642-902 Q&A – Implement an eBGP based solution (16-20)

Section 3 – Implement an eBGP based solution, given a network design and a set of requirements

QUESTION NO: 16
A router is running BGP and receives more than one route for a particular prefix.Assume all the routes for this prefix have the same attributes. Which three path features would be reasons be for the router to ignore some of the routes and not consider them as candidates for the best path? (Choose three.)
A. paths that are marked as synchronized in the show ip bgp output
B. paths that are marked as not synchronized in the show ip bgp output
C. paths for which the NEXT_HOP is accessible
D. paths for which the NEXT_HOP is inaccessible
E. paths from an external BGP (eBGP) neighbor if the local autonomous system (AS) appears in the AS_PATH
F. paths from an internal BGP (iBGP) neighbor if the local autonomous system (AS) appears in the AS_PATH
Answer: BDE

QUESTION NO: 17
BGP contains two paths to a destination. Assuming both routes were originated locally and have an equal weight. what will be the next determining factor in choosing the best path?
A. lowest MED
B. highest local preference
C. lowest neighbor IP address
D. lowest origin code
E. shortest AS-path
Answer: B

QUESTION NO: 18
What technique should be used on BGP edge routers to prevent a multi-homed autonomous system from becoming a transit system?
A. Advertise with a high MED value all networks that are discovered via external BGP.
B. Remove the AS-Path information on all routes in the BGP table prior to advertising externally.
C. Only advertise networks externally if they have been discovered via internal BGP.
D. Use an outgoing distribution list to filter all networks not originating from inside the autonomous system.
E. Set the no-export community attribute on all networks that are advertised externally.
F. Set the origin code to incomplete for all networks that are discovered via external BGP.
Answer: D
Explanation:
The minimum configuration that guarantees you won’t become a transit AS is shown in the following example:
router bgp 65000 neighbor 10.1.1.1 filter-list 1 out ! ip as-path access-list 1 permit ^$ In this example, the outgoing filter list says that all traffic not sourced from the local AS, should not be advertised. This will prevent the announcement of routes that originated from other Autonomous Systems, and prevent traffic destined for networks in another AS from going through your AS. This is useful real-world information to know and if you configure your network with BGP in a multi-homed environment, odds are good that you will want to implement this kind of filter.

QUESTION NO: 19
Which two conditions can cause BGP neighbor establishment to fail? (Choose two.)
A. There is an access list blocking all TCP traffic between the two BGP neighbors.
B. The IBGP neighbor is not directly connected.
C. BGP synchronization is enabled in a transit autonomous system with fully-meshed IBGP neighbors.
D. The BGP update interval is different between the two BGP neighbors.
E. The BGP neighbor is referencing an incorrect autonomous system number in its neighbor statement.
Answer: AE
Explanation:
BGP uses TCP port 179 to establish and maintain neighbor relationships, so any access lists or firewalls must permit this port for BGP to function.
By default, EBGP multi-hop is not enabled, so the EBGP peer must be directly connected in order for the local BGP router to know how to reach the EBGP peer. BGP uses neighbor statements that specify the AS number of the BGP peer. If it is the same number as the local BGP router process, then the BGP router knows that IBGP is used. If the AS number for the specified peer is different, then EBGP is used. Either way, the specified neighbor must be configured correctly, or the BGP peers will not become neighbors.

QUESTION NO: 20
Refer to the exhibit. By default, when RTB passes BGP advertisements from RTA about network 192.168.2.0 to RTC, what address will be listed as the next-hop address? Select the best response.

20
A.10.1.1.1
B.10.1.1.2
C.192.168.1.49
D.192.168.1.50
Answer: D
Explanation:
The next hop attribute is a well-known mandatory attribute, type code 3. In terms of an IGP, such as RIP, the "next hop" to reach a route is the IP address of the router that has announced the route.
The next hop concept with BGP is more complex and takes one of the following three forms: For
EBGP sessions, the next hop is the IP address of the neighbor that announced the route. For IBGP sessions, where routes originated inside the AS, the next hop is the IP address of the neighbor that announced the route. For routes injected into the AS by way of EBGP, the next hop learned from EBGP is carried unaltered into IBGP. The next hop is the IP address of the EBGP neighbor from which the route was learned. When the route is advertised on a multi-access medium, such as Ethernet or Frame Relay, the next hop is usually the IP address of the interface of the router. This will be the interface connected to the media that originated the route.

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