributes may be present on update message. Must be supported; propagation optional.
    ii)Optional :
    Optional attributes are recognized by some implementation of BGP & expected that not recognized byeveryone. Optional attributes are propagated to theirneighbors based on the meanings.
    Further divided into :
    1) Transitive: Optional transitive attributes don’t have to be supported, but must be passed onto peers. Marked as partial if unsupported by neighbor.
    2) Non Transitive: Optional non-transitive attributes don’t have to be supported, and can be ignored. Deleted if unsupported by neighbor.
    Now the different BGP attributes are listed below:
    1)Weight (Attribute Type – Mandatory):
    Weight is a Cisco-defined attribute that is local to a router. The weight attribute is not advertised to neighboring routers. If the router learns about more than one route to the same destination, the route with the highest weight is preferred.
    2)Local preference (Attribute Type – Discretionary ):
    The local preference attribute is used to prefer an exit point from the local autonomous system. Unlike the weight attribute, the local preference attribute is propagated throughout the local AS. If there are multiple exit points from the AS, the local preference attribute is used to select the exit point for a specific route.
    3)AS path (Attribute Type – Mandatory):
    When a route advertisement passes through an autonomous system, the AS number is added to an ordered list of AS numbers that the route advertisement has traversed.
    4)Origin:
    The origin attribute indicates how BGP learned about a particular route. The origin attribute can have one of three possible values:
    a) IGP – The route is interior to the originating AS. This value is set when the network router configuration command is used to inject the route into BGP.
    b)EGP -The route is learned via the Exterior Gateway Protocol (EGP).
    c)Incomplete – The origin of the route is unknown or is learned some other way. An origin of Incomplete occurs when a route is redistributed into BGP.
    5)Multi-exit discriminator (Attribute Type – Non Transitive ):
    The multi-exit discriminator (MED) or metric attribute is used as a suggestion to an external AS regarding the preferred route into the AS that is advertising the metric.
    6)Next-hop (Attribute Type – Mandatory):
    The EBGP next-hop attribute is the IP address that is used to reach the advertising router. For EBGP peers, the next-hop address is the IP address of the connection between the peers.
    7)Community (Attribute Type – Transitive ):
    The community attribute provides a way of grouping destinations, called communities, to which routing decisions (such as acceptance, preference, and redistribution) can be applied. Route maps are used to set the community attribute. The predefined community attributes are as follows:
    –No-export: Do not advertise this route to EBGP peers.
    –No-advertise: Do not advertise this route to any peer.
    –Internet: Advertise this route to the Internet community; all routers in the network belong to it.
    8)Atomic Aggregate (Attribute Type – Discretionary ):
    Notes that route summarization has been performed.
    9)Aggregator (Attribute Type – Transitive ):
    Identifies the router and AS where summarization was performed.
    10)Originator ID (Attribute Type – Non Transitive ):
    Identifies a route reflector.
    11)Cluster List (Attribute Type – Non Transitive ):
    Records the route reflector clusters the route has traversed.

    QUESTION 10
    What are the mandatory, well-known BGP attributes?
    A.    origin, AS-path, next-hop
    B.    AS-path, origin, MED
    C.    AS-path, origin, weight
    D.    AS-path, weight, MED
    Answer: A

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