Terminology

The next section continues the discussions by looking at specific commonalities and differences between IS-IS and OSPF. The most obvious place to continue is to consider the similarities. However, before that, take a quick look at terminologies associated with each protocol. As previously indicated, these protocols originated in different standard bodies, IS-IS in ISO (now the ITU) and OSPF in the IETF, and therefore, each is associated largely with different terminologies. Table 7-1 lists the terminology associated with each protocol and attempts to march them with equivalents in other protocol.

Table 7-1. IS-IS Versus OSPF Terminology

IS-IS	OSPF	Comments
End System	Host
Intermediate System (IS)	Router
Circuit	Link
Subnetwork Point of Attachment (SNPA)	Datalink Address
Protocol Data Unit (PDU)	Packet
Designated Intermediate System	Designated Router (DR)
Not Applicable	Backup Designated Router (BDR)
Intermediate System to Intermediate System Hello PDU (IIH)	Hello packet
Link-State Packet (LSP)	Link-State Advertisement (LSA)	IS-IS routing information is stored in TLVs, which are part of LSPs. LSAs are actually comparable to TLVs used in LSPs, however, each LSA has its own header, whereas TLVs share a common header.
Link-State Packet	Link-State Update	OSPF link-state update is a vehicle for flooding LSAs. LSPs are advertised individually.
Complete Sequence Number Packet (CSNP)	Database Description Packet (DBD)
Partial Sequence Number Packet (PSNP)	Link-State Acknowledgment or Request Packet
Routing Domain	Autonomous System
Level 2 Subdomain	Backbone area
Level 1 Area	Area (non-backbone)
Virtual Link	Virtual Link	Designed in OSPF for connecting a partitioned backbone or connecting an area that is not directly connected to the backbone over another area. In IS-IS, virtual links are used to connect a partitioned Level 1 area over Level 2.
Any Level 2 router	AS boundary Router (ASBR)	In OSPF, the ASBR redistributes external routes into the OSPF domain. Any Level 2 router can distribute externals into the domain. No special name . Note: Cisco IOS Software allows Level 1 routers to redistribute external routes into the IS-IS domain.
System ID	Router ID	The system ID is the key for SPF calculations. An IP address is symbolically carried in LSPs. A recent IETF enhancement of the IS-IS protocol introduces a Router ID TLV (Type 134) for MPLS traffic engineering applications.
Link-State Packet ID (LSPID)	Link-State ID	In IS-IS, the LSP ID is consistent and contains the system ID of the originator, the Pseudonode number, and the LSP fragment number. In OSPF, the contents of the LS ID field depends on the LS type.
No equivalent in IS-IS	Advertising Router

Common Grounds

As previously mentioned, there is a lot of similarity between IS-IS and OSPF. The commonalities are strong from a functional perspective, even though there are also significant architectural differences between them, which will be covered in later sections. Some of the high-level similarities are the following:

• Both protocols are link-state protocols, requiring routers in an area to exchange routing or link-state information. The link-state information is gathered in a Link-State database and provides an abstraction of an area's topology.

• Both protocols use a similar mechanism, known as flooding, for exchanging routing information.

• Both protocols use the concept of designated router on broadcast links to control flooding and constrain resource requirements for many-to-many adjacencies over such media.

• Both protocols use practically the same algorithm, the shortest path first algorithm (Dijkstra's Algorithm) for computing best paths based on information in the Link-State database.

• Both protocols support a two-level routing hierarchy.

• Both protocols support classless routing of IP prefixes.

Highlights of Differences

There are several technical differences between the architectures of the IS-IS and the OSPF protocols and their historical background. This section is intended to highlight some of the major technical differences, as shown in Table 7-2.

Table 7-2. Major Differences Between IS-IS and OSPF

	IS-IS	OSPF
1	IS-IS is an integrated protocol that supports routing of both ISO CLNP and IP Packets.	Designed to route only IP packets.
2	IS-IS encoding requires transimission of IS-IS packets on the data link.	OSPF packets are encapsulated in IP packets and, therefore, are transmitted over the network layer.
3	IS-IS is designed as one of three network layer protocols that supports the ISO connectionless networking environment. Recognized at the data link as an ISO Family Protocol (data-link type FEFE on Ethernet), IS-IS has Network Layer protocol ID 0x83 within the ISO family.	OSPF is not a network layer protocol and runs over IP as protocol type 89.
4	IS-IS routers advertise LSPs containing TLVs with routing information directly to adjacent neighbors.	OSPF uses different LSA types to carry different kinds of routing information. LSAs are packed in update packets for advertising to neighbors.
5	IS-IS packets use TLVs for carrying information allowing easy extensibility of all PDU.	In OSPF, only LSA are extensible.
6	An IS-IS router can skip a TLV type if the implementation does not support it.	All OSPF routers in a network must recognize all enabled extensions or LSA options for proper operation.
7	Multiple TLVs can be nested in an IS-IS Packet with a single header resulting in bandwith efficient transport.	All OSPF LSAs have their own header, such as sequence number, age, and ID of router that generated the information. Only type 1 and 2 LS allow multiple prefixes in each LSA. Because each type 3, 4, and 5 LSAs can hold only a single IP prefix, every destination outside an area will require its own LSA and independent header information.
8	IS-IS supports only broadcast and point-to-point links for all practical purposes and does not support NBMA links, which can be configured as point-to-point or as broadcast if fully meshed.	OSPF supports many link types, such as the following: Point-to-point Broadcast Nonbroadcast multiacess Point-to-multipoint Demand circuits
9	A 3-way adjacency formation is standardized for only broadcast links. Effort is underway in the IETF to standardize a 3-way process on point-to-point links.	OSPF adjacency formation involves a more elaborate multistage process.
10	Initial database synchronization occurs after adjacencies are formed .	Initial database synchronization precedes adjacency formation.
11	IS-IS routers are assocated with a single area. The whole router belongs to the area.	OSPF routers can be attached to multiple areas. Interfaces are assigned to areas.
12	Area boundaries intersect links.	Areas intersect on routers.
13	IS-IS areas are stubs by default. The recently published RFC 2966 standardizes leaking of interarea routes from Level 2 into Level 1.	By default, OSPF areas are not stubs but can be configured as such if necessary.
14	IS-IS supports only reliable flooding on point-to-point links. Flooding on broadcast links is not reliable; however, reliability is achieved by periodic synchronization with the help of the DIS.	OSPF ensures reliable flooding on all links.
15	DIS can be replaced pre-emptively. There is no backup DIS.	The designated router (DR) cannot be preempted. There is a backup DR.
16	Partial route calculation (Partial SPF) is more prevalent in an IS-IS area because IP prefixes are leaves in the SPF tree. In general, this implies less load on the route processor on the average and a plus for large areas.	Partial SPF is limited to interarea and external routes. Any interarea link flap will result in full SPF requiring smaller areas and hierarchical topologies to scale.
17	No native support for IP Multicast routing.	MOSPF extensions provide support for native IP Multicast routing.