Introduction
I was assigned to a research on various Interior Gateway protocols and also their behaviours. I was told to do a research on the advantages as well as the disadvantages of these interior gateway protocols. After doing the research, I did a comparison table of the advantages and also the disadvantages of these protocols.
There were few interior gateway that I managed to do researches on. They are Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Enhanced Interior Gateway Protocol (EIGRP) and also Immediate System-to-Immediate System (IS-IS).
Routing Information Protocol (RIP) is developed to manage router information in a self-contained network. For instance a corporate local area network (LAN) or an interconnected group of such LANs. It keeps the information on the fastest route between computers to computer.
Open Shortest Path First (OSPF) is developed and used within larger autonomous system networks in preference to the Routing Information Protocol (RIP). It keeps track of only the closet router for each destination address. Open Shortest Path First keeps track of a complete topological database of all the connections in the local network.
Enhanced Interior Gateway Protocol (EIGRP) is a more enhanced version of Interior Gateway Protocol or IGRP. EIGRP address the demands of large-scale internetworks and the changes in network technology that has been developed since the implementation of IGRP.
Immediate System-to-Immediate System (IS-IS) is standardized Interior Gateway Protocol or IGP by the Internet Engineering Task Force (IETF) and this routing protocol is commonly used in large service provide networks. It is a link-state routing protocol and it provides fast convergence and also scalability excellence.
Routing Information Protocol
Routing Information Protocol or more commonly known as RIP is developed to manage router information in a self-contained network. For instance a corporate local area network (LAN) or an interconnected group of such LANs. The Routing Information Protocol or RIP is used widely and it is classified by the Internet Engineering Task Force (IETF) as one of several gateway protocols (Interior Gateway Protocol).
How Routing Information Protocol (RIP) Works
In the Routing Information Protocol or RIP, there is a routing database that keeps the information on the fastest route between computer to computer. Then there is also a process of update that will allow each router to tell the routers which is the fastest route from its point of view and then an update algorithm that enables each router to update the database with the fastest route that are communicating with the other neighbouring routers.
In the database, each Routing Information Protocol router on a given network stores a database that will keep the following information for each and every computer in that network :
Here is how the Routing Information Protocol algorithm works :
Routing Information Protocol Messages
Routing Information Protocol updates are as a UDP and it is payload inside an IP datagram. Here is a base format of a Routing Information Protocol message.
command
Address Family ID
IP Address
Zeroes
Zeroes
Metric
Payload...
Here are some of the command types in a protocol :
Routing Information Protocol Timers
Update - The update timer is default at 30 seconds. It controls the interval between the route update advertisements.
Hold-Down - The Hold-Down timer is default at 90 seconds. It is the time taken when a route is withdrawn from the table to prevent a routing loop.
Timeout - The timeout timer is default at 180 seconds. It is the interval a route should stay ‘live' in the routing table. The counter is reset every time the router hears an update for the route.
Flush - The flush timer is default at 120 seconds. It is to determine the time taken to wait and delete a route after it has timed out.
Advantages of Routing Information Protocol
Disadvantages of Routing Information Protocol
Open Shortest Path First (OSPF)
Open Shortest Path First or commonly known as OSPF is a router protocol which is developed and used within larger autonomous system networks in preference to the Routing Information Protocol (RIP). Open Shortest Path First is designed by the Internet Engineering Task Force (IETF) as one of the several Interior Gateway Protocols (IGPs).
When the Open Shortest Path First is used, a host that obtains a change to a routing table or detects a change in the network immediately multicasts the information to all the others hosts in the network. This will make all the other hosts in the network have the same routing table information. OSPF only sends the part that has changed unlike RIP who sends the entire routing table. RIP sens the routing table to a neighbour host every 30 seconds but as for OSPF, it multicasts the updated information only when a change has taken place.
OSPF bases its path descriptions on “link states” rather than counting the number of hops. This will take into account of the additional network information. Other than that, OSPF also lets the user to assign cost metrics to a given host router in order for the network to be subdivided. Routing Information Protocol is supported within Open Shortest Path First for router-to-end station communication.
How OSPF Works
One of the main differences between Open Shortest Path First and Routing Information Protocol is that Routing Information Protocol keeps track of only the closet router for each destination address. Open Shortest Path First keeps track of a complete topological database of all the connections in the local network.
The Open Shortest Path First protocol has a backbone that is responsible for distributing routing information between areas. This consists of all the area border routers, networks not wholly contained in any area and also their attached routers. Here is a figure that shows an example of an internetwork with several areas :
From the figure we can see that routers 4,5,6,10,11 and 12 make up the backbone. Let's say that H1 in area 3 wants to send a packet to Host H2 in Area 2, the packet is sent to router 13 who will forward the packet to router 12 who will send the packet to router 11. Then router 11 will forward the packet along the backbone to area border router 10 who will then send the packet through two intra-area routers which are router 9 and router 7 so that it can be forwarded to Host H2.
The backbone is an Open Shortest Path First area itself and this means that all backbone routers uses the same procedures and algorithms to maintain the routing information within the backbone that any area router would. The backbone topology is invisible to all intra-area routers and so is the individual area topologies to the backbone.
Here is how the Open Shortest Path First algorithm works :
Advantages of Open Shortest Path First
Disadvantages of Open Shortest Path First
Open Shortest Path First Packet Format
1 1 2 4 4 2 2 8 Variable
Version NumberTypePacket lengthRouter IDArea IDChecksumAuthent-ication typeAuthenticationData
Enhanced Interior Gateway Protocol (EIGRP)
Enhanced Interior Gateway Protocol or also known as EIGRP is a more enhanced version of Interior Gateway Protocol or IGRP. EIGRP address the demands of large-scale internetworks and the changes in network technology that has been developed since the implementation of IGRP. Because the metrics for both protocols are directly translatable, so routers that already use IGRP can also use EIGRP. It is as easily comparable as if they were routes that originated in their own autonomous systems.
Routers that are running on EIGRP stores copies of all its neighbour's routing tables so that it can adapt to alternate routes quickly and easily. When there is no appropriate route existed, EIGRP will then query its neighbours to encounter an alternate route. The queries are propagated until an alternate route is encountered. The difference between EIGRP and some of the routing protocols that will send an entire table to their neighbour routers when one routing table entry is changed, EIGRP lets the neighbour routers know that only the specific table is changed. The difference between EIGRP and IGRP is that EIGRP uses the diffusing-update algorithm or commonly known as DUAL which is developed at SRI International.
EIGRP is a hybrid protocol because it incorporates features of a Distance Vector routing protocol and it also features of a Link State routing protocol. Cisco-based networks running multiple network-layer protocols use the Enhanced Interior Gateway Protocol very often.
How Enhanced Interior Gateway Protocol works
Information is saved by a typical distance vector protocol when computing the best path to a destination. The distance which is the total metric or distance such as hop count) and also the vector which is the next hop.
Take figure 1 for example, the routers in the network are running Routing Information Protocol of RIP. Router Two chooses the path to Network A by examining the hop count through each path available.
The path through router three is three hops and the path through router One is two hops. Therefore router Two will chose the path through router One and then the information learned through router Three is then discarded. If there is any problems for instance the path between router One and Network A is down, router Two will lose all connectivity with this destination until it times out the route of its routing table which is three update periods of 90 seconds. Router Three will then re-advertise the route which occurs every 30 seconds in Routing Information Protocol. It will take approximately between 90 and 120 seconds for router Two to then switch the path from router One and router Three and time is not including any hold-down time.
Enhanced Interior Gateway Protocol builds a topology table from each of its neighbour's advertisement instead of counting on full periodic updates to re-converge and converges by either looking for a likely loop=free route in the topology or by querying its neighbour's if no other route is known. Router Two will then save the information received from both router One and router Three. Router Two will then choose the path through router One as it's the best path which is the successor and the part through router Three as a loop-free path which is a feasible successor. When the path through router One becomes unavailable, router Two will examine its topology table and find a feasible successor and then begin using the path through router Three immediately.
From here, we know that EIGRP must provide :
Routing Concepts
Enhanced Interior Gateway Protocol has four fundamental routing concepts. They are neighbour tables, topology tables, route states and also route tagging. Here are the fundamentals routing concepts of EIGRP
* Neighbour tables
When a new neighbour is discovered by the router, the router records the neighbour's address and also the interface as an entry in the neighbour table. A neighbour table exists for each protocol-dependent module and when a neighbour sends a hello packet, it will advertise a hold time which is the amount of time that a router takes to treat a neighbour as reachable and operational. If a hello packet is not received during the hold time, the hold time will expire and DUAL will be informed of the topology change.
Other than that, the neighbour-table also includes the information required by RTP. Sequence numbers are employed to math the acknowledgements with data packets and the last sequence number received from the neighbour is recorded in order for the out-of-order packets to be detected. A transmission list is also used to queue packets for possible retransmission on a per-neighbour bases. Round-trip timers are kept in the neighbour-table entry so that it can estimate an optimal retransmission interval.
* Topology tables
The topology table contains all the destinations that are advertised by the neighbouring routers. The protocol-dependent modules will populate the table and this table will be acted on by the DUAL finite-state machine. Each entry in the topology table will include the destination address as well as a list of the neighbours that have advertised the same destination. For each neighbour the entry records the advertised metrics which is stored by the neighbour in its routing table.
The metric that is used by the router to reach the destination is also associated with the destination. The metric that the router uses in the routing table as well as to advertise to other routers is the amount of the best-advertised metric from all the neighbours and the lick cost to the best neighbour.
* Route States
A topology-table entry for a destination can exist in one out of two states whether it is active or passive. A destination is in the passive state when the router is not performing a recomputation. The router only performs recomputation in the active state. If feasible successors are always available, a destination will never have to go into the active state and therefore avoiding performing recomputation.
Recomputation can occur when a destination has no feasible successors. The router will initiate the recomputation by sending a query packet to each of the neighbouring routers. The neighbouring routers will then send a reply packet to notify the router that it has a feasible successor for the destination. Other than that, the neighbouring router can also send a query packet saying that it is participating in the recomputation. When a destination is in the active state, a router cannot change the destination's routing-table information. Once the router had received a reply from each neighbouring router, the topology-table entry for the destination returns to the passive state and the router can now select a successor.
* Route Tagging
Enhanced Interior Gateway Protocol also supports external routes. Internal routes are originated within an EIGRP AS. So, a directly attached network that is configured to run EIGRP is considered an internal route and is propagated with this information throughout the EIGRP AS. External routes are also learned by other routing protocols or reside in the routing table as static routes. The routes are tagged individually with the identity of their origin.
External routes are tagged with these information :
The route tagging allows the network administrator to change the routing and maintain flexible policy controls. Route tagging is particularly useful in transit ASs where EIGRP interacts with an interdomain routing protocol that implements more global policies. This will result to very scalable, policy-based routing.
Message Types
http://www.inetdaemon.com/tutorials/internet/ip/routing/eigrp/#overview
Advantages of EIGRP
Disadvantages of EIGRP
Intermediate System-to-Intermediate System
Intermediate System-to-Intermediate System or more commonly known as IS-IS routing protocol is standardized Interior Gateway Protocol or IGP by the Internet Engineering Task Force (IETF) and this routing protocol is commonly used in large service provide networks. Intermediate System-to-Intermediate System is a link-state routing protocol and it provides fast convergence and also scalability excellence. This routing protocol is also very efficient in its use of network bandwidth just like all the other link-state protocols. It is used to distribute IP routing information throughout a single autonomous system or AS in an IP network.
Intermediate System-to-Intermediate System routers exchange topology information with their nearest neighbours because it is a link-state routing protocol. These topology information is flooded throughout the AS in order for every router within the AS to have a complete picture of the topology of the AS. The picture is then used to calculate the end-to-end paths through the AS using a variant of the Dijkstra algorithm normally. So, in a link-state routing protocol, the next hop address to which data is forwarded is determined by choosing the best end-to-end path to the eventual destination.
Immediate System-to-Immediate System was first devised as a routing protocol for CLNP. Then it has been extended to include IP routing. This extended version is sometimes called Integrated IS-IS.
The IS-IS routers distributes information about its local state such as usable interfaces and reachable neighbours and also the cost of using each interface to other routers using a link state PDU (LSP) message. The routers uses the received messages to build up an identical database that describes the topology of the AS.
IS-IS Protocol Structure
IS-IS Dataflow Diagram
The receive stage is where all the data enters. The data are user data, error reports, routing information and also control packets. It will pass the user data and error reports to the forward process and pass the routing and information and control packets such as hellos, LSPs and sequence number packets to the update process.
The update stage generates local link information that are flooded to adjacent routers. Plus, the update stage also receives, processes and also forwards link information that is received from adjacent routers. This stage manages the level 1 and level 2 link-state databases and floods level 1 and level 2 LSPs throughout an area. Each LSP that resides in the link-state database has a remaining lifetime, checksum and also a sequence number.
The decision stage runs the shortest-path-first (SPF) algorithm on the link-state database and it also creates the forwarding database. It computes the next-hop information and computes the sets of equal-cost paths and this creates an adjacency set that is used for load balancing. On a cisco router, Intermediate System-to-Intermediate System supports load balancing over and up to six equal-cost paths.
The forward stage will get its input from the receive stage and uses the forwarding database to forward data packets toward their destination. Other than that, it also redirects load sharing and it also generates error reports.
Intermediate System-to-Intermediate System Backbone
The Intermediate System-to-Intermediate System routing protocol do not have a backbone area like the Open Shortest Path First area. The IS-IS backbone is a contiguous collection of level 2 capable routers where each of which can be in a different are. The figure below shows the Intermediate System-to-Intermediate System backbone.
Advantages of IS-IS
Disadvantages of IS-IS
Comparison on Advantage of four Interior Gateway Protocol
Comparison on Disadvantages of four Interior Gateway Protocol
Network Prototype
Conclusion
In conclusion, I have learned a lot from this assignment on the various types of Interior Gateway Protocols. I had learned the advantages and also the disadvantages of each type of the Interior Gateway Protocols and also some of its features based on the research that I had done. I had also learned about the network prototypes that is used in the Interior Gateway Protocols.
The Routing Information Protocol or RIP has a process of update that will allow each router to tell the routers which is the fastest route from its point of view and then an update algorithm that enables each router to update the database with the fastest route that are communicating with the other neighbouring routers.
Open Shortest Path First (OSPF) bases its path descriptions on “link states” rather than counting the number of hops. This will take into account of the additional network information. Other than that, OSPF also lets the user to assign cost metrics to a given host router in order for the network to be subdivided.
Enhanced Interior Gateway Protocol (EIGRP) EIGRP is a hybrid protocol because it incorporates features of a Distance Vector routing protocol and it also features of a Link State routing protocol.
Immediate System-to-Immediate System (IS-IS) distributes information about its local state such as usable interfaces and reachable neighbours and also the cost of using each interface to other routers using a link state PDU (LSP) message.
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