Solved: Re: About Router and Routing Table

Ahmed Malik · ‎09-10-2011

Hello everyone i just joined this community and i been preparing myself for the CCNA exam that i will be taking shortly. Now i am still a learner and my question might sound kinda noobish so do help me out.

First question is regarding Access Router.

Q) Now what i studied is that the main access router has 2 interfaces, one interface has a public ip address to acess internet and the other interface has private ip address for local lan subnet. My question is that the home users that access internet from routers at their home, do they also connect to a main access router and secondly the main access router is it also acting as a DCE? Another thing is that the dotted lines in block diagrams of a network, do they represent virtual paths??

Q) Regarding Routing table, when the user's pc forward a packet to a router with the destination ip address and mac address then how does a router determine which path to choose while looking at its routing table?

Peter Paluch · ‎09-10-2011

Hello Ahmed,

Please do not rush people into answering your questions - if there is somebody willing to answer, he or she certainly will do it but please keep in mind we all are volunteers here and do the answering in our free time.

My question is that the home users that access internet from routers at  their home, do they also connect to a main access router and secondly  the main access router is it also acting as a DCE?

The topology of a provider is a vast topic. In essence, however, you are correct: the home routers, also called CPE routers (Customer Premises Equipment), talk to a routing device, sometimes called the Access Router, located at the ISP. The connection between the CPE router and the access router can be made of various technologies - leased lines, virtual circuits, DSL technologies, SONET/SDH, and even Ethernet or WiFi. However, from an IP protocol perspective, a router talks to another router, so yes: the next device to process the IP packets sent by home users and routed by the CPE router is another (access) router at the ISP.

The DCE is a specific term used for serial lines. When serial lines are used, the DCE is a modem directly connected to the CPE router and located at the customer premises as well (usually in the same rack with the CPE router). The CPE router is in the DTE position. The access router does not act as a DCE device - rather, it also works in the DTE mode, and there must be another modem at the ISP - either standalone or as a built-in device - that works in the DCE mode towards the ISP router.

Another thing is that the dotted lines in block diagrams of a network, do they represent virtual paths??

It depends on the diagram - we would need to see one of them to be able to say for sure. Dotted lines can represent console connections, virtual paths, tunnels and another components - there is no fixed established meaning.

Regarding Routing table, when the user's pc forward a packet to a router  with the destination ip address and mac address then how does a router  determine which path to choose while looking at its routing table?

The router receives the packet encapsulated in a frame because the destination MAC address of the frame is set to the router's MAC address. After that, the entire frame header is thrown away and only the IP packet is further used to make the routing decision.

The router takes the destination IP address from the IP packet and traverses its routing table. It performs a binary AND between the destination IP address and the netmask recorded in the particular entry of the routing table (i.e. it computes the network address from the destination address using a particular prefix length determined by the netmask), and compares the result of this AND to the network address in the same routing table entry. If these two network addresses match, the router has found the way to forward this packet. If these two values do not match, the router tries the next entry in the routing table until it finds a match or hits the end of the routing table. The routing table itself is internally sorted in the descending order, starting from the longest netmasks to the shortest. This way, the router always tries to find the match in the longest prefix, i.e. the most specific match.

Feel welcome to ask further.

Best regards,

Peter

View solution in original post

Peter Paluch · ‎09-10-2011

Hello Ahmed,

The DCE/DTE terminology is inspired by an earlier architecture of data transfers over longer distances, usually provided by means of telephone companies that had the necessary communication infrastructure to carry signals over a geographically large area.

In these networks, you could not connect your digital devices like mainframes, computers, not even routers directly onto a leased line provided by your telco, because that line was unable to carry digital signals as produced by network interfaces directly over longer distances. These digital signals would be attenuated and distorted very soon, and become unintelligible for the receiving party. Therefore, these digital signals had to be converted to analog signals as they could be transported over longer distances more readily. This function of converting digital signals to analog and vice versa was provided by modems.

So, the modems were attached to both sides of a circuit provided by a telco. Their function was to take the digital signal from the attached PC or router, carry in an analog form over the telecommunication circuit, and convert it back to digital signal so that the opposite PC or router could process the data. The circuit provided by the telco was effectively terminated by these modems, as they were connected directly to the circuit's ends. This is the reason that these modems are routinely called as Data Circuit-terminating Equipment, in short, DCE. Note that two modems, or two DCEs, directly talk to each other over the circuit provided by the telco.

The devices connected to these modems are further called Data Terminal Equipment, or DTE. The name is inspired by the early notion of 'terminal', i.e. a device where data could be accessed or processed, and obviously, a PC or a router is just that.

So the connection would be:

DTE (router) ----- DCE (modem) ----- telco data circuit ----- DCE (modem) ----- DTE (router)

Even if the DCE is not provided as a standalone device, i.e. modem, it may still be functionally present, perhaps as some built-in component of an integrated router. For example, the ADSL modem built into a broadband router can be considered a DCE component, and the opposite modem in the DSLAM can be considered another DCE component.

In other words, a DCE is reponsible for converting digital signals to a form that allows to carry it over a particular medium. The DTE is the device that needs to have its data transferred and uses the DCE to adapt the digital signals for the necessary transmission.

I am not sure if this is not more confusing that explanatory - please do ask further

Best regards,

Peter

View solution in original post

Ahmed Malik · ‎09-10-2011

can anyone help me out here??

Peter Paluch · ‎09-10-2011

Hello Ahmed,

Please do not rush people into answering your questions - if there is somebody willing to answer, he or she certainly will do it but please keep in mind we all are volunteers here and do the answering in our free time.

My question is that the home users that access internet from routers at  their home, do they also connect to a main access router and secondly  the main access router is it also acting as a DCE?

The topology of a provider is a vast topic. In essence, however, you are correct: the home routers, also called CPE routers (Customer Premises Equipment), talk to a routing device, sometimes called the Access Router, located at the ISP. The connection between the CPE router and the access router can be made of various technologies - leased lines, virtual circuits, DSL technologies, SONET/SDH, and even Ethernet or WiFi. However, from an IP protocol perspective, a router talks to another router, so yes: the next device to process the IP packets sent by home users and routed by the CPE router is another (access) router at the ISP.

The DCE is a specific term used for serial lines. When serial lines are used, the DCE is a modem directly connected to the CPE router and located at the customer premises as well (usually in the same rack with the CPE router). The CPE router is in the DTE position. The access router does not act as a DCE device - rather, it also works in the DTE mode, and there must be another modem at the ISP - either standalone or as a built-in device - that works in the DCE mode towards the ISP router.

Another thing is that the dotted lines in block diagrams of a network, do they represent virtual paths??

It depends on the diagram - we would need to see one of them to be able to say for sure. Dotted lines can represent console connections, virtual paths, tunnels and another components - there is no fixed established meaning.

Regarding Routing table, when the user's pc forward a packet to a router  with the destination ip address and mac address then how does a router  determine which path to choose while looking at its routing table?

The router receives the packet encapsulated in a frame because the destination MAC address of the frame is set to the router's MAC address. After that, the entire frame header is thrown away and only the IP packet is further used to make the routing decision.

The router takes the destination IP address from the IP packet and traverses its routing table. It performs a binary AND between the destination IP address and the netmask recorded in the particular entry of the routing table (i.e. it computes the network address from the destination address using a particular prefix length determined by the netmask), and compares the result of this AND to the network address in the same routing table entry. If these two network addresses match, the router has found the way to forward this packet. If these two values do not match, the router tries the next entry in the routing table until it finds a match or hits the end of the routing table. The routing table itself is internally sorted in the descending order, starting from the longest netmasks to the shortest. This way, the router always tries to find the match in the longest prefix, i.e. the most specific match.

Feel welcome to ask further.

Best regards,

Peter

Ahmed Malik · ‎09-10-2011

First Off thanks for such a detailed answer. Now what i didnt get is that you said DCE modem is only used in case of serial lines so at my home its only a router and no DCE modem, meaning that there is DCE modem at ISP side only?

Secondly incase of serial lines the DCE modem is present at customer premises too so once the data goes from the DCE ( at customer side) to the ISP than does it again has to pass through a DCE modem too or just a main access router only?

Thanks,

Ahmed

Peter Paluch · ‎09-10-2011

Hello Ahmed,

The DCE/DTE terminology is inspired by an earlier architecture of data transfers over longer distances, usually provided by means of telephone companies that had the necessary communication infrastructure to carry signals over a geographically large area.

In these networks, you could not connect your digital devices like mainframes, computers, not even routers directly onto a leased line provided by your telco, because that line was unable to carry digital signals as produced by network interfaces directly over longer distances. These digital signals would be attenuated and distorted very soon, and become unintelligible for the receiving party. Therefore, these digital signals had to be converted to analog signals as they could be transported over longer distances more readily. This function of converting digital signals to analog and vice versa was provided by modems.

So, the modems were attached to both sides of a circuit provided by a telco. Their function was to take the digital signal from the attached PC or router, carry in an analog form over the telecommunication circuit, and convert it back to digital signal so that the opposite PC or router could process the data. The circuit provided by the telco was effectively terminated by these modems, as they were connected directly to the circuit's ends. This is the reason that these modems are routinely called as Data Circuit-terminating Equipment, in short, DCE. Note that two modems, or two DCEs, directly talk to each other over the circuit provided by the telco.

The devices connected to these modems are further called Data Terminal Equipment, or DTE. The name is inspired by the early notion of 'terminal', i.e. a device where data could be accessed or processed, and obviously, a PC or a router is just that.

So the connection would be:

DTE (router) ----- DCE (modem) ----- telco data circuit ----- DCE (modem) ----- DTE (router)

Even if the DCE is not provided as a standalone device, i.e. modem, it may still be functionally present, perhaps as some built-in component of an integrated router. For example, the ADSL modem built into a broadband router can be considered a DCE component, and the opposite modem in the DSLAM can be considered another DCE component.

In other words, a DCE is reponsible for converting digital signals to a form that allows to carry it over a particular medium. The DTE is the device that needs to have its data transferred and uses the DCE to adapt the digital signals for the necessary transmission.

I am not sure if this is not more confusing that explanatory - please do ask further

Best regards,

Peter

Ahmed Malik · ‎09-10-2011

Thanks alot of clearing out. I just started preparing myself for the ccna and its my 3rd week into this and so i want to clear my concepts thats why i asked. If i face more problems i might bother you again.

Thanks,

Ahmed