Chapter 14: Quiz – Troubleshooting BGP (Answers) CCNPv8 ENARSI

1. What are two issues that may cause the failure of a BGP neighbor relationship from forming? (Choose two.)

  • neighbor statement being wrong
  • neighbor not having a route to the local router
  • path to the neighbor being through a static route
  • the keepalive timer not matching between BGP peers
  • BGP packets being sourced from a loopback interface

Explanation: Some of the reasons that a BGP neighbor relationship might not form are as follows:

  • Neighbor does not have a route to the local router: The two routers forming a BGP peering must have routes to each other.
  • Incorrect neighbor statement: The IP address and ASN in the neighbor ip_address remote-as as_number statement must be accurate.
  • Path to the neighbor is through the default route: A neighbor must be reachable using a route other than the default route.
  • Timers: Timers do not have to match; however, if the minimum holddown from neighbor option is set, it could prevent a neighbor adjacency.

2. A network administrator issues the show bgp ipv4 unicast summary command on a router to check the adjacency state with a neighbor router. The value under the State/PfxRcd field shows 0. Which statement describes the status of adjacency between the two routers?

  • The adjacency is successfully formed.
  • The adjacency is formed, but no message is transmitted.
  • Because the neighbor command is wrong, the adjacency is not formed.
  • Because there is no Layer 3 connectivity between the two routers, the adjacency is not formed.

Explanation: The State/PfxRcd indicates the current state of the BGP peer or the number of prefixes received from the peer. A value of 0 or another number indicates that the adjacency is successfully formed. The value indicates the number of prefixes received from the neighbor. Messages received and sent between routers are indicated under the MsgRcvd and MsgSent fields. BGP uses TCP to communicate. If there is Layer 3 connectivity, the state value is Idle. If Layer 3 connectivity exists and a three-way TCP handshake is complete, an open message is sent. However, if there is no response to the open message, the state value is Active.

3. Refer to the exhibit. A network administrator issues the show bgp ipv4 unicast 10.1.1.0 command on router R1 to verify that the network 10.1.1.0 is in the BGP table. The administrator notices that there are two routes to reach the network. Which BGP path attribute is used to determine the best-path?

Chapter 14: Quiz - Troubleshooting BGP (Answers) CCNPv8 ENARSI 6

  • AIGP
  • weight
  • BGP RID
  • origin code

Explanation: Cisco routers review BGP attributes in the following ranked order when deciding which path is the best-path:

  1. Prefer the highest weight.
  2. Prefer the highest local preference.
  3. Prefer the route originated by the local router.
  4. Prefer the path with the shorter Accumulated Interior Gateway Protocol (AIGP) metric attribute.
  5. Prefer the shortest AS_Path.
  6. Prefer the lowest origin code.
  7. Prefer the lowest multi-exit discriminator (MED).
  8. Prefer an external path over an internal path.
  9. Prefer the path through the closest IGP neighbor.
  10. Prefer the oldest route for EBGP paths.
  11. Prefer the path with the lowest neighbor BGP RID.
  12. Prefer the path with the lowest neighbor IP address.

In this case, since path attributes 1-9 are either not configured, not used, or equal, BGP RID is used to determine the best-path.

4. Which statement describes the BGP evaluation criteria of path attributes during the calculation of the best path for a route?

  • Prefer the lowest weight attribute.
  • Prefer an internal path over an external path.
  • Prefer the lowest multi-exit discriminator attribute.
  • Prefer the path with the highest neighbor IP address.

Explanation: Cisco routers review BGP attributes in the following ranked order when deciding which path is the best-path:

  1. Prefer the highest weight.
  2. Prefer the highest local preference.
  3. Prefer the route originated by the local router.
  4. Prefer the path with the shorter Accumulated Interior Gateway Protocol (AIGP) metric attribute.
  5. Prefer the shortest AS_Path.
  6. Prefer the lowest origin code.
  7. Prefer the lowest multi-exit discriminator (MED).
  8. Prefer an external path over an internal path.
  9. Prefer the path through the closest IGP neighbor.
  10. Prefer the oldest route for EBGP paths.
  11. Prefer the path with the lowest neighbor BGP RID.
  12. Prefer the path with the lowest neighbor IP address.

5. Refer to the exhibit. A network administrator is troubleshooting a BGP adjacency issue between routers R2 and R5. R2 uses loopback0 interface 2.2.2.2 and R5 uses loopback0 interface 5.5.5.5 in the neighbor ip_address remote-as as_number statement in their respective configurations. However, an adjacency does not form. Which two conclusions can be drawn from the troubleshooting process performed on R5? (Choose two.)

Chapter 14: Quiz - Troubleshooting BGP (Answers) CCNPv8 ENARSI 7

  • R2 does not have a route to R5.
  • R5 uses the default route to reach R2.
  • There is no Layer 3 connectivity between R2 and R5.
  • The interface with the IP address 10.1.45.5 on R5 is down.
  • The AS number in the neighbor ip_address remote-as as_number statement is incorrectly configured on R2.

Explanation: Two BGP routers do not have to be directly connected or in the same subnet to form a BGP neighbor relationship; however, Layer 3 connectivity is required. For a router to have Layer 3 connectivity to the neighbor, it needs to have a route in the routing table that points to the neighbor router. In this case, there is no route in the routing table on R5 to reach R2. R5 can reach R2 via the default route configured on R5, BGP does not consider it a valid route for forming an adjacency.

6. A network administrator is troubleshooting a BGP network and issues the show bgp ipv4 unicast command on a BGP router. What is a purpose of using this command?

  • to verify the status of BGP peering
  • to verify IPv4 unicast BGP neighbors
  • to verify the IPv4 routes in the BGP table
  • to display detailed information about all the BGP neighbors

Explanation: The show bgp ipv4 unicast command could be used for multiple reasons but it is very useful for verifying the IPv4 unicast BGP-learned routes or routes locally injected into the BGP table.

7. A network technician is verifying the BGP configuration and neighbor relationships on a Cisco router. The technician suspects that one BGP neighbor lost the peering relationship. Which command should the technician use to verify the network link status?

  • show ipv4 route bgp
  • show ip interface brief
  • show bgp ipv4 unicast neighbor
  • show bgp ipv4 unicast summary

Explanation: The interface with the IP address that is being used to form BGP neighbor relationships must be up/up. The status of an interface can be verified by using the show ip interface brief command.

8. Refer to the exhibit. A network administrator issues the show bgp ipv4 unicast command to check the routes in the BGP table. What does the indication of 0.0.0.0 under Next Hop mean?

Chapter 14: Quiz - Troubleshooting BGP (Answers) CCNPv8 ENARSI 8

  • The route is learned through IGP.
  • 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: As the BGP prefix is installed into the Loc-RIB table, the following BGP PAs are set, depending on the RIB prefix type:

  • Connected network: The next-hop BGP attribute is set to 0.0.0.0, the BGP origin attribute is set to i (IGP), and the BGP weight is set to 32,768.
  • Static route or routing protocol: The next-hop BGP attribute is set to the next-hop IP address in the RIB, the BGP origin attribute is set to i (IGP), the BGP weight is set to 32,768, and the MED is set to the IGP metric.

9. A network administrator receives a call, from a colleague at the overseas branch office, that a BGP route advertised by the company carries an autonomous number of 64520 in the AS_Path attribute. Which statement describes the situation?

  • The ASN is in the experimental ASN range and should not be used at all.
  • The ASN is in the private ASN range and should not be used on the internet.
  • The ASN is in the reserved ASN range and is allowed to be used on the internet.
  • The ASN is in the private ASN range and NAT should be used to translate this ASN to a public ASN to be used on the internet.

Explanation: BGP ASNs have a private range. The 2-byte AS range is 64,512 to 65,534, and the 4-byte AS range is 4,200,000,000 to 4,294,967,294. These ASNs can be used for networks that are single-homed or dual-homed to the same ISP, thereby preserving the public ASNs for networks that are multihomed to multiple ISPs. Although the private ASNs can be used in a customer network, it is imperative that the ASN not be in the AS_Path attribute when the routes are advertised to the internet.

10. Refer to the exhibit. A network administrator issues the show bgp ipv6 unicast | begin Network command on router R1 to verify routes in the IPv6 BGP table. Which statement describes the status of the route 2001:DB8:2::/64?

Chapter 14: Quiz - Troubleshooting BGP (Answers) CCNPv8 ENARSI 9

  • It is redistributed into BGP.
  • It will not be installed in the routing table.
  • It is the best route for the prefix 2001:DB8:2::/64.
  • It can be reached via the local address ::FFFF:2.2.2.2.

Explanation: The show bgp ipv6 unicast | begin Network command can be used to verify the IPv6 unicast routes that have been learned from all neighbors. Its output displays the IPv6 BGP table. The route 2001:db8:2::/64 is the route that was learned from the 2.2.2.2 neighbor. It is not installed in the routing table, as indicated by the absence of the *>. Because the next hop is not reachable, it is not installed . The address ::FFFF:2.2.2.2 is a dynamically generated next hop that was created to replace the original next hop of 2.2.2.2. This occurs because an IPv6 route cannot have an IPv4 next-hop address.

11. A network administrator is troubleshooting a BGP network where some BGP routes are missing from BGP tables. The administrator has verified that BGP peers were formed as expected. What are two possible reasons that routes would not appear in the BGP table or the routing table? (Choose two.)

  • BGP split-horizon rule
  • mismatched authentication being used
  • missing or bad network mask command
  • no Layer 3 connectivity to neighbor routers
  • only EBGP being used to establish adjacency

Explanation: BGP peers are the foundation of BGP information sharing. Without peers, BGP will not learn BGP routes. Some common reasons that BGP routes might be missing from either the BGP table or the routing table include the following:

  • Missing or bad network mask command: An accurate network command is needed to advertise routes.
  • Next-hop router not reachable: To use a BGP route, the next hop must be reachable.
  • BGP split-horizon rule: A router that learns BGP routes through an IBGP peering does not share those routes with another IBGP peer.
  • Better source of information: If the same network is learned from a more reliable source, it is used instead of the BGP-learned information.
  • Route filtering: A filter might be preventing a route from being shared with neighbors or learned from neighbors.

12. Refer to the exhibit. A network administrator issues the show bgp ipv4 unicast | begin Network command on router R5 to verify routes in the BGP table. The administrator realizes that the route 10.1.1.0/24 and the component subnets learned from router R1 with IP address of 1.1.1.1 will not be installed in the routing table. What is a possible solution to the issue?

Chapter 14: Quiz - Troubleshooting BGP (Answers) CCNPv8 ENARSI 10

  • Set the weight to 100 on R5 for these routes.
  • Set the local preference to 200 on R5 for these routes.
  • Create a static route to reach the 10.1.1.0/24 network on R5.
  • Establish fully meshed IBGP peers among these networks.

Explanation: The show bgp ipv4 unicast command can be used to verify routes in the BGP table. For a BGP router to install a BGP route in the routing table, it must be able to reach the next-hop address listed for the network. The > symbols indicate a valid best path to reach the network that has been installed in the routing table. The route 10.1.1.0/24 and its component subnets have no > symbol after the , which means these routes are valid but not the best, and as a result, they are not placed in the routing table. One solution to the issue is to create a static route to reach 10.1.1.0/24 network on R5.


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