Internet communication is associated with the routing of the information from source machine to destination machine. The communication entities such as nodes and workstations create the information packets by the help of protocols. The packets are routed to the appropriate route by the help of routers. There is need of routing protocols to the routers to route the informational packets at appropriate links of the network in Internet world. Basically, routers are the network layer hardware device used to route the packets. As Internet is complex network formed by the collection of different networks interconnected with each other. Due to this fact there is need to route the packets from one network to another network to reach the packets at desired destination.
The router devices requires algorithm to route the packets from source machine of a network to destination machine of same of different network. Routing algorithms are called the routing protocols. These routing protocols define the routing strategies and rules for the packets. According to routing requirements there are many categories of routing protocols used to route the packets.
The IP based network requires Internet routing path from source machine to destination machine and this is determined by criteria such as shortest path. OSPF (Open Shortest Path First) and IS-IS (Intermediate System to Intermediate System) are link state routing protocols. These two routing protocols are Dynamic or adaptive routing protocols. LSR (Link State Routing) protocols are based on the technique of shortest path where the routers find the shortest path for each of the network. There is a database maintained by each of the router under link state routing which is called link state database. The link state database states the topology of the AS (Autonomous System). The routers exchange the routing information by advertisement of the link state among the nodes such as routers. The link state advertisement information of each of the node of AS contains the information of neighbors. The changes of the link state of node are advertised by that node to its neighbors. This way the nodes having the link state database maintain the routing information and also updated information of the link state under the AS.
The cost of the routing is determined under the routing table for each of the link by counting the number of nodes from source to destination nodes in AS by a node with link state routing. OSPF uses the link state routing algorithm to route the packets from source node to destination node of network. Similarly IS-IS is an IGP (Interior Gateway Protocol). This IS-IS routing is used to route the packets throughout under only an AS. It has no scope to route the packets from one AS to another AS therefore, it is called IGP.
Comparative Analysis of OSPF Vs IS-IS
The comparative analysis of OSPF and IS-IS routing protocols are detailed with following factors of usability in routing of packets.
• OSPF and IS-IS both are dynamic source routing protocol based on the link state routing. These two protocols use the Dijkstra’s shortest path algorithm to find the shortest path between source to destination nodes. The Dijkstra’s Shortest Path Algorithm is used to determine the least cost path from the source node to destination node.
• Both OSPF and IS-IS are the Interior Gateway Protocols. It means that both are only able to deliver the packets to nodes of single autonomous system. OSPF and IS-IS routing protocols supports classless Interdomain routing and variable subnet length masking for the network under a autonomous system. Internet Service Providers (ISPs) are using both of the routing protocols to route the packets for Internet users.
• For transport OSPF uses the Internet Protocol 89 where as IS-IS is directly encapsulated with link layer. The header of the transport is mentioned below.
OSPF Transport
IS-IS Encapsulation with Link Layer 2
• Basically, OSPF runs at Internet Layer while IS-IS runs at Data link Layer. Due to this fact, OSPF is used widely with router platform while the IS-IS is less widely used on router platform. As IS-IS runs at link layer so that it is less prone to be attacked. OSPF runs at Internet layer so it is more prone to be attacked. Therefore, security with use of IS-IS routing protocol is high while with the OSPF is low.
• The link support of OSPF is point to multipoint in the network but IS-IS does not supports the point to multi point links in the network. Thus, routing to the packets by OSPF protocol on router is possible with one point to multi point. IS-IS routes the packets from one point to another single point in the network. This is biggest disadvantages or limitation of the IS-IS routing protocol.
• Virtual link is supported by OSPF routing protocol but it is not supported by the IS-IS routing protocol. Due to this lack of IS-IS routing protocol only circuit based links are used by the router when the router is implemented with the IS-IS protocol.
• IS-IS routing protocol is originally developed and designed for the CLNP. This IS-IS has the two forms of routing the packets from source node to destination node. These two forms are stated as.
o Bottom 6 bytes exact match and
o Top part with shortest prefix
OSPF routing protocol is specifically designed for the Internet Protocol. Thus, the working of OSPF routing protocol is attributed with the network layer.
• With IS-IS routing the router election is deterministic. It means that same set of routers are selected or elected for routing the packets. With OSPF the election of router is sticky. It means that designated router is always designated for routing the packets.
• As IS-IS routing protocol runs over the link layer 2 so it supports multiple protocols such as IPX, IPV4 and IPV6. It is considered as more scalable due to fact that the change of the link address does not requires the re-calculation of the shortest path. OSPF runs on the IPV4 and also routes the packets of IPV4. The migration from IPV4 to IPV6 is complicated with OSPF. Therefore, for IPV6 packet routing a separate version of OSPF is required.
• OSPF routing employs different topologies for IPV4 packets and IPV6 packets routing whereas IS-IS has a single topology for both IPV4 and IPV6 packets routing. The area of OSPF is considered as the collection of routers but in case of IS-IS area is collection of the links such as L1 and L2 links. These links act as a border between the domains. Thus IS-IS routing is more flexible than that of OSPF routing.
• OSPF routing defines area 0 which is known as backbone area for advertisement of the link addresses into internal area. But in case of IS-IS the domains are categorized with two different layers such link layer 1 and link layer 2. Therefore, with OSPF routing the all the nodes of the backbone area such as area 0 are covered and by IS-IS only those nodes are covered which have the links with layer 1 or layer 2.
• Router ID is used by the OSPF so that each router is identified by its Identification number in the network. Whereas, IS-IS uses the system ID so that each node is identified with the identification number as defined for the system on the network.
• OSPF is considered as resource intensive on the router and efficient in routing of packets in core network. With IS-IS there is no need of the areas to be connected with designated area 0 for the routing the packets. Thus, IS-IS in this context is considered more flexible routing protocol for an Autonomous System.
• The management and control of IS-IS routing protocol requires more intensive training and education to the entities who manages it. So that the cost of IS-IS is implementation is higher than that of OSPF.
• There is inherent ability to define the overload bits under the IS-IS LSAs. Thus, all other routers by default understand that this is the leaf router and be avoided by other router to take path from that router to reach the intended destination. This, reduces the path calculation time and also reduces the burden of the router processor. But in case of OSPF routing protocol this cannot be defined and it is burden of the router to determine the least cost route.
• The router with IS-IS protocol connects only in single zone but router with OSPF protocol connects two zones. Hence, the complexity of OSPF routing protocol is higher than that of IS-IS. The internal firewall must be with distinct machine under the IS-IS based routing protocol network in an autonomous system but this is not a case in OSPF. So that cost of OSPF is lower with firewall security.
• OSPF supports the physical star topology of the network whereas IS-IS supports the logical topology that is discovered by it.
The comparative analysis of the terms and terminologies used with OSPF and IS-IS routing protocols are summarized under the given below table.
OSPF
IS-IS
Node is termed as host
Node is termed as end system
The routing device is termed as router
Routing device is termed as Intermediate system
The connection is termed as link
The Connection is termed as circuit
Information is termed as packet
Information is termed as protocol data unit
OSPF uses the link state advertisement
IS-IS sues the link state PDU
Link state database is described as database description
Link state database is described as complete sequence number PDU
The location is defined under area
Location is defined under sub domain
Above terminologies are commonly used with OSPF and IS-IS routing standards. These terminologies are specific for each of the mentioned routing protocols.
Conclusion
Internet requires a routing protocol to routes the packets from source node to destination node. There are two types of routing protocols broadly categorized. These two categories of protocols are static and dynamic routing protocols. OSPF and IS-IS routing protocols are dynamic routing protocols. The routing decisions are taken during the working stage of the routers. OSPF and IS-IS are the dynamic routing protocols to route the information packet of a workstation to the destination workstation. These two routing protocols are link state routing protocol and use Dijkstra’s Shortest Path Routing strategies to select the least cost path to route the packet from source node to destination node in same autonomous system.
References
[1] V. Kaur, "Behavior Analysis of OSPF and ISIS Routing Protocol with Service Provider Network", International Journal Of Engineering And Computer Science, 2016.
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[3] M. Bhatia, "Why providers still prefer IS-IS over OSPF when designing large flat topologies!", Routing Freak!, 2011. [Online]. Available: https://routingfreak.wordpress.com/2011/03/05/why-providers-still-prefer-is-is-over-ospf-when-designing-large-flat-topologies/. [Accessed: 06- Nov- 2017].
[4] "Protocols ISIS VS. OSPF", Cisconet.com, 2009. [Online]. Available: http://cisconet.com/routing/isis/119-protocols-isis-vs-ospf.html. [Accessed: 06- Nov- 2017].
[5] R. Perlman, "A comparison between two routing protocols: OSPF and IS-IS", IEEE Network, vol. 5, no. 5, pp. 18-24, 2005.