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CCNP Route FAQ: External BGP

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CCNP Route FAQ: External BGP

Q1. Enterprise Router R1, in ASN 1, connects to ISP Router I1, ASN 2, using eBGP. The single serial link between the two routers uses IP addresses 10.1.1.1 and 10.1.1.2, respectively. Both routers use their S0/0 interfaces for this link. Which of the following commands would be needed on R1 to configure eBGP? (Choose two.)
a. router bgp 2
b. router bgp 1
c. neighbor 10.1.1.2 remote-as 2
d. neighbor 10.1.1.2 Update-source 10.1.1.1
e. neighbor 10.1.1.2 Update-source S0/0

Answer: B and C. The router bgp command lists the local ASN, and the neighbor remote-as command lists the neighbor’s ASN. Because the neighbor relationship uses the IP addresses on the common link, the routers do not need to identify the update source interface, because each will default to use their S0/0 interfaces (in this case) as the update source.

Q2. Enterprise Router R1, in ASN 1, connects to ISP Router I1, ASN 2, using eBGP. There are two parallel serial links between the two routers. The implementation plan calls for each router to base their BGP TCP connection on their respective loopback1 interfaces, with IP addresses 1.1.1.1 and 2.2.2.2, respectively. Which of the following commands would not be part of a working eBGP configuration on Router R1?
a. router bgp 1
b. neighbor 2.2.2.2 remote-as 2
c. neighbor 2.2.2.2 update-source loopback1
d. neighbor 2.2.2.2 multihop 2

Answer: D. Three of the commands list valid commands. The neighbor 2.2.2.2 multihop 2 command is syntactically incorrect; it should be neighbor 2.2.2.2 ebgp-multihop 2.

Figure: Using Loopbacks with Update Source for eBGP

Q3. The following output, taken from a show ip bgp command on Router R1, lists two neighbors. In what BGP neighbor state is neighbor 1.1.1.1?

Neighbor  V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
State/PfxRcd
1.1.1.1   4  1    60     61      26   0   0 00:45:01
0
2.2.2.2   4  3    153   159      26   0   0 00:38:13
1

a. Idle
b. Opensent
c. Active
d. Established

Answer: D. The show ip bgp command lists the BGP neighbor state in the last column of output, listing the literal state, unless in an established state. In that state, the output lists the number of prefixes learned from the neighbor, so a numeric value implies an established state.

Q4. The following output was taken from the show ip bgp command on Router R2. In this case, which of the following commands are most likely to already be configured on R2? (Choose two.)

BGP router identifier 11.11.11.11, local AS number 11
...
Neighbor  V  AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
State/PfxRcd
1.1.1.1   4  1   87     87       0     0    0   00:00:06 Idle
(Admin)
2.2.2.2   4  3   173    183      41    0    0   00:58:47
2

a. router bgp 11
b. neighbor 1.1.1.1 remote-as 11
c. neighbor 2.2.2.2 prefix-limit 1
d. neighbor 1.1.1.1 shutdown

Answer: A and D. The output lists R2’s local ASN as ASN 11, a value that is configured in the router bgp asn command. The line for neighbor 1.1.1.1 lists that router’s ASN as 1, so a neighbor 1.1.1.1 remote-as 1 command should exist on R2 instead of the neighbor 1.1.1.1 remote-as 11 command. The state for neighbor 1.1.1.1 lists “Idle (Admin),” implying that the neighbor 1.1.1.1 shutdown command has been configured. The other answer lists a nonexistent command.

Q5. Which of the following answers is most true about the BGP Update message?
a. It lists a set of path attributes, along with a list of prefixes that use those PAs.
b. It lists a prefix/length, plus the PA settings for that prefix.
c. It lists withdrawn routes, but never in the same Update message as newly advertised routes.
d. A single Update message lists at most a single prefix/length.

Answer: A. The BGP Update message lists a set of PAs, plus any prefixes/lengths that use those PAs. It can also list withdrawn routes in the same Update message as newly advertised routes. It can also list multiple prefixes in a single Update message.

Q6. The following output occurs on Router R1. Which of the following cannot be determined from this output?

R1# show ip route 180.1.1.0 255.255.255.240
Routing entry for 180.1.1.0/28
Known via “bgp 2”, distance 20, metric 0
Tag 3, type external
Last update from 192.168.1.2 00:10:27 ago
Routing Descriptor Blocks:
* 192.168.1.2, from 192.168.1.2, 00:10:27 ago
Route metric is 0, traffic share count is 1
AS Hops 2
Route tag 3

a. The type of BGP peer (iBGP or eBGP) that advertised this route to R1
b. R1’s ASN
c. The next-hop router’s ASN
d. The AS_Path length

Answer: C. The “Known via” text refers to the local router’s (R1’s) router bgp command, which identifies the local router’s ASN. The rest of the output does not identify the neighboring ASN, nor the rest of the AS_Path details. It does list that the route is external, with the text “type external”, and the AS Hops (which is the AS_Path length).

Q7. The following line of output was extracted from the output of the show ip bgp command on Router R1. Which of the following can be determined from this output?

Network Next Hop Metric LocPrf Weight Path
* 130.1.1.0/28 1.1.1.1 0 1 2 3 4 i

a. The route is learned from an eBGP peer.
b. The route has no more than three ASNs in the AS_Path.
c. The route is the best route for this prefix.
d. None of these facts can be positively determined by this output.

Answer: A. The third character in each line for each router is either blank, meaning the route is an eBGP route, or an “i,” meaning an iBGP-learned route. The contents of the AS_Path can be determined (1, 2, 3, 4), but the answer about AS_Path does not suggest 4 ASNs. The best route for each prefix has a “>” in the second character, and this route does not.

Q8. Router R1 has eBGP connections to I1 and I2, routers at the same ISP. The company that owns R1 can use public address range 130.1.16.0/20. The following output lists all the IP routes in R1’s routing table within this range. Which of the following answers would cause R1 to advertise the 130.1.16.0/20 prefix to its eBGP peers? (You should assume default settings for any parameters not mentioned in this question.)

R1# show ip route 130.1.16.0 255.255.240.0 longer-prefixes
! lines omitted...
O 130.1.16.0/24 [110/3] via 10.5.1.1, 00:14:36, FastEthernet0/1
O 130.1.17.0/24 [110/3] via 10.5.1.1, 00:14:36, FastEthernet0/1
O 130.1.18.0/24 [110/3] via 10.5.1.1, 00:14:36, FastEthernet0/1

a. Configure R1 with the network 130.1.16.0 mask 255.255.240.0 command.
b. Configure R1 with the network 130.1.16.0 mask 255.255.240.0 summaryonly command.
c. Redistribute from OSPF into BGP, filtering so that only routes in the 130.1.16.0/20 range are redistributed.
d. Redistribute from OSPF into BGP, filtering so that only routes in the 130.1.16.0/20 range are redistributed, and create a BGP summary for 130.1.16.0/20.

Answer: D. The network command will take the route from the IP routing table and put the equivalent into the BGP table, if that exact route exists. The output does not show a route for 130.1.16.0/20, so the network 130.1.16.0 mask 255.255.240.0 command does not match a specific route. The other answer with a network command is syntactically incorrect. Redistribution without aggregation would redistribute the three routes, but all three subordinate routes would be advertised into eBGP. By also using BGP route summarization, a single route for 130.1.16.0/20 can be advertised.