Q1. What is the range of autonomous system numbers (ASNs) that can be assigned to a network using BGP?
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A. The assignable BGP autonomous system numbers are from 1 to 65,535 (I.e. 65,535 in total). Autonomous system numbers are of 16 bit length. This 2 ^ 16 = 65536 -1 possible ASNs, since ASN of all 0s is not assigned. Out of this, the Internet Assigned Numbers Authority (IANA) has reserved the block of AS numbers 64512 through 65535 for private use (not to be advertised on the global Internet).
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Q2. A router with a BGP autonomous system number of 65001 is peered with another router with the same BGP AS. Which of the following is true?
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A. When two routers belong to the same autonomous system bearing the same AS number, they are considered internal BGP, or iBGP, neighbors. This also means the routes learned between these neighbors use the internal BGP administrative distance of 200, as opposed to the external AD of 20.
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Q. Under which of the following scenarios BGP is recommended?
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When you need to connect to two or more ISPs.
When you are sending traffic through one AS to get to another AS.
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Few recommended scenarios, where you use BGP are:
1. Connect to two or more ISPs
2. The traffic flow out of your network need to be managed to suit the requirements (Policy) of your organization.
3. The traffic need to be sent through one AS to get to another AS.
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Q. What kind of BGP session is established between two routers that are adjacent, but in two different Autonomous Systems?
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External BGP (eBGP) is used to establish session and exchange route information between two or more autonomous systems. Internal BGP (iBGP) is used by routers that belong to the same Autonomous System (AS).
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Q. Which of the following is true with respect to BGP routing?
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Routing decisions are made based on network Policy.
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Routers running BGP in an AS use network Policy to choose the best path. Metrics are not used in BGP. Remember that Internet is made of autonomous systems (AS) that are connected together based on Policies specific to each AS. Also, AS numbers (ASN) are assigned by AINA and are unique over the Internet. In an internet (not big I) the ASNs can be assigned by the corporation itself that is implementing internet.
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Q. Which of the following is NOT a type of message that a BGP common Header will contain?
A. OPEN message
B. UPDATE message
C. ROUTEBUSY message
D. KEEPALIVE message
Correct Answer: C
Explanation:
The following are the four possible message types in a BGP header:
Type 1: OPEN message - This is the first message sent after TCP session is established.
Type 2: UPDATE message - An UPDATE message contains a new route or a route to be withdrawn or both. Note that only one new route can be advertised with one UPDATE message.
Type 3: NOTIFICATION message - this message is sent if an error occurs during a BGP session. This message can be used to troubleshoot the problem.
Type 4: KEEPALIVE message - KEEPALIVE message is used to confirm that the connection between the neighboring routers is still active.
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Q. Which of the following are well-known mandatory attributes used in BGP? [Choose all that apply].
A. AS_PATH
B. NEXT_HOP
C. ORIGIN
D. COMMUNITIES
Correct Answer: A,B,C
Explanation:
Well-Known mandatory attributes must appear in all BGP update messages. The well-known mandatory messages are:
1. AS_PATH: BGP messages carry the sequence of AS numbers indicating the complete path a message has traversed.
2. NEXT_HOP: This attribute indicates the IP address of the next-hop destination router.
3. ORIGIN: This attribute tells the receiving BGP router, the BGP type of the original source of the NLRI information.
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Q. Which of the following statements is true with respect BGP?
A. Any two routers that have been connected and supposed to make TCP connection in order to exchange BGP routing information are called peers, or neighbors.
B. Any two physically adjacent routers are called peers, or neighbors.
C. Any two routers that have formed a TCP connection in order to exchange BGP routing information are called peers, or neighbors.
D. Any routers belonging to the same AS are called peers, or neighbors.
Correct Answer: C
Explanation:
Any two routers that have formed a TCP connection in order to exchange BGP routing information are called peers, or neighbors. BGP peers initially exchange their full BGP routing tables. After this exchange, incremental updates are sent as the routing table changes. BGP keeps a version number of the BGP table, which should be the same for all of its BGP peers. The version number changes whenever BGP updates the table due to routing information changes. Keep alive packets are sent to ensure that the connection is alive between the BGP peers and notification packets are sent in response to errors or special conditions.
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Q10. Which of the following command remove entries from the BGP routing table and reset BGP sessions?
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A. clear ip bgp
B. clear ip bgp 0.0.0.0
C. clear bgp *
D. clear ip bgp *
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Correct Answer: D
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Explanation:
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The command,
clear ip bgp * -clears all the entries from the BGP routing table and reset BGP sessions. This command is used after every configuration change to ensure that the change is activated and that peer routers are informed.
Another command,
clear ip bgp <address>
ex: clear ip bgp 172.31.0.0 -removes the specified network from the BGP table.