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Differences Between IS-IS and OSPF

Latest reply: Dec 29, 2018 09:21:55 927 5 7 0 1

Hello everyone,

Today I will share with you the differences between IS-IS and OSPF.
IS-IS supports only point-to-point and broadcast subnets. OSPF supports P2P, broadcast, P2MP, and NBMA networks.
OSPF areas are divided based on interfaces. IS-IS is divided into areas based on route routes.
The conditions for establishing an adjacency are different.
The Hello protocol used by IS-IS is simple and OSPF is complex. In addition, the IS-IS check is loose, and the Hello time and Dead time between the master and slave devices do not need to be the same, but the OSPF requirements are the same.
The OSPF adjacency on a point-to-point link is more reliable than that on an IS-IS network.
IS-IS uses the two-way handshake mechanism to form an adjacency relationship. However, there are three handshake mechanisms.
In OSPF, different DRs on a common router form an adjacency relationship. IS-IS adjacencies are established.
The DR and DIS election processes are different.
The DIS election of IS-IS is simple and predictable. The DIS with the highest priority is the DIS, and the DIS with the priority of 0 may also be the DIS.
To ensure a small change, OSPF DR election is complex and unpredictable. The DR with the highest priority is not necessarily the DR. The DR with the priority of 0 cannot become the DR. The DR fails, the BDR immediately takes the responsibility of the DR. The IS-IS does not back up the DIS. As a result, the DIS fails and the DIS is re-elected.
There are many types of OSPF LSAs and the database structure is complex. Therefore, it is difficult to locate faults. IS-IS LSPs include only router LSPs and pseudonode LSPs. The database structure is simple and faults can be located easily.
The synchronization process is different.
The OSPF LSA lifetime is increased from 0 (0~ 60 minutes, and the period cannot be configured). IS-IS decreased from the maximum value (20 minutes ~0, configurable period)
OSPF uses the prefix as the SPT node. IS-IS uses the prefix as a leaf.
The SPF algorithm of ISSI is simple. In addition, IS-IS uses the prefix as the SPT leaf. Therefore, when the leaf changes, you can use partial route calculation (PRC) to update the leaf without performing the SPF calculation.

That is all I want to share with you! Thank you!

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yiyi0519
Created Dec 29, 2018 03:15:15

Both IS-IS and Open Shortest Path First (OSPF) are link state protocols, and both use the same Dijkstra algorithm for computing the best path through the network. As a result, they are conceptually similar. Both support variable length subnet masks, can use multicast to discover neighboring routers using hello packets, and can support authentication of routing updates.

While OSPF was natively built to route IP and is itself a Layer 3 protocol that runs on top of IP, IS-IS is an OSI Layer 2 protocol.[4] It is at the same layer as Connectionless Network Protocol (CLNP). The widespread adoption of IP may have contributed to OSPF's popularity. IS-IS does not use IP to carry routing information messages. OSPF version 2, on the other hand, was designed for IPv4. IS-IS is neutral regarding the type of network addresses for which it can route. This allowed IS-IS to be easily used to support IPv6. To operate with IPv6 networks, the OSPF protocol was rewritten in OSPF v3 (as specified in RFC 2740).

Both OSPF and IS-IS routers build a topological representation of the network. This map indicates the subnets which each IS-IS router can reach, and the lowest-cost (shortest) path to a subnet is used to forward traffic.

IS-IS differs from OSPF in the way that "areas" are defined and routed between. IS-IS routers are designated as being: Level 1 (intra-area); Level 2 (inter area); or Level 1–2 (both). Routing information is exchanged between Level 1 routers and other Level 1 routers of the same area, and Level 2 routers can only form relationships and exchange information with other Level 2 routers. Level 1–2 routers exchange information with both levels and are used to connect the inter area routers with the intra area routers.

In OSPF, areas are delineated on the interface such that an area border router (ABR) is actually in two or more areas at once, effectively creating the borders between areas inside the ABR, whereas in IS-IS area borders are in between routers, designated as Level 2 or Level 1–2. The result is that an IS-IS router is only ever a part of a single area.

IS-IS also does not require Area 0 (Area Zero) to be the backbone area through which all inter-area traffic must pass. The logical view is that OSPF creates something of a spider web or star topology of many areas all attached directly to Area Zero and IS-IS by contrast creates a logical topology of a backbone of Level 2 routers with branches of Level 1–2 and Level 1 routers forming the individual areas.

IS-IS also differs from OSPF in the methods by which it reliably floods topology and topology change information through the network. However, the basic concepts are similar.[citation needed]

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. For example, in order to support IPv6, the IS-IS protocol was extended to support a few additional TLVs, whereas OSPF required a new protocol draft (OSPFv3). In addition to that, IS-IS is less "chatty" and can scale to support larger networks. Given the same set of resources, IS-IS can support more routers in an area than OSPF. This has contributed to IS-IS as an ISP-scale protocol.[citation needed]

The TCP/IP implementation, known as "Integrated IS-IS" or "Dual IS-IS", is described in RFC 1195.
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4am
Created Dec 29, 2018 03:23:04

IS-IS is usually used on carriers, and OSPF is usually used on enterprise networks.
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4am
Created Dec 29, 2018 08:04:22

Open Shortest Path First, which ensures that no loop occurs.
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YOO
Created Dec 29, 2018 08:55:51

There are many types of OSPF LSAs and the database structure is complex. Therefore, it is difficult to locate faults.
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chouhao
Created Dec 29, 2018 09:21:55

Both OSPF and IS-IS routers build a topological representation of the network. This map indicates the subnets which each IS-IS router can reach, and the lowest-cost (shortest) path to a subnet is used to forward traffic.
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