OSPF is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior routing protocols, operating within a single autonomous system (AS). It is defined as OSPF Version 2 for IPv4. The updates for IPv6 are specified as OSPF Version 3. OSPF is perhaps the most widely used interior gateway protocol (IGP) in large enterprise networks.
Comparing OSPF vs ISIS
• Introduction to OSPF
• Introduction to ISIS
• Similarities between OSPF and ISIS
• Differences between OSPF and ISIS
• OSPF and ISIS Terminology
• OSPF and ISIS Transport
• Why Prefer ISIS over OSPF ?
• Which Interior Gateway Protocol(IGP) will you choose ?
•Considerations for ISIS and OSPF
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Introduction to OSPF
Open Shortest Path First
Developed by IETF (OSPF Working Group) for IP – RFC1247
OSPF is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR)
algorithm and falls into the group of interior routing protocols, operating within a single
autonomous system (AS). It is defined as OSPF Version 2 for IPv4. The updates for IPv6 are
specified as OSPF Version 3. OSPF is perhaps the most widely used interior gateway protocol
(IGP) in large enterprise networks.
It gathers link state information from available routers and constructs a topology map of the
network. The topology is presented as a routing table to the Internet layer which routes packets
based solely on their destination IP address.
It computes the shortest-path tree for each route using a method based on Dijkstra's algorithm.
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Introduction to OSPF(Cont.)
OSPF does not use a transport protocol, such as UDP or TCP, but encapsulates its data directly in
IP packets with protocol number 89(Protocol OSPF).
Designed to respond quickly to topology changes but using minimal protocol traffic.
OSPF supports complex networks with multiple routers, including backup routers, to balance
traffic load on multiple links to other subnets. Neighboring routers in the same broadcast
domain or at each end of a point-to-point telecommunications communicate with each other via
the OSPF protocol.
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Introduction to OSPF(Cont.)
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• OSPF can be used to distribute routing information between routers that belongs to same
Introduction to ISIS
Intermediate System to Intermediate System
The protocol was defined in ISO/IEC 10589:2002 as an international standard within the Open
Systems Interconnection (OSI) reference design.
IS-IS is a routing protocol designed to move information efficiently within a computer network, a
group of physically connected computers or similar devices. It accomplishes this by determining
the best route for datagrams through a packet-switched network.
IS-IS is a link-state routing protocol, operating by reliably flooding link state information
throughout a network of routers. Each IS-IS router independently builds a database of the
network's topology, aggregating the flooded network information.
Like the OSPF protocol, IS-IS uses Dijkstra's algorithm for computing the best path through the
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Introduction to ISIS(Cont.)
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• ISIS can be used to distribute routing information between routers that belongs to same
Similarities: OSPF and ISIS
• Both routing protocols are link-state protocols and Interior Gateway Protocols(IGP).
• Both can be used to distribute routing information between routers that belongs to same
• Both use the same Dijkstra algorithm for computing the best path through the network.
• Both support variable length subnet masks, can use multicast to discover neighboring routers
using hello packets, and can support authentication of routing updates.
• Both are classless routing protocols and support CIDR, VLSM and discontinuous network.
• Both elect a designated router in the broadcast networks and support unlimited number of hop
• And both of the routing protocol support authentication mechanism.
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Differences: OSPF and ISIS
• OSPF is under Internet Engineering Task Force(IETF) standard, whereas ISIS is under International
Telecommunication Standard(ITU), formerly International Organization for Standardization(ISO)
• OSPF supports NBMA and point-to-multipoint links, IS-IS does not.
• The widespread adoption of IP may have contributed to OSPF's popularity. IS-IS does not use IP to
carry routing information messages.
• Integrated IS-IS runs directly over OSI layer 2 (Datalink layer) and is therefore safe against attacks
such as: the IP package spoofing but also against certain Denial of Service (DoS) attacks. OSPF, on
the contrary, runs over the Internet protocol (IP) with protocol number 89. This means that OSPF
has to make do with an additional IP header, but this IP header increases the overhead of a routing
• OSPF designates a backbone area (area 0) for inter-area advertisements; IS-IS organizes the
domain into two layers.
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Differences: OSPF and ISIS(Cont.)
• OSPF routers can belong to multiple areas, IS-IS routers belong to exactly one area.
• OSPF elects a DR and BDR, whereas IS-IS elects only a single DR called DIS.
• OSPF has a larger set of extensions and optional features specified in the protocol standards.
However IS-IS is easier to expand: its use of type-length-value (TLV) data allows engineers to
implement support for new techniques without redesigning the protocol.
• Most other routing protocols required modification in order to support IPv6, whereas
because IS-IS is network-protocol neutral, it can support IPv6 right out of the gate.
• OSPF uses Router ID, whereas IS-IS uses System ID to identify each router on the network.
• OSPF can support virtual links, IS-IS cannot (because it rides L2 directly)
• OSPF supports more link types than IS-IS. The point-to-point and broadcast connections are
supported by both routing protocols. OSPF also supports Nonbroadcast Multiacess, Point-to-
Multipoint, and Demand Circuits Link types.
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Terminology: OSPF and ISIS
OSPF Terminology ISIS Terminology
Router Intermediate System(IS)
Designated Router Designated Intermediate system(DIS)
Host End System
Datalink Address Subnetwork Point of Attachment(SNPA)
Packet Protocol Data Unit(PDU)
Hello Packet(HP) Intermediate System to Intermediate System
Link State Advertisement(LSA) Link State Packet(LSP)
Database Description Packet(DBD) Complete Sequence Number Packet(CSNP)
Link State Acknowledgement order Request Packet Partial Sequence Number Packet(PSNP)
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Terminology: OSPF and ISIS(Cont.)
OSPF Terminology ISIS Terminology
Autonomous System Routing Domain
Backbone Area Level-2 Sub-Domain
Area Leval-1 Area
Autonomous System Boundary Router(ASBR) Leval-2 Intermediate System(L2IS)
Router Id System ID
Link-State ID Link-State Packet ID
IP Destination Address + IP Protocol Number Network Service Access Point(NSAP)
IP Destination Address NET(Network Entity Title)
Subnet = IP network Subnet = data link
Virtual Link Virtual Link
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Transport: OSPF and ISIS
OSPF uses IP Protocol 89 (Layer 3) for transport
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Data link header
OSPF packet data
Transport: OSPF and ISIS(Cont.)
ISIS uses Connection-Less Network Service(CLNS) Protocol(Layer 2)
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Data link header ISIS packet
ISIS packet data
Why prefer ISIS over OSPF?
• IS-IS provides the inherent security by running natively over the Layer 2.
• With ISIS it is easier to extend then OSPF. ISIS can run both IPv4 and IPv6 without doing that
much changes. On the other side to run IPv6 you have to use OSPFv3.
• The IS-IS spec makes it easier to put an IS-IS reachability as leaf nodes as all routers are
identified by a different set of TLVs. This information while its available in OSPF is slightly
tricky as the node information is mixed with the link information.
• OSPF uses 32 bit IP addresses for routing. With IS-IS, you can use either IP or CLNP addresses
individually, or in DUAL mode, both simultaneously.
• As a service provider running latest high end router platform and doing upgradation of code
to support a new TLV in an IGP is much better than having change there entire IGP to support
any new feature.
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Why prefer ISIS over OSPF?(Cont.)
• IS-IS uses just one Link State Packet (LSP) per level for exchanging the routing information.
This LSP contains many TLVs, each of which represents a piece of routing information. OSPF
on the other hand, needs to originate multiple LSAs, one for each type and as a consequence
is a lot more chattier and hence not suitable for large flat networks.
• Also one of the most important reason for using ISIS is Partial Route Calculation used in ISIS,
with this you can have large number of routers as compared to OSPF. Routers in ISIS do not
run Shortest Path First when there is change in prefix but they run Shortest Path First when
then is a change in topology. So, because of this larger number of routers can be fit in one ISIS
• ISIS is more scalable, has less overhead and also support IPv4 and IPv6 in a single instance
and protocol and includes multi topology.
• Computation for large area is much less CPU and memory intensive than for OSPF.
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Which IGP will you choose?
OSPF(Open Shortest Path First)
• OSPF is suited to functional enhancements and topological flexibilities built in IPv6.
• Routing platform adjust quickly to link failure and other changes made by converting to new
routing structure in few seconds.
• Based on link cost factor can construct routing tables associated with each routing interface.
• This IGP is widely used by large enterprise network.
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Which IGP will you choose?(Cont.)
IS-IS (Intermediate System to Intermediate System)
• Offers more scalability.
• Can accommodate large number of routers in one area.
• Can work in both domains IPv4 and IPv6, possible with the multi-topology.
• ISIS has advantage over OSPFv2 and OSPFv3 which are restricted to IPv4 and IPv6.
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• IS-IS transports on data link layer
• Not Possible to attack IS-IS using IP.
Independent of IP addressing
• NSAP addressing scheme of IS-IS avoid dependencies on IP.
• IS-IS has been used for long time by world’s major Internet Service Providers
• Vendors give more importance to IS-IS reliability, scalability and the features.
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Migration to IPv6
• Adding IPv6 means OSPFv2 and OSPFv3 in network
o Two protocols, two sets of identical configuration
• IS-IS simply requires the addition of the IPv6 address family
o Most networks operate single topology for IPv4 and IPv6
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