IP Routing
Here you will find answers to IP Routing Questions
Question 1
Refer to the exhibit. In this network, R1 has been configured to advertise a summary route, 192.168.0.0/22, to R2. R2 has been configured to advertise a summary route. 192.168.0.0/21. to R1. Both routers have been configured to remove the discard route (the route to null created when a summary route is configured) by setting the administrative distance of the discard route to 255.
What will happen if R1 receives a packet destined to 192.168.3.1?
A. The packet will loop between R1 and R2
B. It is not possible to set the administrative distance on a summary to 255
C. The packet will be forwarded to R2, where it will be routed to null0
D. The packet will be dropped by R1, since there is no route to 192.168.3.1.
Answer: A
Question 2
While troubleshooting a network, you need to verify the liveness of hosts in the subnet 192.168.1.64/26. All of the hosts are able to reply to ping requests. How would you confirm the existing nodes using one single command?
A. ping 192.168.1.255
B. ping with sweep option
C. ping 192.168.1.127
D. ping 192.168.1.64
E. ping with broadcast option
Answer: C
Explanation
The 192.168.1.27 is the broadcast address of the 192.168.1.64/26 sub-network so by sending a ping request to this address all the hosts in this subnet will reply (to the broadcast address). But it is not quite right nowadays as all the Cisco’s routers which have IOS version 12.0 or above will simply drop these pings. If you wish to test this function then you have to turn on the “ip directed-broadcast” function (which is disabled by default from version 12.0).
The purpose of the “ip directed-broadcast” command is to enable forwarding of directed broadcasts. When this is turned on for an interface, the interface will respond to broadcast messages that are sent to its subnet. Cisco introduced this command in IOS version 10 (and it is enabled by default) but they soon realized this command was being exploited in denial of service attacks and disabled it from version 12.0.
As you can guess, a ping to the broadcast address requires all hosts in that subnet to reply and it consumes much traffic if many are sent. A type of this attack is “smurf attack”, in which the attacker tries to borrow the victim’s IP address as the source address and sends ICMP packets to the broadcast address of the network. When all the hosts in that subnet hear the ICMP request, they will reply to the computer which the attacker borrowed the IP address from.
You can try this function by enabling “ip directed-broadcast” command in interface mode. Then from the directly connected router issue the ping to the broadcast address of that subnet (or ping 255.255.255.255).
Question 3
Which mechanism can you use to achieve sub-second failover for link failure detection when a switched Ethernet media is used and loss of signal is not supported by the link provider?
A. OSPF standard hellos
B. Cisco Discovery Protocol link detection
C. Bidirectional Forwarding Detection
D. Fast Link Pulse
E. autonegotiation
Answer: C
Explanation
BFD is a detection protocol designed to provide fast forwarding path failure detection times for all media types, encapsulations, topologies, and routing protocols. In addition to fast forwarding path failure detection, BFD provides a consistent failure detection method for network administrators. Because the network administrator can use BFD to detect forwarding path failures at a uniform rate, rather than the variable rates for different routing protocol hello mechanisms, network profiling and planning will be easier, and reconvergence time will be consistent and predictable
(Reference: http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/fs_bfd.html)
Question 4
Half of your network uses RIPv2 and the other half runs OSPF. The networks do not communicate with each other. Which two of these factors describe the impact of activating EIGRP over each separate part? (Choose two)
A. EIGRP will not be accepted when configured on the actual RIPv2 routers.
B. OSPF will no longer be used in the routing table, because you only have EIGRP internal routes running.
C. OSPF will no longer be used in the routing table, because you only have EIGRP external routes running.
D. RIPv2 will populate its RIP database but not its routing table, because you only have EIGRP external routes running.
E. RIPv2 will populate its RIP database but not its routing table, because you only have EIGRP internal routes running.
F. OSPF database will have RIPv2 routes.
Answer: B E
Question 4
Based on the exhibit presented. What will be the objective of this route map when applied to traffic passing through a router?
| route-map direct-traffic permit 10 match ip address 100 set next-hop 10.1.1.1 ……………. access-list 100 permit ip any host 10.1.14.25 access-list 100 permit ip 10.2.0.0 0.0.255.255 any |
A. Take any packet sourced from any address in the 10.2.0.0/16 network or destined to 10.1.14.25 and set the next hop to 10.1.1.1
B. Take any packet sourced from any address in the 10.2.0.0/16 network and destined to 10.1.14.25 and set the next hop to 10.1.1.1
C. Nothing; extended access lists are not allowed in route maps used for policy-based routing
D. Drop any packet sourced from 10.2.0.0/16
Answer: A
Can some1 explain question1?
Why loop.
Thnx
Why loop in question 1?
R1 would have the most specific route to 192.168.3.1 so it wont be forwarded to R2. I should go directly to Null0.
Can somebody explain?
Q1. Both routers have been configured to remove the discard route. 192.168.3.1 is not exist . both routers will use summary route till TTL reach 0
I tried question 1 in my lab and there is no loop, tried with ospf eigrp and got the same result. Aswer is D.
Question-1 answer is explaining here
http://astorinonetworks.com/2011/07/20/summary-routes-to-null0-the-protocols-that-love-them/
Thanks gun,
The explanation is good but question 1 never mention anything about default routes, when using default routes you will have a loop, but again q1 doesn’t tell anything about default routes.
The correct answer is D. When R1 gets the packet, it will have a route from R2 for 192.168.0.0/21. That covers 192.168.0.1 – 192.168.7.255. Therefore, the packet gets routed to R2. R2 has a route for 192.168.0.0/22 from R1 which covers 192.168.0.1 – 192.168.3.255 so R2 will send it back to R1.
Keep in mind, this only works if you have properly disabled the EIGRP summary route to null0 as the question states. If you did not do that, R1 would have routed the packet to null0 due to the summary route.
P.S — Here are the configurations and CLI output to prove the answer is A. This is done on IOS 12.4T train of code.
R1
—
interface Loopback0
ip address 192.168.1.1 255.255.255.0
!
interface FastEthernet0/0
ip address 10.1.2.1 255.255.255.0
ip summary-address eigrp 1 192.168.0.0 255.255.252.0 255
!
router eigrp 1
network 10.0.0.0
network 192.168.1.0
no auto-summary
R2
—
interface Loopback0
ip address 192.168.4.2 255.255.255.0
!
interface FastEthernet0/0
ip address 10.1.2.2 255.255.255.0
ip summary-address eigrp 1 192.168.0.0 255.255.248.0 255
!
router eigrp 1
network 10.0.0.0
network 192.168.4.0
no auto-summary
Show Output
—————–
Look at R1’s routing table…notice it does NOT have the summary route to null0 for 192.168.0.0/22 because we have disabled it. It does have a summary route to 192.168.0.0/21 from R2 it is receiving…
R1#show ip route | b Gateway
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.2.0 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, Loopback0
D 192.168.0.0/21 [90/156160] via 10.1.2.2, 00:03:45, FastEthernet0/0
Now look at R2’s routing table. Notice it does NOT have a summary route to null0 for the 192.168.0.0/21 because we disabled it. It does however have a route for 192.168.0.0/22 received from R1.
R2#sh ip route | b Gateway
Gateway of last resort is not set
C 192.168.4.0/24 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.2.0 is directly connected, FastEthernet0/0
D 192.168.0.0/22 [90/156160] via 10.1.2.1, 00:05:36, FastEthernet0/0
Now, what will happen? As I said — When R1 pings 192.168.3.1 it will look in it’s routing table, and it will see the EIGRP route 192.168.0.0/21 from R2 so it will send the packet to R2 since 192.168.3.1 is part of that summary. R2 will get the packet and look in it’s routing table. It will find an EIGRP route for 192.168.0.0/22 from R1 and route the packet back to R1 since 192.168.3.1 is within that summary range. This continues in a loop until the IP TTL expires.
Traceroute from R1. Notice it goes to R2 then right back to R1…Hope this helps!
————————
R1#trace 192.168.3.1
Type escape sequence to abort.
Tracing the route to 192.168.3.1
1 10.1.2.2 0 msec 0 msec 4 msec
2 10.1.2.1 0 msec 0 msec 4 msec
3 * * *
I think the only thing to remind is:
Indeed, When you change the administrative distance of the discard route to 255, this prevents the route to be installed in the routing table
BUT
THIS NOT PREVENT the route to be advertise to the other peer !
From the Cisco website:
“You can configure a summary aggregate address for a specified interface. If there are any more specific routes in the routing table, EIGRP will advertise the summary address out the interface with a metric equal to the minimum of all more specific routes”
And don’t forget YOU HAVE a directly connected route 192.168.1/24 in R1 and .4/24 in R2.
Then your summary is advertised, and the packet loop between R1 and R2.
If you do the test with GNS3, you will see via “show ip eigrp topology” that you FD of your local subnet 192.168.1.0/24 in R1 become the metric of the summary route “192.168.0.0/22” advertise to R2 ! And vice-versa.
Joe is right, i made the test also 🙂
The Answer is totally “A”
Here is an Abstract of my show commands
——————————————————
R1# sh ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(192.168.1.254)
Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,
r – reply Status, s – sia Status
P 10.1.2.0/24, 1 successors, FD is 281600
via Connected, Ethernet0
P 10.0.0.0/8, 1 successors, FD is 281600
via Summary (281600/0), Null0
P 192.168.0.0/21, 1 successors, FD is 284160
via 10.1.2.2 (284160/28160), Ethernet0
P 192.168.0.0/22, 1 successors, FD is 28160
via Summary (28160/0), Null0
P 192.168.1.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0
R1#sh ip route 192.168.3.1
Routing entry for 192.168.0.0/21, supernet
Known via “eigrp 1”, distance 90, metric 284160, type internal
Redistributing via eigrp 1
Last update from 10.1.2.2 on Ethernet0, 00:10:46 ago
Routing Descriptor Blocks:
* 10.1.2.2, from 10.1.2.2, 00:10:46 ago, via Ethernet0
Route metric is 284160, traffic share count is 1
Total delay is 1100 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
#######################################################
R2#sh ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(192.168.4.254)
Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,
r – reply Status, s – sia Status
P 10.1.2.0/24, 1 successors, FD is 281600
via Connected, Ethernet0
P 10.0.0.0/8, 1 successors, FD is 281600
via Summary (281600/0), Null0
P 192.168.0.0/22, 1 successors, FD is 284160
via 10.1.2.1 (284160/28160), Ethernet0
P 192.168.0.0/21, 1 successors, FD is 28160
via Summary (28160/0), Null0
P 192.168.4.0/24, 1 successors, FD is 28160
via Connected, FastEthernet0
R2#sh ip route 192.168.3.1
Routing entry for 192.168.0.0/22, supernet
Known via “eigrp 1”, distance 90, metric 284160, type internal
Redistributing via eigrp 1
Last update from 10.1.2.1 on Ethernet0, 00:06:27 ago
Routing Descriptor Blocks:
* 10.1.2.1, from 10.1.2.1, 00:06:27 ago, via Ethernet0
Route metric is 284160, traffic share count is 1
Total delay is 1100 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
Hope this help in comprehension.
http://www.4shared.com/folder/HSwcnsCH/_online.html
Pass4share 205Q dums.
yup Q1 answer is A. joe astorino and p3rh0ps comments describes why. i’ve confirmed it in today’s exam.
Hi guys.
I need the dumps of 642 263 …. can you help me out
Hi,
I`m preparing for lab exam. Going to try it on Feb 2013, if someone want to share experience i can help and explain ccie technology
skype neogsm1
http://dl.dropbox.com/u/59575250/13_12_2012%2021_01_28.jpeg
Hi all,
I still confused with this question, can anyone please give explain the answer..
Thank’s