What Is Router ? - Basic Guide To Routing
Before we learn more about how to configure cisco router, we need to understand better about some basic rules routing. Also of course we must understand the numbering system IP, subnet, netmasking ... etc.
Example cases:
Host X à 128.1.1.1 (Class B network ip en 128.1.xx)
Y à host 128.1.1.7 (class B IP network id 128.1.xx)
Host Z à 128.2.2.1 (class B IP network id 128.2.xx)
In the case above, the host of X and Y can communicate with the host directly, but both hosts X and Y can not communicate with host Z, because they have different network ID. How so Z can communicate with X and Y? use a router!
An example of using subnetting
Host P à 128.1.208.1 subnet mask 255.255.240.0
Host Q à 128.1.208.2 subnet mask 255.255.240.0
Host R à 128.1.80.3 subnet mask 255.255.240.0
Well, when subnetting is used, then the two hosts are connected to the same network segment can communicate only if both the network id and subnetid his sesuai.Pada case above, P and Q can communicate with directly, R has the same network id with P and Q but has a different subnetidyang. Thus R can not communicate directly with P and Q. How so R can communicate with the P and Q? use a router!
So the router function, is easy to say, connect two different networks, precisely directs the best route to achieve the expected network.
In implementation, a router is often used to connect networks between institutions or companies that each have a network with different network id. Another example that is currently popular is when your company will be connected to the internet. Then the router will work stream of data packets from your company to other institutions via the Internet, of course, your network number would be calm with perushaaan you go.
If you simply connect the 2 pieces networks, in fact you can also use Windows NT based pc or linux. By providing 2 pieces and a network card settings, you actually have to make practical router. But of course with all its limitations.
More about routing
The data from the devices connected to the Internet is sent in a datagram, the data packet defined by the IP. Datagram has a destination address of data packets; Internet Protocol check this address to submit original datagram from the device to the destination device. If the datagram destination address is located a home network device, datagram directly delivered to the destination device. If the datagram destination address was not found in the same network, datagram forwarded to the appropriate router (the best available router).
IP router (usually called routers only) are devices that perform functions on the IP datagram to continue the network layer. Router has more than one network Default (network interface) and can forward datagram from one interface to another interface. For each datagram is received, the router checks if the datagram is addressed to him. If it is not addressed to the router, the datagram sent to the transport layer.
If the datagram not addressed to the router, which will examine is that it has a forwarding table to decide where the datagram should be addressed. Forwarding table is a table consisting of pairs of IP addresses (host address or network address), following the router address, and the interface where the datagram out.
If you do not find even a single line in the appropriate forwarding table with a destination address, the router will give a message to the sender that the address in question can not be achieved. This incident may be analogous with the message "return to sender" on mail. A router can also tell that he was not the best router to a destination, and suggested the use of other routers. With the three functions contained on this router, hosts on the Internet can connect to each other.
Static and Dynamic
In general, routing coordination mechanisms can be divided into two: static routing and dynamic routing. In static routing, the entries in the router's forwarding table is filled and removed manually, while the dynamic routing changes are made through routing protocols. Static routing is the simplest routing settings that can be done on computer networks. Using pure static routing in a network means to fill each entry in the forwarding table at each router in the network.
The use of static routing in a small network of course is not a problem; only a few entries that need to be filled in the forwarding table at each router. But you certainly can imagine what if must complete the forwarding table in each router that number is not small in a large network. Especially if you are assigned to fill the entries in the router on the Internet are very numerous and growing every day. Of trouble once!
Dynamic routing is the way used to release the obligation to fill these entries manually forwarding table. Routing protocol routers set so that it can communicate with each other and give each other routing information that can change the contents of a forwarding table, depending on the network. In this way, routers know the final state of the network and able to continue the datagram in the right direction.
Interior Routing Protocol
In the early 1980s, the Internet is limited to the ARPANET, Satnet (ARPANET extension that uses satellites), and several local networks connected by gateways. In its development, the Internet requires a hierarchical structure to anticipate that the network has become big. Internet and then broken up into several autonomous systems (AS) and now the Internet consists of thousands of U.S.. Every American has a mechanism for information exchange and routing own collection.
Protocol used to exchange routing information in the United States are classified as interior routing protocol (IRP). Routing information collected is then submitted to the U.S. in the form of reachability information. Reachability information issued by the United States contains information about networks that can be achieved through the U.S. and the U.S. is an indicator connected to the Internet. Submission of reachability information between the U.S. carried out using a protocol which has been classified as an exterior routing protocol (ERP).
IRP is used as standard in the Internet today is the Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). In addition to these two protocols have also routing protocols that are proprietary but widely used in the Internet, the Internet Gateway Routing Protocol (IGRP) from Cisco Systems. IGRP protocol later expanded into the Extended IGRP (EIGRP). All of the above routing protocols use metrics as a basis for determining the best path can be taken by the datagram. Metrics associated with the "costs" contained in each link, which can be the throughput (data rate), delay, connection costs, and reliability of the link.
I. Routing Information Protocol
RIP (acronym, pronounced as rip) included in the distance-vector protocol, a protocol that is very simple. Distance-vector protocol is also called Bellman-Ford protocol, because it comes from the shortest distance calculation algorithm by RE Bellman, and described in the form of algorithms-first distributed by Ford and Fulkerson.
Each router with distance-vector protocol when it first started only know how routing to itself (local information) and did not know where he is a network topology. Then router sends the local information in the form of distance-vector to all the links that connect directly to him. Router that receives the routing information to calculate distance-vector, adding distance-vector with the link metric information received, and put it in a forwarding table entry if it is considered one of the best lines.
Routing information after the addition of metric and then sent back to the router interface, and is performed every certain time interval. And so on so that all routers in the network knows the network topology.Distance-vector protocols have a weakness that can be seen if there is a link in the network disconnected. Two possible failures that may occur is a bounce effect and count-until-no-till (counting to infinity). Bounce effects can occur in networks that use different metrics at least a link. Broken link can cause routing loops, so the datagram that passes through a particular link only circling between two routers (bounce) until the age (time to live) datagram is finished.
Counting-to-no-till occurs because the router was too late to inform you that a link is lost. This delay caused the router to send and receive distance-vector and calculate the metrics to the maximum limit of distance-vector metrics achieved. Link is found broken after the distance-vector reached the maximum metric. When calculating this metric routing loop occurs, even for a longer time than in case of bounce effect ..
RIP does not adopt the distance-vector protocol for granted, but by doing some additions to the algorithm for routing loops that occur can be minimized. RIP Split horizon is used to minimize the bounce effect. The principle used simple split horizon: if node A deliver datagram to the destination X through node B, then to B does not make sense to reach the goal X by A. Thus, A does not need to tell B that X can be accomplished B through A.
To prevent cases of counting-to-no-till, Triggered RIP Update method. RIP has a timer to know when the router must re-routing information. If there is a change in the network, while the timer has not run out, the router must still send the routing information as prompted by the changes (triggered updates). Thus, routers in the network can quickly find out the change and minimize the possibility of routing loops occur.
RIP is defined in RFC-1058 using the metrics between 1 and 15, while 16 considered as no-till. Route with distance-vector 16 is not included in the forwarding table. These limits prevent the 16 metric calculates the time-to-no-till is too long. RIP packets are normally sent every 30 seconds or sooner if there is triggered updates. If within 180 seconds a route is not updated, the router route delete it from the forwarding table. RIP does not have any information about the subnet route. Router must consider each route has received the same subnet with the router's subnet. Thus, RIP does not support Variable Length Subnet Masking (VLSM).
RIP version 2 (RIP-2 or RIPv2) attempt to generate some improvements over RIP, which is support for VLSM, using the authentication, the next hop information (next hop), and multicast. Adding subnet mask information in each route to make the router does not have to assume that the route has the same subnet mask with the subnet mask is used to him.
RIP-2 also uses authentication in order to know which routing information that can be trusted. Authentication is required at the routing protocols to make the protocol becomes more secure. RIP-1 does not use authentication so that people can give false routing information. Next hop information in RIP-2 is used by routers to inform a route but the route to reach does not pass through routers that provide information, but the other router. The following usage usually hop on the border between the United States.
RIP-1 uses a broadcast address to send routing information. As a result, this packet is received by all hosts within that subnet and add the host's workload. RIP-2 can send multicast packets using the IP 224.0.0.9, so not all hosts need to receive and process the routing information. Only routers that use RIP-2 that receives the routing information without the need to compromise other hosts in the subnet.
RIP is a simple routing protocol, and this is the reason why RIP is implemented in most networks. Set the RIP routing is not complicated to use and gives results quite acceptable, especially if rare network link failures. However, for large networks and complex, RIP may not be enough. In this case, the RIP routing calculation often takes a long time, and lead to routing loops. For such networks, most computer network specialist to use the protocol included in the link-state group.
II. Open Shortest Path First (OSPF)
Link-state technology developed in the ARPAnet to produce a distributed protocol is much better than distance-vector protocol. Instead of exchanging distance (distance) to the destination, each router in the network has a network map that can be updated quickly after any topology change. This map is used to calculate the route which is more accurate than using distance-vector protocol. These technological developments eventually led to a protocol Open Shortest Path First (OSPF) which was developed by the IETF for use on the Internet. Even now the Internet Architecture Board (IAB) has been recommended as a substitute for OSPF, RIP.
The principle of link-state routing is very simple. As an alternative route to calculate the "best" distributed by, all routers have a network map and count all the best route from the map. Network maps are stored in a database and each record in the database specifies a link in the network. The records were sent by a router that is connected directly with each link.
Because each router need to have a network map that illustrates the last condition is a complete network topology, any changes in the network must be followed by changes in the data base link-state located at each router. Link status change is detected by the router will change the database link-state router, then the router sends these changes to other routers.
Protocol used to transmit these changes must be fast and reliable. This can be achieved by flooding protocol. In the flooding protocol, the message sent is a change from the database and the message sequence number. By simply sending database changes, the time required for sending and processing messages sent fewer dibandingdengan entire data base. Message sequence number is needed to determine whether the message is received more recent than that contained in the database. This serial number handy in case of broken links to be connected again.
At the end, and there is a link to a separate network, the two parts of the database into a different network.
When the link is broken back to life, the data base on all routers must be identified. This data base will not return the same as sending a link-state messages only. The process of data base on the equation of the neighboring router is called the turn adjacency. Two routers adjacent neighborhood called when the database link-state both have the same. In this process the two routers are not exchanging data base because it will take a long time.
The process consists of adjacency bring fasa.Fasa first two, both routers exchange database description is a summary of the database that each router. Each router then compares the description database received by the data base has. In the second phase, each router is asking neighbors to send the records of different databases, ie if the router has no such record, or record serial number owned less than that delivered by the description of the data base. After this process, the router and update multiple records are then sent to other routers via flooding protocol.
Link-state protocol is better than distance-vector protocol is caused by several things: the time required to converge faster, and more importantly, this protocol does not produce routing loops. This protocol supports the use of several metrics at once. Throughput, delay, cost, and reliability are metrics commonly used in the network. In addition, this protocol can also produce many paths to a destination. Suppose that router A has two lines with the same metrics to host B. Protocol can enter into both of these pathways in a forwarding table so that the router is able to divide the load between the two path.
Draft OSPF uses link-state protocols with some additional functions. The functions of added include support multi-access networks, such as X.25 and Ethernet, and divide large networks becoming some areas.
Has been described above that each router in link-state protocols necessary to establish neighbor adjacency with the router. On multi-access network, each router's neighbors can be more than one. In these circumstances, every router in the network needs to form adjacency with all other routers, and is inefficient. OSPF adjacency streamline this by introducing the concept of designated routers and backup designated routers. All routers need only adjacent to the designated router, so that only the designated router is adjacent to all other routers. Designated backup router will take over the designated router fails.
The first step in a multi-access network is to select the designated router and backup. This election is inserted into the Hello protocol, the OSPF protocol to find neighbors routers in each link. After the election, then routers to form adjacency with the designated router and backup. Each network changes, the router sends a message using the flooding protocol to the designated router, and the designated router that sends the message to other routers in the link.
Designated router backup also listen to the messages sent to the designated router. If the designated router fails, the backup designated router then becomes the new designated and elected a new backup router. Because the new designated router has been adjacent to the other routers, no longer necessary database identification process that requires such a long time.
In a large network would need a large database to store all the network topology. This leads to a memory requirement larger router and route calculation time is longer. To anticipate this, OSPF uses the concept of area and backbone. Network is divided into several areas that are connected to the backbone. Each area is considered as a separate network and routers in it only needs to have a network topology map in that area. Routers located at the border between the area just send a summary of the links contained in the area and did not send a broadcast topology to other areas. Thus, the calculation becomes much simpler route.
Example cases:
Host X à 128.1.1.1 (Class B network ip en 128.1.xx)
Y à host 128.1.1.7 (class B IP network id 128.1.xx)
Host Z à 128.2.2.1 (class B IP network id 128.2.xx)
In the case above, the host of X and Y can communicate with the host directly, but both hosts X and Y can not communicate with host Z, because they have different network ID. How so Z can communicate with X and Y? use a router!
An example of using subnetting
Host P à 128.1.208.1 subnet mask 255.255.240.0
Host Q à 128.1.208.2 subnet mask 255.255.240.0
Host R à 128.1.80.3 subnet mask 255.255.240.0
Well, when subnetting is used, then the two hosts are connected to the same network segment can communicate only if both the network id and subnetid his sesuai.Pada case above, P and Q can communicate with directly, R has the same network id with P and Q but has a different subnetidyang. Thus R can not communicate directly with P and Q. How so R can communicate with the P and Q? use a router!
So the router function, is easy to say, connect two different networks, precisely directs the best route to achieve the expected network.
In implementation, a router is often used to connect networks between institutions or companies that each have a network with different network id. Another example that is currently popular is when your company will be connected to the internet. Then the router will work stream of data packets from your company to other institutions via the Internet, of course, your network number would be calm with perushaaan you go.
If you simply connect the 2 pieces networks, in fact you can also use Windows NT based pc or linux. By providing 2 pieces and a network card settings, you actually have to make practical router. But of course with all its limitations.
More about routing
The data from the devices connected to the Internet is sent in a datagram, the data packet defined by the IP. Datagram has a destination address of data packets; Internet Protocol check this address to submit original datagram from the device to the destination device. If the datagram destination address is located a home network device, datagram directly delivered to the destination device. If the datagram destination address was not found in the same network, datagram forwarded to the appropriate router (the best available router).
IP router (usually called routers only) are devices that perform functions on the IP datagram to continue the network layer. Router has more than one network Default (network interface) and can forward datagram from one interface to another interface. For each datagram is received, the router checks if the datagram is addressed to him. If it is not addressed to the router, the datagram sent to the transport layer.
If the datagram not addressed to the router, which will examine is that it has a forwarding table to decide where the datagram should be addressed. Forwarding table is a table consisting of pairs of IP addresses (host address or network address), following the router address, and the interface where the datagram out.
If you do not find even a single line in the appropriate forwarding table with a destination address, the router will give a message to the sender that the address in question can not be achieved. This incident may be analogous with the message "return to sender" on mail. A router can also tell that he was not the best router to a destination, and suggested the use of other routers. With the three functions contained on this router, hosts on the Internet can connect to each other.
Static and Dynamic
In general, routing coordination mechanisms can be divided into two: static routing and dynamic routing. In static routing, the entries in the router's forwarding table is filled and removed manually, while the dynamic routing changes are made through routing protocols. Static routing is the simplest routing settings that can be done on computer networks. Using pure static routing in a network means to fill each entry in the forwarding table at each router in the network.
The use of static routing in a small network of course is not a problem; only a few entries that need to be filled in the forwarding table at each router. But you certainly can imagine what if must complete the forwarding table in each router that number is not small in a large network. Especially if you are assigned to fill the entries in the router on the Internet are very numerous and growing every day. Of trouble once!
Dynamic routing is the way used to release the obligation to fill these entries manually forwarding table. Routing protocol routers set so that it can communicate with each other and give each other routing information that can change the contents of a forwarding table, depending on the network. In this way, routers know the final state of the network and able to continue the datagram in the right direction.
Interior Routing Protocol
In the early 1980s, the Internet is limited to the ARPANET, Satnet (ARPANET extension that uses satellites), and several local networks connected by gateways. In its development, the Internet requires a hierarchical structure to anticipate that the network has become big. Internet and then broken up into several autonomous systems (AS) and now the Internet consists of thousands of U.S.. Every American has a mechanism for information exchange and routing own collection.
Protocol used to exchange routing information in the United States are classified as interior routing protocol (IRP). Routing information collected is then submitted to the U.S. in the form of reachability information. Reachability information issued by the United States contains information about networks that can be achieved through the U.S. and the U.S. is an indicator connected to the Internet. Submission of reachability information between the U.S. carried out using a protocol which has been classified as an exterior routing protocol (ERP).
IRP is used as standard in the Internet today is the Routing Information Protocol (RIP) and Open Shortest Path First (OSPF). In addition to these two protocols have also routing protocols that are proprietary but widely used in the Internet, the Internet Gateway Routing Protocol (IGRP) from Cisco Systems. IGRP protocol later expanded into the Extended IGRP (EIGRP). All of the above routing protocols use metrics as a basis for determining the best path can be taken by the datagram. Metrics associated with the "costs" contained in each link, which can be the throughput (data rate), delay, connection costs, and reliability of the link.
I. Routing Information Protocol
RIP (acronym, pronounced as rip) included in the distance-vector protocol, a protocol that is very simple. Distance-vector protocol is also called Bellman-Ford protocol, because it comes from the shortest distance calculation algorithm by RE Bellman, and described in the form of algorithms-first distributed by Ford and Fulkerson.
Each router with distance-vector protocol when it first started only know how routing to itself (local information) and did not know where he is a network topology. Then router sends the local information in the form of distance-vector to all the links that connect directly to him. Router that receives the routing information to calculate distance-vector, adding distance-vector with the link metric information received, and put it in a forwarding table entry if it is considered one of the best lines.
Routing information after the addition of metric and then sent back to the router interface, and is performed every certain time interval. And so on so that all routers in the network knows the network topology.Distance-vector protocols have a weakness that can be seen if there is a link in the network disconnected. Two possible failures that may occur is a bounce effect and count-until-no-till (counting to infinity). Bounce effects can occur in networks that use different metrics at least a link. Broken link can cause routing loops, so the datagram that passes through a particular link only circling between two routers (bounce) until the age (time to live) datagram is finished.
Counting-to-no-till occurs because the router was too late to inform you that a link is lost. This delay caused the router to send and receive distance-vector and calculate the metrics to the maximum limit of distance-vector metrics achieved. Link is found broken after the distance-vector reached the maximum metric. When calculating this metric routing loop occurs, even for a longer time than in case of bounce effect ..
RIP does not adopt the distance-vector protocol for granted, but by doing some additions to the algorithm for routing loops that occur can be minimized. RIP Split horizon is used to minimize the bounce effect. The principle used simple split horizon: if node A deliver datagram to the destination X through node B, then to B does not make sense to reach the goal X by A. Thus, A does not need to tell B that X can be accomplished B through A.
To prevent cases of counting-to-no-till, Triggered RIP Update method. RIP has a timer to know when the router must re-routing information. If there is a change in the network, while the timer has not run out, the router must still send the routing information as prompted by the changes (triggered updates). Thus, routers in the network can quickly find out the change and minimize the possibility of routing loops occur.
RIP is defined in RFC-1058 using the metrics between 1 and 15, while 16 considered as no-till. Route with distance-vector 16 is not included in the forwarding table. These limits prevent the 16 metric calculates the time-to-no-till is too long. RIP packets are normally sent every 30 seconds or sooner if there is triggered updates. If within 180 seconds a route is not updated, the router route delete it from the forwarding table. RIP does not have any information about the subnet route. Router must consider each route has received the same subnet with the router's subnet. Thus, RIP does not support Variable Length Subnet Masking (VLSM).
RIP version 2 (RIP-2 or RIPv2) attempt to generate some improvements over RIP, which is support for VLSM, using the authentication, the next hop information (next hop), and multicast. Adding subnet mask information in each route to make the router does not have to assume that the route has the same subnet mask with the subnet mask is used to him.
RIP-2 also uses authentication in order to know which routing information that can be trusted. Authentication is required at the routing protocols to make the protocol becomes more secure. RIP-1 does not use authentication so that people can give false routing information. Next hop information in RIP-2 is used by routers to inform a route but the route to reach does not pass through routers that provide information, but the other router. The following usage usually hop on the border between the United States.
RIP-1 uses a broadcast address to send routing information. As a result, this packet is received by all hosts within that subnet and add the host's workload. RIP-2 can send multicast packets using the IP 224.0.0.9, so not all hosts need to receive and process the routing information. Only routers that use RIP-2 that receives the routing information without the need to compromise other hosts in the subnet.
RIP is a simple routing protocol, and this is the reason why RIP is implemented in most networks. Set the RIP routing is not complicated to use and gives results quite acceptable, especially if rare network link failures. However, for large networks and complex, RIP may not be enough. In this case, the RIP routing calculation often takes a long time, and lead to routing loops. For such networks, most computer network specialist to use the protocol included in the link-state group.
II. Open Shortest Path First (OSPF)
Link-state technology developed in the ARPAnet to produce a distributed protocol is much better than distance-vector protocol. Instead of exchanging distance (distance) to the destination, each router in the network has a network map that can be updated quickly after any topology change. This map is used to calculate the route which is more accurate than using distance-vector protocol. These technological developments eventually led to a protocol Open Shortest Path First (OSPF) which was developed by the IETF for use on the Internet. Even now the Internet Architecture Board (IAB) has been recommended as a substitute for OSPF, RIP.
The principle of link-state routing is very simple. As an alternative route to calculate the "best" distributed by, all routers have a network map and count all the best route from the map. Network maps are stored in a database and each record in the database specifies a link in the network. The records were sent by a router that is connected directly with each link.
Because each router need to have a network map that illustrates the last condition is a complete network topology, any changes in the network must be followed by changes in the data base link-state located at each router. Link status change is detected by the router will change the database link-state router, then the router sends these changes to other routers.
Protocol used to transmit these changes must be fast and reliable. This can be achieved by flooding protocol. In the flooding protocol, the message sent is a change from the database and the message sequence number. By simply sending database changes, the time required for sending and processing messages sent fewer dibandingdengan entire data base. Message sequence number is needed to determine whether the message is received more recent than that contained in the database. This serial number handy in case of broken links to be connected again.
At the end, and there is a link to a separate network, the two parts of the database into a different network.
When the link is broken back to life, the data base on all routers must be identified. This data base will not return the same as sending a link-state messages only. The process of data base on the equation of the neighboring router is called the turn adjacency. Two routers adjacent neighborhood called when the database link-state both have the same. In this process the two routers are not exchanging data base because it will take a long time.
The process consists of adjacency bring fasa.Fasa first two, both routers exchange database description is a summary of the database that each router. Each router then compares the description database received by the data base has. In the second phase, each router is asking neighbors to send the records of different databases, ie if the router has no such record, or record serial number owned less than that delivered by the description of the data base. After this process, the router and update multiple records are then sent to other routers via flooding protocol.
Link-state protocol is better than distance-vector protocol is caused by several things: the time required to converge faster, and more importantly, this protocol does not produce routing loops. This protocol supports the use of several metrics at once. Throughput, delay, cost, and reliability are metrics commonly used in the network. In addition, this protocol can also produce many paths to a destination. Suppose that router A has two lines with the same metrics to host B. Protocol can enter into both of these pathways in a forwarding table so that the router is able to divide the load between the two path.
Draft OSPF uses link-state protocols with some additional functions. The functions of added include support multi-access networks, such as X.25 and Ethernet, and divide large networks becoming some areas.
Has been described above that each router in link-state protocols necessary to establish neighbor adjacency with the router. On multi-access network, each router's neighbors can be more than one. In these circumstances, every router in the network needs to form adjacency with all other routers, and is inefficient. OSPF adjacency streamline this by introducing the concept of designated routers and backup designated routers. All routers need only adjacent to the designated router, so that only the designated router is adjacent to all other routers. Designated backup router will take over the designated router fails.
The first step in a multi-access network is to select the designated router and backup. This election is inserted into the Hello protocol, the OSPF protocol to find neighbors routers in each link. After the election, then routers to form adjacency with the designated router and backup. Each network changes, the router sends a message using the flooding protocol to the designated router, and the designated router that sends the message to other routers in the link.
Designated router backup also listen to the messages sent to the designated router. If the designated router fails, the backup designated router then becomes the new designated and elected a new backup router. Because the new designated router has been adjacent to the other routers, no longer necessary database identification process that requires such a long time.
In a large network would need a large database to store all the network topology. This leads to a memory requirement larger router and route calculation time is longer. To anticipate this, OSPF uses the concept of area and backbone. Network is divided into several areas that are connected to the backbone. Each area is considered as a separate network and routers in it only needs to have a network topology map in that area. Routers located at the border between the area just send a summary of the links contained in the area and did not send a broadcast topology to other areas. Thus, the calculation becomes much simpler route.