1. What is the correct BGP command to summarize the addresses from 172.16.0.0 through 172.16.15.0 into a single address that will be advertised by BGP?
- summary-address 172.16.0.0 0.15.255.255 summary-only
- summary-address 172.16.0.0 255.255.240.0 summary-only
- aggregate-address 172.16.0.0 0.15.255.255 summary-only
- aggregate-address 172.16.0.0 255.255.240.0 summary-only
Explanation: Route summarization on BGP edge routers via dynamic method is configured by specifying an aggregation network prefix. The aggregate-address command advertises the aggregated route (the supernet) in addition to the specific original component network prefixes. By using the optional summary-only keyword, the component network prefixes in the summarized network range are suppressed.
2. A network administrator is configuring a prefix list to match networks for BGP route filtering. Which two networks match the prefix match specification 10.224.0.0/12 ge 24 le 28? (Choose two.)
- 10.224.10.0/12
- 10.240.10.0/24
- 10.230.15.0/24
- 10.192.20.0/26
- 10.239.255.240/28
Explanation: Prefix lists provide another method of identifying networks in a routing protocol. A prefix list identifies a specific IP address, network, or network range and allows for the selection of multiple networks with a variety of prefix lengths by using a prefix match specification. A prefix match specification contains two parts: a high-order bit pattern and a high-order bit count, which determines the high-order bits in the bit pattern that are to be matched. The high-order bit pattern is also known as the network address and the high-order bit count as the mask length. The network portion of the matching networks is 10.224.0.0 and higher. The mask length of the matching networks must be greater than or equal to /24 and less than or equal to /28.
3. Refer to the exhibit. A network administrator is troubleshooting BGP configuration and wants to verify routes that carry the 100 AS in the AS_Path value. Which regular expression should the administrator use in the command show bgp ipv4 unicast regex regex-pattern?
- show bgp ipv4 unicast regex .100.
- show bgp ipv4 unicast regex $100
- show bgp ipv4 unicast regex 100_
- show bgp ipv4 unicast regex ^100^
Explanation: In troubleshooting BGP, regular expressions (regex) can be used to parse through the large number of available ASNs. Regular expressions are based on query modifiers used to select the appropriate content. The regex pattern 100_ indicates to only include the lines that contain the exact phrase of 100. Some regex query modifiers are as follows:
- (underscore) – Matches a space
- ^ (caret) – Indicates the start of a string
- $ (dollar sign) – Indicates the end of a string
- . (period) – Matches a single character, including a space
4. What is the purpose of the BGP maximum prefix feature?
- to restrict the number of routes received from a peer
- to restrict the bandwidth used for a specific set of routes
- to restrict the number of peers that will advertise the same route
- to restrict the number of routes sent to a peer with low bandwidth
Explanation: The BGP maximum prefix feature restricts the number of routes that are received from a BGP peer.
5. A network administrator is configuring an ACL to match networks for BGP route filtering. The administrator creates an ACE permit ip 10.0.128.0 0.0.63.0 255.255.255.0 0.0.0.0. Which network matches the ACE?
- 10.0.63.0/24
- 10.0.130.0/24
- 10.0.192.0/24
- 10.0.128.0/25
Explanation: One method for conditional matching of routes in BGP is through extended ACLs. When using extended ACLs for this purpose, the source fields match against the network portion of the route, and the destination fields match against the network mask. The matching networks are 10.0.128.0/24 through 10.0.191.0/24.
6. A network administrator is configuring a BGP router with the neighbor 10.12.1.2 maximum-prefix 12 command. Which three actions will the router take when the peer at 10.12.1.2 advertises more than 12 routes?(Choose three.)
- closes the BGP session
- sends a syslog message
- moves the peer to the ldle state
- sends a warning message to the peer
- closes the BGP session for 60 seconds and then restarting the BGP peering process
- transitions the BGP session to listening mode but does not install any new routes into the BGP table
Explanation: The prefix restrictions can be put on a BGP neighbor by using the BGP address family configuration command neighbor ip-address maximum-prefix prefix-count [warning-percentage][**restart** *time*] [warning-only]. When a peer advertises more routes than the maximum prefix count, the router moves the neighbor to the Idle (PfxCt) state in the finite-state machine (FSM), closes the BGP session, and sends out the appropriate syslog message.
7. Refer to the exhibit. A network administrator is configuring BGP route filtering on R1. The router ID for R1 is configured with lookback address 192.168.1.1. Which three routes are allowed from the BGP neighbor router 10.12.1.2 to inject into the BGP table of R1? (Choose three.)
- 192.168.1.1/32
- 192.168.2.2/32
- 192.168.3.3/32
- 10.12.1.0/24
- 64.100.3.0/25
- 64.100.2.192/26
Explanation: Per the distribute list filtering ACL-IN, two loopback networks (192.168.2.2/32 and 192.168.3.3/32) are allowed because they match the first ACE in the ACL-IN. The network 64.100.3.0/25 is allowed because it matches the second ACE in the ACL-IN. The networks 192.168.1.1/32 and 10.12.1.0/24 are local routes. The network 64.100.2.192/26 would not be allowed because it does not match entries in the ACL-IN.
8. Refer to the exhibit. A network administrator issues the show bgp ipv4 unicast 172.16.0.0 command to check the route information in the BGP table. Which statement describes the characteristic of the advertisement of this route?
R3# show bgp ipv4 unicast 172.16.0.0
BGP routing table entry for 172.16.0.0/20, version 25
Paths: (1 available, best #1, table default)
Not advertised to any peer
Refresh Epoch 2
65200, (aggregated by 65200 192.168.2.2)
10.23.1.2 from 10.23.1.2 (192.168.2.2)
Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best
rx pathid: 0, tx pathid: 0x0
- The route is advertised through IGP.
- The route is advertised through a static route.
- The route is advertised without including path attributes before the aggregation.
- The route is advertised with the aggregate-address 172.16.0.0 255.255.240.0 command.
Explanation: As indicated in the BGP table, the route is an aggregate route 172.16.0.0/20 learned from the neighbor 192.168.2.2. The atomic-aggregate attribute indicates loss of path attributes, such as AS_Path in this scenario.
9. A network administrator is configuring a BGP router with the neighbor 10.12.1.2 maximum-prefix 20 75 command. What will happen when the peer at 10.12.1.2 advertises 15 routes?
- The router sends a warning message to the peer.
- The router drops any further advertisements from the peer.
- The router moves the peer to the Idle state and closes the BGP session.
- The router transitions to listening mode and does not install any new routes into the BGP table.
Explanation: The prefix restrictions can be put on a BGP neighbor by using the BGP address family configuration command neighbor ip-address maximum-prefix prefix-count [warning-percentage][**restart** *time*] [warning-only]. When a peer advertises more routes than the maximum prefix count, the router moves the neighbor to the Idle (PfxCt) state in the finite-state machine (FSM), closes the BGP session, and sends out the appropriate syslog message. A warning is not generated before the prefix limit is reached. By adding a warning percentage (set to 1 to 100) after the maximum prefix count, a warning message will be sent when the percentage is reached.
10. Which statement describes a characteristic of a BGP peer group?
- Member routers are of the same model.
- Member routers form a fully meshed connection.
- Member routers share the same outbound routing policy.
- Member routers share the same outbound and inbound routing policy.
Explanation: The routers in a BGP peer group contain the same outbound routing policy. However, they can have a unique inbound routing policy.
11. Which three options are well-known BGP community sets? (Choose three.)
- Internet
- Test_Only
- No_Export
- No Transit
- Transit_Only
- No Advertise
Explanation: Three common well-known communities are as follows:
- Internet: This is a standardized community for identifying routes that should be advertised on the Internet.
- No_Advertise: Routes with this community should not be advertised to any BGP peer (IBGP or EBGP).
- No_Export: When a route with this community is received, the route is not advertised to any EBGP peer. Routes with this community can be advertised to IBGP peers.
12. Refer to the exhibit. When considering the route map configuration for BGP, what is the result when inbound traffic is received with a source IP that matches ACL1?
- The incoming network traffic will go through the whole route map.
- The optional action is processed, and the route map completes.
- If the incoming network traffic does not match ACL2 and ACL3, it will be dropped.
- The optional action is processed, and route map continues to test with ACL2.
Explanation: A route map uses the command syntax route-map route-map-name [permit | deny][*sequence-number*]. The rules that apply to route map statements are as follows:
- If a processing action is not provided, the default value permit is used.
- If a sequence number is not provided, the sequence number is incremented by 10 automatically.
- If a matching statement is not included, an implied all prefixes is associated with the statement.
- Processing within a route map stops after all optional actions have processed and after matching a conditional matching criterion.
“Do I Know This Already?” Quiz Answers:
1. Which BGP command advertises a summary route to prevent link-flap processing by downstream BGP routers?
- aggregate-address network subnet-mask as-set
- aggregate-address network subnet-mask summary-only
- summary-address network subnet-mask
- summary-address network mask subnet-mask
Explanation: The command aggregate-address network subnet-mask summary-only creates a BGP aggregate and suppresses the component routes.
2. What is the BGP atomic aggregate?
- A BGP path attribute used to indicate that a prefix should not be advertised to a peer
- A BGP path attribute that indicates a loss of path attributes
- The amount of time that a peer’s routes should not be installed due to a flapping link
- The BGP routes that are suppressed during summarization
Explanation: The BGP atomic aggregate is a path attribute that indicates a loss of path attributes when a prefix is summarized.
3. Which extended ACL entry allows any network in the 172.16.x.x network with a /24 to /32 prefix length?
- permit ip 172.16.0.0 0.0.255.255 255.255.255.0 0.0.0.255
- permit ip 172.16.0.0 255.255.0.0 0.0.255.255 0.0.0.0
- permit ip 172.16.0.0 255.255.0.0 255.255.255.0 0.0.0.255
- permit ip 172.16.0.0 0.0.255.255 0.0.255.255 0.0.0.0
Explanation: The correct source fields match against the 172.16.0.0 network range, and the destination fields match against any mask between /24 and /32.
4. Which command displays only the network prefixes that originate from AS 40 or AS 45?
- show bgp ipv4 unicast regexp _40|45$
- show bgp ipv4 unicast regexp ^40|45
- show bgp ipv4 unicast regexp _4(0|5)$
- show bgp ipv4 unicast regexp _[40,45]$
Explanation: Please refer to Example 12-31.
5. True or false: A BGP AS_Path ACL and a prefix list can be applied to a neighbor at the same time.
- True
- False
Explanation: A distribution list and a prefix list cannot be used at the same time for a neighbor. All other filtering techniques can be combined.
6. Which of the following is not a well-known BGP community?
- No_Advertise
- Internet
- No_Export
- Private_Route
Explanation: The other communities listed here-No_Advertise, Internet, and No_Export-are common global communities.
7. A router has been configured with the command neighbor 10.12.1.2 maximum-prefix 100. What happens when the BGP peer advertises 101 prefixes to it?
- The 101st prefix overwrites the 1st prefix in the Loc-RIB table.
- The 101st prefix is discarded.
- The BGP session is shut down.
- The 101st prefix is received and installed in the Loc-RIB table, and a warning message is generated.
Explanation: When a peer advertises more routes than the maximum prefix count, the peer shuts down the BGP session with the status PfxCt.
8. What is the primary difference between a BGP peer group and a peer template?
- They can have different inbound routing policies.
- They can have different outbound routing policies.
- They can have different BGP authentication settings.
- They can have different BGP timers.
Explanation: BGP peer groups requires all of the routers to have the same outbound routing policy. A BGP peer template allows for routers to have different outbound routing policies.