CCNA, CCNP Made Easy !!! Learn indepth while you learn......

[Insight of the week] DSL Modem......

2009-12-16T13:11:00.002+05:30

My previous posts....

My Previous Insights...

What is a DSL Modem?

DSL and ADSL modems are transceiver devices that assist in the connection of a router or a single computer to a DSL phone line so that the consumer can use an ADSL service. DSL modems are also referred to as ATU-R, Network termination broadband adapters, network termination broad band access (NTBBA) in some countries or simply DSL transceivers.

DSL modems are also used as a gateway to connect and share DSL service with several machines. In situations as such the modem is referred to as a DSL router or residential gateway

Operating Systems and DSL Modems

DSL modems are not recognized by Microsoft Windows or other operating systems. This is because the computer and transceiver are viewed as separate nodes in LAN, rather than the transceiver being a device controlled or restricted by the computer, like keyboards, webcams or the mouse. Routers are configured via a Web page. The Web page can be obtained via the modem through the Ethernet the router is wired to. DSL modems, however, seldom require configuration. This because the computer networks that they are a part of has basically done it all for them. The DSL modems are only acting as a medium, sending ones and zeros from one computer network location to another, which is the telephone line.

How Do DSL Modems Work?

DSL modems consist of a functional block. This block performs framing. ADSL modems work as a bridge and do not require an IP address. However, it might consist of one for management functions.
DSL modems transform high-frequency tones to transmit tones to a DSLAM. DSL modems also receive and demodulate the tones from the DSLAM for the purpose of serving computers.
Most DSL modems are outside of the computer and connected via wire to the computer's Ethernet port. Sometimes DSL modems are connected via a USB port. Like voice band modems that are located inside computers, there are also internal DSL modems available (with PCI interface) that are also located inside computers; however, internal DSL modems are uncommon.
DSL modems are basically for specific protocols, because they don't work on all lines even when they are from the same company. However, because of the tremendous Internet speed that DSL modems offer, in addition to the ability to use both the telephone and computer simultaneously, many consumers prefer the use of DSL modems. DSL filters are required to be connected to the phone lines so that both can be accomplished without interruption.
The frequency of DSL modems is 25 kHz, extending beyond 1MHz, which prevents interference with voice service.........

Some Links to Learn More....

http://computer.howstuffworks.com/modem3.htm
http://www.wisegeek.com/what-is-a-dsl-modem.htm

Videos

http://www.youtube.com/watch?v=Tz2d17gWZnc

EIGRP Stub Sim Question

2009-12-10T12:30:00.000+05:30

Routing Hot questions [Click Here]

EIGRP Stub Sim Question

Question:

By increasing the first distant office, JS manufactures has extended their business. They configured the remote office router (R3) from which they can reach all Corporate subnets. In order to raise network stableness and lower the memory usage and broadband utilization to R3, JS manufactures makes use of route summarization together with the EIGRP Stub Routing feature. Another network engineer is responsible for the implementing of this solution. However, in the process of configuring EIGRP stub routing connectivity with the remote network devices off of R3 has been missing.

Presently JS has configured EIGRP on all routers in the network R2, R3, and R4. Your duty is to find and solve the connectivity failure problem with the remote office router R3. You should then configure route summarization only to the distant office router R3 to complete the task after the problem has been solved.
The success of pings from R4 to the R3 LAN interface proves that the fault has been corrected and the R3 IP routing table only contains two 10.0.0.0 subnets.

Answer and Explanation:

First we have to figure out why R3 and R4 can not communicate with each other. Use the show running-config command on router R3

Notice that R3 is configured as a stub receive-only router. The receive-only keyword will restrict the router from sharing any of its routes with any other router in that EIGRP autonomous system. This keyword will also prevent any type of route from being sent.
Therefore we will remove this command and replace it with the eigrp stub command:

R3#configure terminal
R3(config)#router eigrp 123
R3(config-router)#no eigrp stub receive-only
R3(config-router)#eigrp stub
R3(config-router)#end

Now R3 will send updates containing its connected and summary routes to other routers. Notice that the eigrp stub command equals to the eigrp stub connected summary because the connected and summary options are enabled by default.
Next we will configure router R3 so that it has only 2 subnets of 10.0.0.0 network. Use the show ip route command on R3 to view its routing table

R3#show ip route

Because we want the routing table of R3 only have 2 subnets so we have to summary sub-networks at the interface which is connected with R3, the s0/0 interface of R4. There is one interesting thing about the output of the show ip route shown above: the 10.2.3.0/24, which is a directly connected network of R3. We can't get rid of it in the routing table no matter what technique we use to summary the networks. Therefore, to make the routing table of R3 has only 2 subnets we have to summary other subnets into one subnet.
In conclusion, we will use the ip summary-address eigrp 123 10.0.0.0 255.0.0.0 at the interface s0/0 of R4 to summary.

R4>enable
R4#configure terminal
R4(config)#interface s0/0
R4(config-if)#ip summary-address eigrp 123 10.0.0.0 255.0.0.0

Now we jump back to R3 and use the show ip route command to verify the effect, the output is shown below:

(But please notice that the ip addresses and the subnet masks in your real exam might be different so you might use different ones to solve this question)
Just for your information, notice that if you use another network than 10.0.0.0/8 to summary, for example, if you use the command ip summary-address eigrp 123 10.2.0.0 255.255.0.0 you will leave a /16 network in the output of the show ip route command.

But in your real exam, if you don't see the line "10.0.0.0/8 is a summary,....Null0" then you can summary using the network 10.2.0.0/16. This summarization is better because all the pings can work well.
Finally don't forget to use the copy running-config startup-config command on routers R3 and R4 to save the configurations.

R4(config-if)#end
R4#copy running-config startup-config

[EIGRP] CCNP Routing Hot Questions !!!!!!!

2009-12-06T18:45:00.001+05:30

HERE YOU WILL FIND EIGRP Exam Questions

Question 1:

Which three statements about the EIGRP routing protocol are true? (Choose three)
A - EIGRP sends periodic hello packets to the multicast IP address 224.0.0.9
B - EIGRP sends periodic hello packets to the multicast IP address 224.0.0.10
C - EIGRP supports five generic packet types. including hello, update, query, reply, and ACK packets
D - EIGRP supports five generic packet types, including hello, database description (DBD), link-state request (LSR), link-state update (LSU), and LSAck
E - E. EIGRP will form a neighbor relationship with another peer even when their K values are mismatched
F - A. EIGRP will not form a neighbor relationship with another peer when their K values are mismatched

Answer: B, C, F

Question 2:

After DUAL calculations, a router has identified a successor route, but no routes have qualified as a feasible successor. In the event that the current successor goes down, what process will EIGRP use in the selection of a new successor?
A - EIGRP will find the interface with the lowest MAC address
B - The route will transition to the active state
C - The route will transition to the passive state
D - EIGRP will automatically use the route with the lowest feasible distance (FD)
E - EIGRP will automatically use the route with the lowest advertised distance (AD)

Answer: B

Question 3:

Refer to the exhibit. Routers R1 and R2 have established a neighbor relationship and are exchanging routing information. The network design requires that R1 receive routing updates from R2, but not advertise any routes to R2. Which configuration command sequence will successfully accomplish this task?

A - R1(config)# router eigrp 1

     R1(config-router)# passive-interface serial 0
B - R2(config)# router eigrp 1
     R2(config-router)# passive-interface serial 0
C - R1(config)# access-list 20 deny any
     R1(config)# router eigrp 1
     R1(config-router)# distribute-list 20 out serial 0
D - R2(config)# access-list 20 deny any
     R2(config)# router eigrp 1
     R2(config-router)# distribute-list 20 out serial 0
E - R1(config)# access-list 20 permit any
     R1(config)# router eigrp 1
     R1(config-router)# distribute-list 20 in serial 0
F - R2(config)# access-list 20 permit any
    R2(config)# router eigrp 1
    R2(config-router)# distribute-list 20 in serial 0

Answer: C

Explanation:

We can not use passive-interface to accomplish this task because the "passive-interface..." command (in EIGRP or OSPF) will shut down the neighbor relationship of these two routers (no hello packets are exchanged). And to filter routing updates we should configure a distribute list on R1 with an access list that deny all and apply it to the outbound direction so that R1 can receive but can not send routing updates.

Question 4:

EIGRP has been configured to operate over Frame Relay multipoint connections. What should the bandwidth command be set to?
A - the CIR rate of the lowest speed connection multiplied by the number of circuits
B - the CIR rate of the lowest speed connection
C - the CIR rate of the highest speed connection
D - the sum of all the CIRs divided by the number of connections

Answer: A

Explanation:

If the multipoint network has different speeds allocated to the VCs, take the lowest CIR and simply multiply it by the number of circuits. This is because in Frame-relay all neighbors share the bandwidth equally, regardless of the actual CIR of each individual PVC, so we have to get the lowest speed CIR rate and multiply it by the number of circuits. This result will be applied on the main interface (or multipoint connection interface).

Question 5:

Refer to the exhibit. EIGRP is configured on all routers in the network. On a basis of the show ip eigrp topology output provided, what conclusion can be derived?
A - Router R1 can send traffic destined for network 10.6.1.0/24 out of interface FastEthernet0/0
B - Router R1 is waiting for a reply from the neighbor 10.1.2.1 to the hello message sent out before it declares the neighbor unreachable
C - Router R1 is waiting for a reply from the neighbor 10.1.2.1 to the hello message sent out inquiring for a second successor to network 10.6.1.0/24
D - Router R1 is waiting for a reply from the neighbor 10.1.2.1 in response to the query sent out about network 10.6.1.0/24

Answer: D

Explanation:

From the output, we notice that there is an active route (A) and the reply status flag (r) was set. An active EIGRP route is the state when a network change occurs and a feasible successor is not found by a EIGRP router for a given route (10.6.1.0/24); and the reply status flag (r) means that R1's queries were sent out to the neighbors asking for routing information to the 10.6.1.0/24 network but hasn't received a reply yet. Therefore the answer A - router R1 can send traffic destined for network 10.6.1.0/24 is not correct because router R1 can't find a path to that network. Answers B and C are not correct because R1 doesn't send a hello message but a query asking for routing information to the desired network.

EIGRP SIMULATION

Watch out the simulation with Full Explanation in my next post !!!!!

Latest CCNA , CCNP Routing Dumps

2009-12-02T20:02:00.000+05:30

Thought this will be helpful for all those who take the exams......

Downloads below......

CCNA

CCNP Routing

CCNP ISCW

CCNP ONT

Thanks,

Kalyan.....

DHCP Continued.....What happens when your DSL modem connects to internet????

2009-11-25T23:48:00.001+05:30

Lets go little in deep and dig what is happening when you try to connect to internet using a DSL modem.

For more understanding, I would recommend to spend few minutes to read the next paragraph, and then go ahead and watch the rest of the page about what is happening deep inside wire.

Let me walk through the STEP BY STEP path taken by the dsl router before it opens for a full fledged internet access:

•    Firstly the modem will start sending DHCP request and wait for the DCHP offer from the server.

•    In the mean time the Ethernet card(MAC) will not be silent and it will keep on be flooding packets for knowing the MAC address for the immediate port.

•    Secondly there is one more thing happening at this time, the DSL modem will send an IGMPv2 join message to the ISP.

Before I complicate the process with introducing two new protocols let me explain the fact that the DSL modem does hell lot of background process before it acquires an IP for it.

•    The Netbios-ns protocol will use UDP: 137 and registers itself with the layer-2 network with the host name(Here if I connect my laptop with modem then the name would be my_laptop_kalyan, nothing but the name I gave for my laptop in windows).
•    Along side this, the SSDP (sinple service discovery protocol will get the services given by ISP, the main thing to be noted here is that the protocol does not need a defined static IP for this communication, it does with out IP), SSDP provides a mechanism which network clients can use to discover network services. Clients can use SSDP with little or no static configuration.

Ahh !!! Now we can see a reply from the dhcp server………..

•    The DHCP server will send dhcp offer containing the ip-address for the client.
•    The address will be registered and once again the ARP will be resolved for knowing MAC addresses(remember we can see frequent arp requests flowing in card, just because of the arp timeouts).

I hope I have explained the process at least to some level of accuracy, please feel free to comment and correct mistakes…….

                  ~~!!!!!!!!!Now lets go ahead and see how the packets look !!!!!!!!!!!!~~

1.    As soon as you turn on your modem you can see the DHCP REQUEST being sent for an ip address to the nearest DHCP Server(located most probably at your ISP).

Let’s go deep into the wire and see what happens at this stage,

Above we can infer,

1.    Ip address of client 0.0.0.0 unknown and that of dest:255.255.255.255 as broadcast.
2.    Dhcp sends an request from client using UDP Port: 68(src) and Port: 67(dst)
3.    dhcp msg type: Request
4.    There are many parameters requested by the boot protocol to the dhcp server, some of the main items to be kept in mind are,
1.    subnet mask
2.    domain name
3.    dns server
4.    nbns server
5.    vebdor specific information
6.    Static route

Dynamic Host Control Protocol.....

2009-11-22T18:08:00.017+05:30

POST 1: Just a very small introduction about DHCP

The DHCP server manages a pool of IP addresses and information about client configuration parameters such as default gateway, domain name, the DNS servers, other servers such as time servers, and so forth. On receiving a valid request, the server assigns the computer an IP address, a lease (length of time the allocation is valid), and other IP configuration parameters, such as the subnet mask and the default gateway. The query is typically initiated immediately after booting, and must complete before the client can initiate IP-based communication with other hosts. Depending on implementation, the DHCP server may have three methods of allocating IP-addresses:

dynamic allocation: A network administrator assigns a range of IP addresses to DHCP, and each client computer on the LAN has its IP software configured to request an IP address from the DHCP server during network initialization. The request-and-grant process uses a lease concept with a controllable time period, allowing the DHCP server to reclaim (and then reallocate) IP addresses that are not renewed (dynamic re-use of IP addresses).
automatic allocation: The DHCP server permanently assigns a free IP address to a requesting client from the range defined by the administrator. This is like dynamic allocation, but the DHCP server keeps a table of past IP address assignments, so that it can preferentially assign to a client the same IP address that the client previously had.
static allocation: The DHCP server allocates an IP address based on a table with MAC address/IP address pairs, which are manually filled in (perhaps by a network administrator). Only requesting clients with a MAC address listed in this table will be allocated an IP address.

Bootp uses manual mapping of mac address-ip address. DHCP is introduced to overcome the issues with bootp.

Please add your comments and share extra information you have. So that it will be helpful for the other viewers....

Download the below setup to your pc and check how dhcp server works !!!!!

Sample Lab Environment illustration

Download Click

In the above lab setup you can find,

Routers R5,4,3,2 are acting as PCs having two connections to each ethernet switch.Each PC will be having two ports expecting dhcp assigned address from dhcp_server_7200.
The dhcp_server_7200 is having two address pools with 10.0.0.0 network and 192.168.1.0 network
respectively.

Once the network is ON, each PC will be getting an ip address from each pool.
The 192.168.1.0 pool is having very less lease time of 10mins "0 0 10", so the server will always be busy reallocating the
address.

The setup is still open for many enhancements !!
1.DHCP Relay.
2.DHCP Accounting.

~Kalyan
kaprakashr@gmail.com'

Remember you should have GNS3 to use the setup, in the download link.

Download Click

~Kalyan
kaprakashr@gmail.com'