IS-IS and OSPF – What the heck?

The increasing popularity of IS-IS and OSPF over the years has drawn significant attention to the relative merits and demerits of one with respect to the other. The blog entries that follow would present an elaborate comparision between the two routing protocols to explain how the features and functionalities of one differs from the other.

Since both these routing protocols originated in different standard bodies, IS-IS in ISO and OSPF in the IETF, there exists some difference in the terminologies used.


End System – Host
Intermediate System – Router
Circuit – An adjacency on one link
SNPA Address – Data link Address
Protocol Data Unit (PDU) – Packet
Designated Intermediate System (DIS) – Designated Router (DR)
IS to IS Hello PDU (IIH) – Hello Packet
Not Applicable – Backup Designated Router (BDR)
Link State Packet(LSP) – Link State Advertisement (LSA)
Link State Packet – Link State Update
Complete Sequence Number Packet(CSNP) – Database Description packet
Partial Sequence Number Packet(PSNP) – Link state ACK or Request Packet
Routing Domain – AS
Level 2 Subdomain – Backbone Area
Level 1 Area – Non Backbone Area
Level 1/2 IIH PDU – Simple Hello Packet
Level 1/2 LSP – No Distinction
L1L2 router – ABR System ID – Router ID
Link State Packet ID(LSPID) – Link State ID
Pseudonode LSP – Network LSA

Router LSAs, Summary LSAs, Network LSAs, ASBR Summaries, AS-external LSAs are equivalent of TLVs carried in LSPs in IS-IS. The difference is that each LSA has its own header whereas the TLVs share a common header.

IS-IS Terms with no OSPF equivalent:

TLV – Type-Length-Value tuple. These carry most of the information in IS-IS PDUs.

OSPF Terms with no IS-IS equivalent:

Advertising Router – Router that originated the advertisement. In IS-IS, this is the LSP’s originator.
Backup Designated Router – Router which takes over in case the DR goes down. In IS-IS, there is no Backup DIS and the DIS election takes place again in case the former goes down or is no more available.


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