642-902 Q&A – Implement an eBGP based solution (21-25)

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

QUESTION NO: 21
During BGP configuration on a router that has peered with other BGP speakers, the BGP command aggregate-address 172.32.0.0 255.255.252.0 is issued. However, the peers do not receive this aggregate network in BGP advertisements. Also, the router does not have this aggregate network in its BGP table. Which option indicates a possible reason this command did not cause the router to advertise the aggregate network to its peers? Select the best response.
A. Interface NULL 0 is likely shutdown.
B. The BGP command no synchronization is missing.
C. The BGP command no auto-summary is missing.
D. Subnets of 172.32.0.0/22 do not exist in the BGP table.
E. The IGP running on this router does not have network 172.32.0.0/22 installed. F. The next hop IP address must be a loopback address.
Answer: D

Explanation:
BGP allows the aggregation of specific routes into one route using the “aggregate-address address mask” command. Aggregation applies to routes that exist in the BGP routing table. This is in contrast to the network command, which applies to the routes that exists in IP routing table.
Aggregation can be performed if at least one or more of the specific routes of the aggregate address exists in the BGP routing table.

QUESTION NO: 22
The 192.168.0.0 network is not being propagated throughout the network. Observe the BGP configuration commands from the advertising router. What is the reason the 192.168.0.0 route is not being advertised?
router bgp 65111
neighbor 172.16.1.1 remote-as 65111 neighbor 172.16.2.1 remote-as 65112 network 192.168.0.0
network 10.0.0.0
!
ip route 192.168.0.0 255.255.0.0 null0
Select the best response.
A. The network 192.168.0.0 statement is missing mask 255.255.0.0
B. The network 192.168.0.0 statement is missing mask 0.0.255.255.
C. The network 10.0.0.0 statement is missing mask 255.0.0.0.
D. The network 10.0.0.0 statement is missing mask 0.255.255.255.
E. The auto-summary configuration is missing.
Answer: A

QUESTION NO: 23
Refer to the exhibit. Currently the two eBGP links between AS100 and AS200 have an average inbound load of 65% and 20% respectively. After further investigation, traffic to 10.10.1.16/28 accounts for 45%, and traffic to 10.10.1.32/28 and to 10.10.1.48/28 each account for 20% of the inbound load. The BGP attributes are currently set at their default values in both autonomous systems.
If you want to influence how AS200 sends traffic to AS100, which eBGP configurations would you configure in AS100 to influence AS200 to use the eBGP links more evenly? (Choose two.)
Select 2 response(s).

23
A. neighbor 192.168.30.2 route-map as_50 out
B. neighbor 192.168.20.2 route-map as_50 out
C. route-map as_50 permit 10 match ip address 50
set metric 150
access-list 50 permit 10.10.1.16 0.0.0.240
D. route-map as_50 permit 10 match ip address 50
set metric 150
access-list 50 permit 10.10.1.32 0.0.0.240
Answer: BC

QUESTION NO: 24
Refer to the exhibit. Network 10.0.0.0/8 is being advertised to autonomous system 65550 via both external links. Which statement about the preferred path to the 10.0.0.0/8 network is true?

24
A. Router R1 will be preferred because its neighbor has the higher autonomous system number.
B. Router R1 will be preferred because it has the lower neighbor IP address.
C. Router R1 will be preferred because it has a lower local preference.
D. Router R2 will be preferred because its neighbor has a lower autonomous system number.
E. Router R2 will be preferred because it has the higher neighbor IP address.
F. Router R2 will be preferred because it has a higher local preference.
Answer: F
Explanation:
The preferred path to 10.0.0.0/8 network is R2 because it has a higher local preference.
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 example, since the weights remained the same (default) value the next thing that is looked at is the highest local preference.

QUESTION NO: 25
Refer to the exhibit. Which two statements are true about the partial configuration that is provided? (Choose two.) Select 2 response(s).

25
A. All the configured neighbors are in autonomous system 100.
B. The peer group shortens the IBGP configuration.
C. The peer group shortens the EBGP configuration.
D. Only the outgoing filters are applied to BGP updates.
E. Three AS-path filters are applied to each BGP neighbor.
Answer: AB

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One thought on “642-902 Q&A – Implement an eBGP based solution (21-25)

  1. The correct answer for question 23 is actually B,D.

    The reason is that in order to more evenly distribute traffic across the links, you need to move the traffic from subnet 10.10.1.32 /28, from the A->E link, to the B->F link. With that in mind, choose answer D (instead of C) will give traffic from 10.10.1.32 /28 a metric of 150 over the A->E link. This is higher than the default metric that subnet would have over the B->F link (default metric 0).

    This answer has been confirmed by test takers.

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