1. Which transport layer port is used in BGP peering?
- UDP port 289
- TCP port 289
- UDP port 179
- TCP port 179
Explanation: The BGP routing protocol uses an established TCP port 179 connection to maintain connectivity and transmit updates to BGP neighbors.
2. Which routing protocol is used to exchange routing information between autonomous systems on the Internet?
- IS-IS
- EIGRP
- OSPF
- BGP
Explanation: Exterior routing protocols exchange routing information between autonomous systems. BGP is the only exterior routing protocol in use today on the Internet.
3. Refer to the exhibit. A network administrator is configuring BGP on the router RT1. Which command set will allow RT1 to establish a neighbor relationship with RT2?
- RT1(config)# router bgp 65100
RT1(config-router)# network 128.107.0.0 mask 255.255.0.0
RT1(config-router)# neighbor 10.1.1.2 remote-as 65200 - RT1(config)# router bgp 65100
RT1(config-router)# network 128.107.0.0 mask 255.255.0.0
RT1(config-router)# neighbor 10.1.1.1 remote-as 65200 - RT1(config)# router bgp 65100
RT1(config-router)# network 128.107.0.0 mask 255.255.0.0
RT1(config-router)# neighbor 192.168.2.2 remote-as 65200 - RT1(config)# router bgp 65200
RT1(config-router)# network 64.104.0.0 mask 255.255.0.0
RT1(config-router)# neighbor 10.1.1.2 remote-as 65100
Explanation: The basic BGP configuration steps include the following:
- Initialize the BGP routing process with the global command router bgp as-number.
- Identify the IP address and autonomous system number associated with a BGP neighbor by the BGP router configuration command neighbor ip-address remote-as as-number.
- Identify the specific network prefixes to be installed into the BGP table with the network statements.
4. Which two characteristics are associated with IBGP? (Choose two.)
- AS numbers reserved for IBGP sessions are from 65000 to 65400.
- The IBGP sessions are established among IBGP routers in the same AS.
- An IBGP session is established between an edge router and an ISP router.
- When IBGP prefixes are installed into the RIB, they are assigned an AD of 200.
- The AD value assigned to an IBGP prefix is dependent upon whether the connected router is in the same AS.
Explanation: Internal BGP (IBGP) sessions are established between routers that are in the same AS or that participate in the same BGP confederation. IBGP prefixes are assigned an administrative distance (AD) of 200 upon being installed into the RIB of the BGP router.
5. What is used by BGP to determine the best path to a destination?
- cost
- hop count
- attributes
- administrative distance
Explanation: BGP uses attributes, such as AS-path, to determine the best path to a destination.
6. What is the first BGP message type that is sent after a TCP session is established between BGP peers?
- hello
- keepalive
- notification
- open
- update
Explanation: In the Connect state of the BGP FSM, BGP initiates the TCP connection. If the three-way TCP handshake is completed, the established BGP session process resets the ConnectRetryTimer and sends the Open message to the neighbor.
7. What is a characteristic of an IBGP session type?
- Neighbors may be located anywhere within multiple autonomous systems, even several hops away from each other.
- A session typically occurs between routers in different autonomous systems with multiple ISPs.
- It can be established between routers in the same AS or that participate in the same BGP confederation.
- It occurs between routers in two different autonomous systems.
Explanation: IBGP sessions are established with IBGP routers that are in the same AS or that participate in the same BGP confederation. IBGP prefixes are assigned an administrative distance (AD) of 200 upon installation in the router RIB.
8. A network administrator issues the show bgp ipv4 unicast summary command to verify the BGP session after basic BGP configuration is completed. Which three pieces of information are found in the BGP session summary? (Choose three.)
- the AS number of the peer
- the BGP router ID of the peers
- the peer synchronization configuration
- the routes that are redistributed into BGP
- the IGP that is configured on the BGP peer
- the number of exchanged prefixes with a neighbor
Explanation: The BGP session summary report includes information as follows:
- Neighbor – the IP address of the BGP peer
- V – the BGP version spoken by the BGP peer
- AS – the autonomous system number of the BGP peer
- MsgRcvd – the count of messages received from the BGP peer
- MsgSent – the count of messages sent to the BGP peer
- TblVer – the last version of the BGP database sent to the peer
- InQ – the number of messages queued to be processed by the peer
- OutQ – the number of messages queued to be sent to the peer
- Up/Down – the length of time the BGP session is established or the current status if the session is not in an established state
- State/PfxRcd – the current state of the BGP peer or the number of prefixes received from the peer
9. Which method is used to provide a loop-free routing BGP table for IBGP routers in a single AS?
- AS_Path attribute
- full mesh connection requirement
- identical weight attribute on all IBGP routers
- static route establishment between each connected pair of IBGP routers
Explanation: IBGP peers do not prepend their ASN to AS_Path because the NLRI would fail the validity check and would not install the prefix into the IP routing table. No other method exists for detecting loops with IBGP sessions and RFC 4271 prohibits the advertisement of an NLRI received from an IBGP peer to another IBGP peer. RFC 4271 states that all BGP routers in a single AS must be fully meshed to provide a complete loop-free routing table and prevent traffic from being dropped.
10. Refer to the exhibit. A network administrator issues the show bgp ipv4 unicast | begin Network command to check the routes in the BGP table. What does the symbol ? at the end of a route indicate?
- The route is redistributed into BGP.
- The route is learned through a static route.
- The route is the best route for the network prefix.
- The route is originated from a connected network to the router.
Explanation: The origin is a well-known mandatory BGP path attribute used in the BGP best-path algorithm. A value of i represents an IGP, e indicates EGP, and ? indicates a route that was redistributed into BGP.
11. What change was made to BGP to address the expected depletion of autonomous system numbers?
- the ability to incorporate hierarchical network design
- the use of a 2-octet autonomous system number
- the use of a 4-octet autonomous system number
- the ability to use RFC 1918 address space
Explanation: Autonomous system number (ASN) was originally 2 bytes (16-bit range) long, which made 65,535 ASNs possible. Due to exhaustion, RFC 4893 expanded the ASN field to accommodate 4 bytes (32-bit range). This allows for 4,294,967,295 unique ASNs.
12. Which two statements describe the configuration differences when MP-BGP is applied using the IPv6 protocol compared to IPv4 protocol? (Choose two.)
- IPv6 uses multicast to establish neighbor sessions.
- IPv4 addresses cannot be used to define a BGP RID.
- Routers with only IPv6 addressing must have the BGP RID statically defined.
- Routers with only IPV6 addressing must use AS numbers beyond 65535.
- The IPv6 address family must be initialized and the neighbor activated.
Explanation: The BGP configuration rules in IPv4 apply to IPv6, except that the IPv6 address family must be initialized, and the neighbor is activated. Routers with only IPv6 addressing must statically define the BGP RID to allow sessions to form.
“Do I Know This Already?” Quiz Answers:
1. Which of the following autonomous system(s) are private? (Choose two.)
- 64,512 through 65,535
- 65,000 through 65,535
- 4,200,000,000 through 4,294,967,294
- 4,265,000 through 4,265,535,016
Explanation: ASNs 64,512 through 65,535 are private ASNs within the 16-bit ASN range, and 4,200,000,000 through 4,294,967,294 are private ASNs within the extended 32-bit range.
2. Which BGP attribute must be recognized by all BGP implementations and advertised to other autonomous systems?
- Well-known mandatory
- Well-known discretionary
- Optional transitive
- Optional non-transitive
Explanation: Well-known mandatory attributes must be recognized by all BGP implementations and included with every prefix advertisement.
3. True or false: BGP supports dynamic neighbor discovery by both routers.
- True
- False
Explanation: BGP neighbors are statically defined. There is a feature that supports dynamic discovery by one peer (which is beyond the scope of this book), but the other router must still statically configure the remote BGP peer.
4. True or false: BGP sessions are always one hop away from a neighbor.
- True
- False
Explanation: BGP supports multi-hop neighbor adjacency
5. True or false: The IPv4 address family must be initialized to establish a BGP session with a peer using IPv4 addressing.
- True
- False
Explanation: The IPv4 address family is automatically initialized by default on IOS based devices.
6. Which command is used to view the BGP neighbors and their hello interval?
- show bgp neighbors
- show bgp afi safi neighbors
- show bgp afi safi summary
- show afi bgp interface brief
Explanation: The command show bgp afi safi neighbors displays all the neighbors, their capabilities, session timers, and other useful troubleshooting information.
7. How many tables does BGP use for storing prefixes?
- One
- Two
- Three
- Four
Explanation: BGP uses three tables for storing BGP prefixes: Adj-RIB-in, Loc-RIB, and Adj-RIB-out.
8. True or false: A route learned from an eBGP peer is advertised to an iBGP neighbor.
- True
- False
Explanation: A route learned from an eBGP peer is advertised to an iBGP peer.
9. True or false: A route learned from an iBGP peer is advertised to an iBGP neighbor.
- True
- False
Explanation: A route learned from an iBGP peer is not advertised to an iBGP peer. This is a loop-prevention mechanism as the AS_Path is not prepended by an iBGP advertisement.
10. Which of the following are considering iBGP scalability enhancements? (Choose two.)
- Route reflectors
- BGP route aggregation
- BGP confederations
- BGP alliances
Explanation: BGP route reflectors and confederations provide a method of scaling iBGP without requiring full-mesh connectivity within an AS.
11. True or false: The IPv6 address family must be initialized to establish a BGP session with a peer using IPv6 addressing.
- True
- False
Explanation: The IPv6 address family does not exist by default on IOS-based devices.
12. True or false: IPv6 prefixes can be advertised only across a BGP session established with IPv6 addresses.
- True
- False
Explanation: BGP is considered a routing application and can exchange network prefixes (such as IPv6 prefixes) with a different session protocol (such as IPv4.)