642-902 Q&A – Implement an eBGP based solution (1-5)

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

QUESTION NO: 1
Which command displays the IBGP and EBGP neighbors that are configured?
A. show ip bgp
B. show ip bgp paths
C. show ip bgp peers
D. show ip bgp summary
Answer: D
Explanation:
Show ip bgp summary command displays the summary of all BGP connections.

QUESTION NO: 2
Refer to the exhibit. Router RT-1 and router RT-2 both advertise network 131.25.0.0/16 to router RT-3 via internal BGP. What is the reason that router RT-3 chose router RT-1 as its best path to network 131.25.0.0/16. Select the best response.

2
A. It advertises the best AS-path.
B. It advertises the best origin code.
C. It advertises the best MED.
D. It advertises the best local preference.
E. It has a better router ID.
F. It advertises a lower autonomous system.
Answer: E
Explanation:
After BGP receives updates about different destinations from different autonomous systems, it chooses the best path to reach a specific destination.
The following process summarizes how BGP chooses the best route on a Cisco router.

  • Prefer the route with the highest weight. (The weight attribute is proprietary to Cisco and is local to the router only.)
  • If multiple routes have the same weight, prefer the route with the highest local preference value. (The local preference is used within an autonomous system.) If multiple routes have the same local preference, prefer the route that the local router originated. A locally originated route has a next hop of 0.0.0.0 in the BGP table.
  • If none of the routes were locally originated, prefer the route with the shortest autonomous system path.
  • If the autonomous system path length is the same, prefer the lowest origin code (IGP < EGP < incomplete).
  • If all origin codes are the same, prefer the path with the lowest MED. (The MED is exchanged between autonomous systems.) The MED comparison is made only if the neighboring autonomous system is the same for all routes considered, unless the bgp always-compare-med command is enabled If the routes have the same MED, prefer external paths to internal paths.
  • If synchronization is disabled and only internal paths remain, prefer the path through the closest IGP neighbor, which means that the router prefers the shortest internal path within the autonomous system to reach the destination (the shortest path to the BGP next hop).
  • For EBGP paths, select the oldest route to minimize the effect of routes going up and down (flapping).
  • Prefer the route with the lowest neighbor BGP router ID value.
  • If the BGP router IDs are the same, prefer the router with the lowest neighbor IP address.

In this case, since everything else is equal the router ID will be used, and the specific router ID that is chosen is 165.1.1.1 (lowest one), which is RT- 3.

QUESTION NO: 3
Refer to the exhibit. On the basis of the configuration that is provided, how would the BGP updates that come from router R1 be replicated inside autonomous system 65200? Select the best response.

3
A. All BGP updates that are received on router R2 will be sent to routers R3 and R4. Routers R3 and R4 will then forward those BGP updates to router R5.
B. All BGP updates that are received on router R2 will not be sent to routers R3 and R4. C. All BGP updates that are received on router R2 will be sent directly to router R5.
D. None of the BGP updates that are received on router R2 will ever be received by router R5.
Answer: D

QUESTION NO: 4
Refer to the exhibit diagram and configuration. RTB is summarizing its networks from AS 64100 with the aggregate-address command. However, the show ip route command on RTA reveals the RTB individual networks as well as its summary route. Which option would ensure that only the summary route would appear in the routing table of RTA? Select the best response.

4
A. Delete the four network statements and leave only the aggregate-address statement in the BGP configuration.
B. Add the keyword summary-only to the aggregate-address command.
C. Add a static route with a prefix of 192.168.24.0 255.255.252.0 pointing to the null0 interface.
D. Create a route map permitting only the summary address.
Answer: B

QUESTION NO: 5
Which statement is true about IBGP routers? Select the best response.
A. They must be fully meshed.
B. They can be in a different AS.
C. They must be directly connected.
D. They do not need to be directly connected.
Answer: D
Explanation:
When two routers establish a TCP enabled BGP connection, they are called neighbors or peers.
Each router running BGP is called a BGP speaker. Peer routers exchange multiple messages to open and confirm the connection parameters, such as the version of BGP to be used. If there are any disagreements between the peers, notification errors are sent and the connection fails. To be a neighbor of BGP no need to be directly connected.

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