IS-IS vs OSPF

1. IS-IS VERSUS OSPF
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2. SIMILARITIES OF THESE TWO PROTOCOL ONE BY ONE
– IS-IS Protocol and OSPF are both Link-State Interior Gateway Protocols.
– IS-IS Protocol and OSPF are both uses Dijkstra SPF (Shortest Path First)
algorithm.
– These protocols have LSDBs and the LSDB similar structure
– Both support VLSM (Variable Length Subnet Mask)
– Both are very scalable protocols
– Both has a fast convergence time after changes
– Both are open standards
– Both have hierarchical structures, there are two hierarchy level
– Both protocols support LANs and point-to-point protocols in similar ways
– Both protocol use periodic Hello messages for neighbour establishment

3. DIFFERENCES OF THESE TWO IMPORTANT PROTOCOLS
– IS-IS Protocol works above Layer 2 next to IP, OSPF works on top of IP,
– IS-IS Protocol routers belong to one area, OSPF routers belong multiple areas, links
are in areas in OSPF
– IS-IS Protocol uses DIS, OSPF uses DR and BDR,
– OSPF has virtual-links, ISIS does not support this,
– OSPF uses opaque LSA extensions, ISIS can be extended more by TLVs
– OSPF has efficient packet types, ISIS has untidy structure because of TLVs, but more
granular,
– In OSPF small changes causes small packets, in ISIS even if small change, entire LSPs
are sent.

4. OSPF IS-IS
Top of IP Above L2, Next to IP
Routers on Multiple Areas Routers on one Area
DR and BDR DIS
Virtual-link Support No Virtual-link
Open LSA Extension TLVs
Effective Packet Types More Granular (Different TVLs)
Small Change, Small Packets Small Change, Entire LSP

5. OSPF IS-IS
Host End System (ES)
Router Intermediate System (IS)
Link Circuit
Packet Protocol data Unit (PDU)
Designated Router (DR) Designated IS (DIS)
Back-up DR (BDR) No Backup DIS
Link-State Advertisement (LSA) Link-State PDU (LSP)
Hello Packet IHH PDU
Database Description (DD) CSNP
Area Sub Domain (Area)
Backbone Area Level 2 sub Domain
Non Backbone Area Level 1 Area
ABR L1/2 Router
ASBR Any IS

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