Question - 1 Figure shows a simple two-router network. Router 0 has one serial interface and three LAN interfaces. Looking at Figure, can you see which interface Router 0 will use to forward an IP data-gram to a host with an IP address of 10.10.10.10?
I
am showing now is called the “longest match rule,” which means that IP
will look through a routing table for the longest match compared to the
destination address of a packet. Let’s take a look.
By
using the command show ip route, we can see the routing table (map of
the internetwork) that Lab_A uses to make forwarding decisions:
Router 0#sh ip route
[output cut]
Gateway of last resort is not set
C 10.10.10.0/24 is directly connected, FastEthernet0/0
C 10.10.20.0/24 is directly connected, FastEthernet0/1
C 10.10.30.0/24 is directly connected, FastEthernet0/0/0
C 10.10.40.0/24 is directly connected, Serial 0/1/0
Gateway of last resort is not set
C 10.10.10.0/24 is directly connected, FastEthernet0/0
C 10.10.20.0/24 is directly connected, FastEthernet0/1
C 10.10.30.0/24 is directly connected, FastEthernet0/0/0
C 10.10.40.0/24 is directly connected, Serial 0/1/0
The C in the routing table output means that the networks listed are “directly connected,”
Note- You mus know these following code.
The router will packet-switch the packet to interface FastEthernet 0/0, and this interface
will frame the packet and then send it out on the network segment. To reiterate on the longest match rule, IP would look for 10.10.10.10 in this example, and if that is not found in the table, then IP would search for 10.10.10.0, then 10.10.0.0, and so on until a route is found.
Codes Meaning
C - connected
S - static
I - IGRP
R - RIP
M - mobile
B - BGP
D - EIGRP,
EX - EIGRP external,
O - OSPF,
IA - OSPF inter area
N1 - OSPF NSSA external type 1,
N2 - OSPF NSSA external type 2
E1 - OSPF external type 1,
E2 - OSPF external type 2,
E - EGP
i - IS-IS,
L1 - IS-IS level-1,
L2 - IS-IS level-2,
ia - IS-IS inter area
* - candidate default,
U - per-user static route,
o - ODR
P - periodic downloaded static route
By
looking at the figure and the output of the routing table, can you tell
what IP will do with a received packet that has a destination IP
address of 10.10.10.10? The router will packet-switch the packet to interface FastEthernet 0/0, and this interface
will frame the packet and then send it out on the network segment. To reiterate on the longest match rule, IP would look for 10.10.10.10 in this example, and if that is not found in the table, then IP would search for 10.10.10.0, then 10.10.0.0, and so on until a route is found.
Question - 2 Based on the output of the next routing table, which interface will a packet with a destination address of 10.10.10.14 be forwarded from?
Lab_A#sh ip route
[output cut]
Gateway of last resort is not set
C 10.10.10.16/28 is directly connected, FastEthernet0/0
C 10.10.10.8/29 is directly connected, FastEthernet0/1
C 10.10.10.4/30 is directly connected, FastEthernet0/2
C 10.10.10.0/30 is directly connected, Serial 0/0
First,
you can see that the network is subnetted and each interface has a
different mask.10.10.10.14 would be a host in the 10.10.10.8/29 subnet
connected to the FastEthernet0/1 interface.
A stub indicates that the networks in this design have only one way out to reach all other networks.
Figure 1: Configuring a gateway of last resort
Let’s take a look at the output of a show ip route command and compare that to the network in Figure 2 and see if you can find a problem:
Router#sh ip route
[output cut]
Gateway of last resort is 172.19.22.2 to network 0.0.0.0
C 172.17.22.0 is directly connected, FastEthernet0/0
C 172.18.22.0 is directly connected, Serial0/0
S* 0.0.0.0/0 [1/0] via 172.19.22.2
Figure -1 : What is the destination address of a frame, from HostA ?
Figure - 5
Question- 3 What is stub router and select the stub router in given scenario ?
A stub indicates that the networks in this design have only one way out to reach all other networks.
Example -
R2 and R3 routers that I have connected to the Corp router, they are considered stub routers because they have only one way out to reach all other networks.
Question- 4 How can we add the gateway of last resort on the router to ISP ?
There’s another command you can use to help you in your inter-network if you have configured a gateway of last resort—the ip default-network command, Figure 1 shows a network that needs to have a gateway of last resort statement configured.
Here are three commands (all providing a default route solution) for adding a gateway of last resort on the router to the ISP.
Gateway(config)#ip route 0.0.0.0 0.0.0.0 217.124.6.1
Gateway(config)#ip route 0.0.0.0 0.0.0.0 s0/0
Gateway(config)#ip default-network network
The first two are the same command—one just uses the next hop and one the exit interface.
You will find no difference in this configuration. However, if you set
them both for some reason, the exit interface would be used. Do you know
why? Directly connected routes have an administrative distance of 0,
but in this example, you’d see absolutely no functional difference
between the two commands.
Question - 5 What happens if you misconfigured a default route?
[output cut]
Gateway of last resort is 172.19.22.2 to network 0.0.0.0
C 172.17.22.0 is directly connected, FastEthernet0/0
C 172.18.22.0 is directly connected, Serial0/0
S* 0.0.0.0/0 [1/0] via 172.19.22.2
You can see by looking at the figure and the directly connected routes in the routing table that the WAN link is on network 172.18.22.0 and
that the default route is forwarding all packets to the 172.19.22.0
network. It will never work, so the problem is a misconfigured static
(default) route.
Figure 2: Misconfigured default route
Figure 2: Misconfigured default route
Question - 6 If you have the routing table output as shown in the following lines, what happens if the router receives a packet from 10.1.6.100 destined for host 10.1.8.5?
Router#sh ip route
[output cut]
Gateway of last resort is 10.1.5.5 to network 0.0.0.0
R 10.1.3.0 [120/1] via 101.2.2, 00:00:00, Serial 0/0
C 10.1.2.0 is directly connected, Serial0/0
C 10.1.5.0 is directly connected, Serial0/1
C 10.1.6.0 is directly connected, Fastethernet0/0
R* 0.0.0.0/0 [120/0] via 10.1.5.5, 00:00:00 Serial 0/1
This
is a tad different than what I’ve shown you up until now because the
default route is listed as R*, which means it’s a RIP-injected route.
This Is because someone configured the ip default-network command on a
remote router as well as configuring RIP, causing RIP to advertise this
route through the inter-network as a default route. Since the destination
address is 10.1.8.5 and there is no route to network 10.1.8.0, the
router would use the default route and send the packet out serial 0/1.
Question - 7 Figure-1 shows a LAN connected to Router A, which is, in turn, connected via a WAN link to Router B. Router B has a LAN connected with an HTTP server attached.
Figure - 1
Figure -1 : What is the destination address of a frame, from HostA ?
The destination address of a frame, from HostA, will be the MAC address of the Fa0/0 interface of the RouterA router.
Figure - 1 : what is the destination address of a packet from Host A ?
The destination address of a packet will be the IP address of the network interface card (NIC) of the HTTP server.
Figure - 1 : What is the value of destination port no. in the segment header ?
The destination port number in the segment header will have a value of 80.
Question - 8 Figure 2 shows a network with only one router but two switches. What happens when Host A sends data to the HTTPS server?
Figure - 2
Figure -2 : What is the destination address of a frame, from HostA ?
The destination address of a frame, from HostA, will be the MAC address of the Fa0/0 interface of the RouterA router.
Figure - 2 : what is the destination address of a packet from Host A ?
The destination address of a packet will be the IP address of the network interface card (NIC) of the HTTPS server.
Figure - 2 : What is the value of destination port no. in the segment header ?
The destination port number in the segment header will have a value of 443.
Question - 9 Take a look at Figure 3. What will happen if the LAN interface of Router C goes down.
Figure - 3
Router
C will use ICMP to inform Host A that Host C can’t be reached, and it
will do this by sending an ICMP destination unreachable message to Host A.
Question - 10 Look at the output of a router’s routing table: What do we see here? If I were to tell you that the corporate router received an IP packet with a source IP address of 192.168.214.20 and a destination address of 192.168.22.3, what do you think the router will do with this packet?
Router#sh ip route
--------[output cut]------
R 192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0
R 192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0
R 192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0
C 192.168.20.0 is directly connected, Serial0/0
C 192.168.214.0 is directly connected, FastEthernet0/0
“The
packet came in on the FastEthernet 0/0 interface, but since the routing
table doesn’t show a route to network 192.168.22.0 (or a default
route), the router will discard the packet and send an ICMP destination
unreachable message back out to interface FastEthernet 0/0. The reason
it does this is because that’s the source LAN where the packet
originated from.
Question -11 Now, let’s check out another Figure - 4
Figure - 4
Figure
- 4 : In order to begin communicating with the Sales server, Host B
sends out an ARP request. How will the devices exhibited in the topology
respond to this request?
Since
MAC addresses must stay on the local network, the Router B will respond
with the MAC address of the Fa0/0 interface and Host B will send all
frames to the MAC address of the Router B Fa0/0 interface when sending
packets to the Sales server.
Figure - 4 : Host
B has received an ARP reply. Host B will now build a packet, then place
this packet in the frame. What information will be placed in the header
of the packet that leaves Host B, if Host B is going to communicate to
the Sales server?
Since
we’re now talking about packets, not frames, the source address will be
the IP address of Host B and the destination address will be the IP
address of the Sales server.
Figure - 4 : At
last, the Router A router has received the packet and will send it out
Fa0/0 onto the LAN toward the server. What will the frame have in the
header as the source and destination addresses?
The
source MAC address will be the Router A's Fa0/0 interface, and the
destination MAC address will be the Sales server’s MAC address.
Figure - 4 : Host
A is displaying two web documents from the Sales server in two browser
windows at the same time. How did the data find its way to the correct
browser windows?
TCP port numbers are used to direct the data to the correct application window.
Question - 12 Figure 5 shows a basic network, and Host B needs to get email.
Figure - 5
Figure - 5 : Which address will be placed in the destination address field of the frame when it leaves Host A?
The answer is that Host 4 will use the destination MAC address of the Fa0/0 interface of the Router B router.
Figure
- 5 : Host B needs to communicate with Host A. Which OSI layer 3 source
address will be found in the packet header when it reaches Host A?
At
layer 3, the source IP address will be Host B and the destination
address in the packet will be the IP address of Host A. Of course, the
destination MAC address from Host B will always be the Fa0/0 address of
the Router_B router, And since we have more than one router, we’ll need a
routing protocol that communicates between both of them so that traffic
can be forwarded in the right direction to reach the network in which
Host 1 is attached.
Figure
- 5 : Host B is transferring a file to the email server connected to
the Router A router. What would be the layer 2 destination address
leaving Host B? And what will be the source MAC address when the frame
is received at the email server?
Layer
2 destination address leaving Host B will be the MAC address of the
Fa0/0 interface of the Router_B router and that the source layer 2
address that the email server will receive will be the Fa0/0 interface
of the Router_A router..
Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.
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