1. OSPF

2. IS-IS - Comparison with OSPF
Both IS-IS and 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.

3. IS-IS - Comparison with OSPF
To operate with IPv6 networks, the OSPF protocol was rewritten in OSPF v3 (as specificed in RFC 2740).

4. IS-IS - Comparison with OSPF
IS-IS differs from OSPF in the way that "areas" are defined and routed between

5. IS-IS - Comparison with OSPF
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.

6. IS-IS - Comparison with OSPF
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.

7. IS-IS - Comparison with OSPF
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.

8. IS-IS - Comparison with OSPF
Given the same set of resources, IS-IS can support more routers in an area than OSPF

9. Open Shortest Path First - OSPF router types
The router type is an attribute of an OSPF process. A given physical router may have one or more OSPF processes. For example, a router that is connected to more than one area, and which receives routes from a BGP process connected to another AS, is both an area border router and an autonomous system boundary router.

10. Open Shortest Path First - OSPF router types
Each router has an identifier, customarily written in the dotted decimal format (e.g., ) of an IP address. This identifier must be established in every OSPF instance. If not explicitly configured, the highest logical IP address will be duplicated as the router identifier. However, since the router identifier is not an IP address, it does not have to be a part of any routable subnet in the network, and often isn't to avoid confusion.

11. Open Shortest Path First - OSPF v3 Packet Formats
The "Main OSPF Packet Header" is the same for all 5 types of packets (with exception of the Type field) whereas the following sub-headers will vary from type to type and are shown below the Main OSPF Packet Header.

12. Open Shortest Path First - OSPF v3 Packet Formats
Type 3: The OSPF Link State Request Packet

13. Open Shortest Path First - OSPF v3 Packet Formats
Type 4: The OSPF Link State Update Packet

14. Open Shortest Path First - OSPF v3 Packet Formats
This "Options Field" is used in OSPF Hello packets, Database Description packets, and certain LSAs (router-LSAs, network-LSAs, inter-area-router-LSAs, and link-LSAs).

15. Open Shortest Path First - OSPF v3 Packet Formats
(Note: Previous OSPF versions {v1 & v2} DO NOT support all of the options/fields listed here.)

16. Open Shortest Path First - OSPF v3 Packet Formats
x-bit: This is currently deprecated. It was previously used by MOSPF.

17. Open Shortest Path First - OSPF in broadcast and non-broadcast networks
For non-broadcast multiple-access networks (NBMA), RFC 2328 defined the following two official modes for OSPF:

18. OSPF
'Open Shortest Path First' ('OSPF') is a link-state routing protocol for Internet Protocol (IP) networks. It uses a link-state routing protocol|link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system (Internet)|autonomous system (AS). It is defined as OSPF Version 2 in RFC 2328 (1998) for IPv4. The updates for IPv6 are specified as OSPF Version 3 in RFC 5340 (2008).

19. OSPF
OSPF is perhaps the most widely used interior gateway protocol (IGP) in large enterprise networks. IS-IS, another link-state dynamic routing protocol, is more common in large service provider networks. The most widely used exterior gateway protocol is the Border Gateway Protocol (BGP), the principal routing protocol between autonomous systems on the Internet.

20. OSPF - Overview
OSPF was designed to support variable-length subnet masking (VLSM) or Classless Inter-Domain Routing (CIDR) addressing models.

21. OSPF - Overview
OSPF detects changes in the topology, such as link failures, and Convergence (routing)|converges on a new loop-free routing structure within seconds. It computes the shortest path tree for each route using a method based on Dijkstra's algorithm, a shortest path first algorithm.

22. OSPF - Overview
The OSPF routing policies to construct a route table are governed by link cost factors (external metrics) associated with each routing interface. Cost factors may be the distance of a router (round-trip time), network throughput of a link, or link availability and reliability, expressed as simple unitless numbers. This provides a dynamic process of traffic load balancing between routes of equal cost.

23. OSPF - Overview
An OSPF network may be structured, or subdivided, into routing areas to simplify administration and optimize traffic and resource utilization. Areas are identified by 32-bit numbers, expressed either simply in decimal, or often in octet-based dot-decimal notation, familiar from IPv4 address notation.

24. OSPF - Overview
Each additional area must have a direct or virtual connection to the backbone OSPF area

25. OSPF - Overview
OSPF does not use a TCP/IP transport protocol (UDP, TCP), but is encapsulated directly in IP datagrams with protocol number 89. This is in contrast to other routing protocols, such as the Routing Information Protocol (RIP), or the Border Gateway Protocol (BGP). OSPF handles its own error detection and correction functions.

26. OSPF - Overview
OSPF reserves the multicast addresses for IPv4 or FF02::5 for IPv6 (all SPF/link state routers, also known as AllSPFRouters) and for IPv4 or FF02::6 for IPv6 (all Designated Routers, AllDRouters), as specified in RFC 2328 and RFC 5340.

27. OSPF - Overview
For routing multicast IP traffic, OSPF supports the Multicast Open Shortest Path First protocol (MOSPF) as defined in RFC 1584.RFC 1584, Multicast Extensions to OSPF, J. Moy, The Internet Society (March 1994) Cisco does not include MOSPF in their OSPF implementations. PIM (Protocol Independent Multicast) in conjunction with OSPF or other IGPs, (Interior Gateway Protocol), is widely deployed.

28. OSPF - Overview
The OSPF protocol, when running on IPv4, can operate securely between routers, optionally using a variety of authentication methods to allow only trusted routers to participate in routing. OSPFv3, running on IPv6, no longer supports protocol-internal authentication. Instead, it relies on IPv6 protocol security (IPsec).

29. OSPF - Overview
All IP prefix information has been removed from the link-state advertisements and from the Hello discovery packet making OSPFv3 essentially protocol-independent

30. OSPF - Neighbor relationships
OSPF uses both unicast and multicast to send hello packets and link state updates.

31. OSPF - Neighbor relationships
Provided that OSPF is configured correctly, OSPF forms neighbor relationships only with the routers directly connected to it

32. OSPF - Area types
While most OSPF implementations will right-justify an area number written in a format other than dotted decimal format (e.g., area 1), it is wise to always use dotted-decimal formats

33. OSPF - Backbone area
Note that in OSPF the term Autonomous System Boundary Router (ASBR) is historic, in the sense that many OSPF domains can coexist in the same Internet-visible autonomous system, RFC1996 .

34. All OSPF areas must connect to the backbone area
OSPF - Backbone area
All OSPF areas must connect to the backbone area

35. OSPF - Proprietary extensions
Several vendors (Cisco, Allied Telesis, Juniper, Alcatel-Lucent, Huawei, Quagga), now implement the two extensions below to stub and NSSA area. Although not covered by RFC, they are considered by many to be standard features in OSPF implementations.

36. OSPF - Proprietary extensions
:Redistribution into an NSSA area creates a special type of LSA known as type 7, which can exist only in an NSSA area. An NSSA ASBR generates this LSA, and an NSSA ABR router translates it into type 5 LSA which gets propagated into the OSPF domain.

37. OSPF - Proprietary extensions
In such a case, the ASBR does send externals into the totally stubby area, and they are available to OSPF speakers within that area

38. OSPF - Transit area
A transit area is an area with two or more OSPF border routers and is used to pass network traffic from one adjacent area to another. The transit area does not originate this traffic and is not the destination of such traffic.

39. OSPF - Path preference
OSPF uses path cost as its basic routing metric, which was defined by the standard not to equate to any standard value such as speed, so the network designer could pick a metric important to the design

40. OSPF - Traffic engineering
OSPF-TE is an extension to OSPF extending the expressivity to allow for traffic engineering and use on non-IP networks (RFC 3630). More information about the topology can be exchanged using opaque link-state advertisement|LSA carrying type-length-value elements. These extensions allow OSPF-TE to run completely out of band of the data plane network. This means that it can also be used on non-IP networks, such as optical networks.

41. OSPF - Traffic engineering
OSPF-TE is used in GMPLS networks as a means to describe the topology over which GMPLS paths can be established. GMPLS uses its own path setup and forwarding protocols, once it has the full network map.

42. OSPF - Traffic engineering
In the Resource Reservation Protocol (RSVP), OSPF-TE is used for recording and flooding RSVP signaled bandwidth reservations for label switched paths within the link-state database.

43. OSPF - Other extensions
RFC 3717 documents work in optical routing for IP, based on constraint-based extensions to OSPF and IS-IS.

44. OSPF - Internal router
An internal router is a router that has OSPF neighbor relationships with interfaces in the same area.

45. OSPF - Designated router
Do not confuse the DR with an OSPF router type

46. OSPF - Designated router
* If the priority setting on an OSPF router is set to 0, that means it can NEVER become a DR or BDR (Backup Designated Router).

47. OSPF - Designated router
* The priority values range between , with a higher value increasing its chances of becoming DR or BDR.

48. OSPF - Designated router
* IF a HIGHER priority OSPF router comes online AFTER the election has taken place, it will not become DR or BDR until (at least) the DR and BDR fail.

49. OSPF - Designated router
DRs and BDRs are always setup/elected on OSPF broadcast networks

50. OSPF - Backup designated router
A backup designated router (BDR) is a router that becomes the designated router if the current designated router has a problem or fails. The BDR is the OSPF router with second highest priority at the time of the last election.

51. OSPF - OSPF v3 Packet Formats
The Main OSPF Packet Header is the same for all 5 types of packets (with exception of the Type field) whereas the following sub-headers will vary from type to type and are shown below the Main OSPF Packet Header.

52. OSPF - OSPF v3 Packet Formats
::This Options Field is used in OSPF Hello packets, Database Description packets, and certain LSAs (router-LSAs, network-LSAs, inter-area-router-LSAs, and link-LSAs).

53. OSPF - OSPF v3 Packet Formats
::(Note: Previous OSPF versions DO NOT support all of the options/fields listed here.)

54. OSPF - OSPF v3 Packet Formats
:::x-bit: This is currently deprecated. It was previously used by MOSPF.

55. OSPF - Implementations
* Allied Telesis implements OSPFv2 OSPFv3 in Allied Ware Plus (AW+)

56. OSPF - Implementations
* Bird Internet routing daemon|BIRD implements both OSPFv2 and OSPFv3

57. OSPF - Implementations
* GNU Zebra, a GNU General Public License|GPL routing suite for Unix-like systems supporting OSPF

58. OSPF - Implementations
* NetWare implements OSPF in its Multi Protocol Routing module.

59. OSPF - Implementations
* OpenBSD includes an OpenOSPFD implementation within the OpenBGPD protocol.

60. OSPF - Implementations
* XORP, a routing suite implementing RFC2328 (OSPFv2) and RFC2740 (OSPFv3) for both IPv4 and IPv6

61. OSPF - Implementations
* Windows NT 4.0 Server, Windows 2000 Server and Windows Server 2003 implement OSPFv2 in the Routing and Remote Access Service, although the functionality was removed in Windows Server 2008.

62. OSPF - Applications
There are some historical accidents that made IS-IS the preferred IGP for ISPs, but ISP's today may well choose to use the features of the now-efficient implementations of OSPF, after first considering the pros and cons of IS-IS in service provider environments.

63. OSPF - Applications
As mentioned, OSPF can provide better load-sharing on external links than other IGPs. When the default route to an ISP is injected into OSPF from multiple ASBRs as a Type I external route and the same external cost specified, other routers will go to the ASBR with the least path cost from its location. This can be tuned further by adjusting the external cost.

64. OSPF - Applications
The only real limiting factor that may compel major ISPs to select IS-IS over OSPF is if they have a network with more than 850 routers. There is mention of an OSPF network with over 1000 routers, but that is quite uncommon and the network must be specifically designed to minimize overhead to achieve stable operation.

65. Link-state advertisement - OSPF v3 LSA Packet Header Formats
As per [ Appendix A.4] of RFC 5340 (OSPFv3 for IPv6) depending upon the LS Type, there are nine major LSA Packet formats as follows (actually eight as one has been deprecated):

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