BGP Questions 5
Here you will find answers to BGP Questions – Part 5
Above is the output from show ip bgp neighbors command. What is line 21 stating about the BGP connection?
|BGP neighbor is 172.16.254.3, remote AS 150, internal link
1. BGP version 4, remote router ID 172.16.254.3
2. BGP state = Established, up for 19:24:07
3. Last read 00:00:06, hold time is 180, keepalive interval is 60 seconds
4. Neighbor capabilities:
5. Route refresh:advertised and received(new)
6. Address family IPv4 Unicast:advertised and received
7. Graceful Restart Capability:advertised and received
8. Remote Restart timer is 120 seconds
9. Address families preserved by peer
10. IPv4 Unicast
11. Received 4231 messages, 0 notifications, 0 in queue
12. Sent 4167 messages, 0 notifications, 0 in queue
13. Default minimum time between advertisement runs is 5 seconds
14. For address family:IPv4 Unicast
15. BGP table version 159559, neighbor version 159559
16. Index 90, Offset 11, Mask 0×4
17. Route refresh request:received 0, sent 0
18. 10031 accepted prefixes consume 441364 bytes
19. Prefix advertised 29403, suppressed 0, withdrawn 9801
20. Number of NLRIs in the update sent:max 242, min 0
21. Connections established 2; dropped 1
22. Last reset 19:26:54, due to NSF peer closed the session
23. Connection state is ESTAB, I/O status:1, unread inout bytes:0
24. Local host:18.104.22.168, Local port:11005
25. Foreign host:172.16.254.3, Foreign port:179
A. the number of consecutive TCP connections to the specified remote neighbor
B. the number of times the router has established a TCP connection
C. the number of total TCP connections that the router has
D. the number of neighbors that the router has
According to http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_s2g.html#wp1040913, “Connections established” is the number of times a TCP and BGP connection have been successfully established while “dropped” is the number of times that a valid session has failed or been taken down.
Refer to the exhibit. Routing updates for the 192.168.1.0 network are being received from all three neighbors. Which statement is correct regarding the result of the configuration shown?
A. The router will prefer the next hop of 172.16.1.1 for packets destined for the 192.168.1.0 network.
B. The router will prefer the next hop of 172.26.1.1 for packets destined for the 192.168.1.0 network.
C. The router will advertise the 192.168.1.0 network only to 172.30.1.1.
D. The router will advertise the 192.168.1.0 network only to 172.26.1.1.
E. The router will prefer the next hop of 172.26.1.1 for packets except those destined for the
The local-preference of the next hop 172.26.1.1 is set to 200, which is higher than the default value (100) so this path is preferred for packets destined to the 192.168.1.0 network.
Refer to the exhibit. Which statement is true about the 22.214.171.124/24 prefix?
A. If another path advertises the 126.96.36.199/24 path and has the default local preference, that path is more preferred.
B. The command neighbor send-community is configured on BGP neighbor 10.10.23.3.
C. The route 10.10.23.3 is not being advertised to other BGP neighbors.
D. Route 188.8.131.52/24 is learned by an IBGP peer.
By default, the community attributes are not advertised to BGP neighbors. But in the output we see the “Community: 100:250″ which means the command “neighbor … send-community” was used to send community attributes of the local router to the neighbor. For more information about this command please read my BGP next-hop-self, community no-export & send-community – GNS3 Lab.
Refer to the exhibit. Currently the two eBGP links between AS100 and AS200 have an average inbound load of 65% and 20% respectively. After further investigation, traffic to 10.10.1.16/28 accounts for 45%, and traffic to 10.10.1.32/28 and to 10.10.1.48/28 each account for 20% of the inbound load. The BGP attributes are currently set at their default values in both autonomous systems.
If you want to influence how AS200 sends traffic to AS100, which eBGP configurations would you configure in AS100 to influence AS200 to use the eBGP links more evenly? (Choose two.)
A. neighbor 192.168.30.2 route-map as_50 out
B. neighbor 192.168.20.2 route-map as_50 out
C. route-map as_50 permit 10
match ip address 50
set metric 150
access-list 50 permit 10.10.1.16 0.0.0.240
D. route-map as_50 permit 10
match ip address 50
set metric 150
access-list 50 permit 10.10.1.32 0.0.0.240
Answer: B C
Note: The wildcard masks in the access-list are not correct, they should be 0.0.0.15 instead of 0.0.0.240.
First let’s recall about MED. If you want to influence external neighbors about the path it sends traffic then MED, also called the metric, should be used. A lower MED value is preferred over a higher value. The default MED value is 0.
Also, an important point I wish to notice in this question is: there is a “deny all” statement at the end of each access-list. In other words, all the routes that do not match the access-list used by the route-map will be filtered out. For example in answer C, only network 10.10.1.16/28 is matched by the access-list and all other networks will be dropped (not advertised to EBGP).
Therefore in answer C, only traffic to 10.10.1.16/28 will be go through routerA (45%). All other traffic must go through routerB (40%) -> links are used more evenly -> C is correct.
There are some debates saying D is correct but in fact it is not. If answer D is applied for router A, the MED for 10.10.1.32 set to 150 would make traffic for this route go through router B (because the MED default value is 0, which is lower than 150). The access-list will also filter out network 10.10.1.16 -> router E can not send traffic for network 10.10.1.16 to router A. Now all traffic must go through router B (85%).
(For detailed information about how to use route map with MED, please read my BGP Route map and MED – GNS3 Lab)
Refer to the exhibit. A client has asked you to consult on an eBGP loading question. Currently the AS 100 eBGP links have an average outbound load of 65% and 20% respectively. On further investigation, traffic from 10.10.24.0 accounts for 45%, and 10.10.25.0 and 10.10.32.0 accounts for 20% each of the outbound load. The customer wants to spread the load between the two eBGP links more evenly. The BGP attributes are currently set at their default values.
If you are located at AS 100 and want to influence how AS 100 sends traffic to AS 200, what BGP attribute could you configure to cause AS 100 outbound traffic to load the eBGP links more evenly?
A. On router A, set the default local-preference to 50.
B. On router B, set the default metric to 150.
C. On router B, configure a route map for 10.10.25.0/24 with a local preference of 150 linked to neighbor 192.168.30.2.
D. On router B, set the default local-preference to 150.
To make the eBGP links more evenly we should use the link B-F for network 10.10.25.0/24 so that the total traffic going through B-F link is about 40%. In this case we should apply a route map on B to set the local preference of 10.10.25.0/24 to a higher value than 100. But notice that we must use a second clause to permit other traffic if not they will be filtered out.
Note: The default value for local preference is 100. A path with higher local preference is preferred.
Refer to the exhibit. AS 65500 is not advertising the prefix 192.168.12.0/22 to its provider. AS 65500 is running OSPF as its IGP.
Which of the following additions to the configuration is most likely to solve the problem?
A. RouterA(config)#ip cef
B. RouterA(config)#ip route 192.168.12.0 255.255.255.252 null 0
C. RouterA(config-router)#ebgp multihop 1
D. RouterA(config-router)#redistribute ospf 1
E. RouterA(config-router)#neighbor 192.168.14.253 next-hop-self
F. RouterA(config-router)#neighbor 192.168.14.253 local-as 65500
The synchronization rule states “A BGP router should not use, or advertise to an external neighbor, a route learned by IBGP, unless that route is local or is learned from the IGP”. Notice that IGP here can be a static route.
In this case, unless there is an entry about network 192.168.12.0 in the routing table of RouterA, RouterA will not advertise this network to its EBGP -> B is correct (even if this static route points to Null0).
Note: Although this question states that OSPF is being used as IGP but for some reasons, network 192.168.12.0/22 is not advertised to RouterA -> RouterA does not have this route in its routing table.