Talk:Open Shortest Path First

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[edit] Initial Comments

TYPE 5 does equal AS External, HELLO???????? —Preceding unsigned comment added by 206.126.163.20 (talk) 16:32, 23 January 2008 (UTC)

type 5, eh??

Summary Net Link States (Area 1)

Link ID ADV Router Age Seq# Checksum 0.0.0.0 2.2.2.2 20 0x80000001 0x75C0 6.0.0.0 2.2.2.2 13 0x80000001 0x2709 —Preceding unsigned comment added by 206.126.163.20 (talk) 16:29, 23 January 2008 (UTC)



The remark concerning Type 5 advertisements and Stub Areas is in fact correct. Type 5 carries AS-external routing information, and a Stub Area only has one connection towards the backbone, thereby eliminating the need for a detailled routing information. Stub Areas are usually only fed a default route and maybe the AS-internal routes (Type 1-4), but they don't need to know about the AS-external routes. Sigkill 02:06, 1 July 2006 (UTC)

"Intra-area routing goes via the backbone"

Is this really correct? It would seem more logical if inter-area routing was done via the backbone. --K. Sperling 10:33, July 20, 2005 (UTC)

True, and is corrected meanwhile. Sigkill 02:06, 1 July 2006 (UTC)

"In contrast to RIP or BGP, OSPF does not use TCP or UDP but uses IP directly, using IP protocol 89."

This sounds incorrect Routers are layer 3 devices so how can they use Transport Layer, Layer 4, services?

While some routing protocols use L4 transports (such as tcp/179 for BGP or udp/520 for RIP), others bring their own L4 protocol (such as (E)IGRP with L4-protcol number 88 or OSPF with L4-protocol number 89). You should distinguish between the actual routing task running on Layer 3, and the inter-router communication, which uses higher-level protocols. Sigkill 02:06, 1 July 2006 (UTC)
I'd hesitate to call the protocol number layer 4. By protocol number, I refer to a field in the IP packet header, which identifies what the packet carries if not pure IP. It can be TCP or UDP, but also ICMP or OSPF or EIGRP. OSPF has its own reliable delivery mechanism with retransmissions and other features built into the protocol, but with lower overhead than TCP. Be careful before trying to force routing protocols into OSI layers, because (1) IP routing protocols were designed without OSI compliance as a goal, and (2) additional ISO documents, such as "OSI Routeing [sic] Framework", identify routing protocols as layer management protocols of the network layer. Layer management is defined in an annex to the basic OSI reference model. Hcberkowitz 21:15, 14 June 2007 (UTC)

Could a list of entry level Products which support OSPF be added, so others can learn by 'doing'?

Added meanwhile. Sigkill 02:06, 1 July 2006 (UTC)


I suggest adding a "transit" area description. Actually the OSPF areas are either transit or stubby (not mentioning the fancy types building on these elementary types). A backbone is a special case of a transit area. Other areas may be transit as well and they can also import external routes via LSA-5. Also I think it would be nice to describe the basic LSA types and the states of routers when creating neighborhood/adjacencies. I can try to provide this content it the maintainer agrees to :) Paluchpeter 00:18, 29 December 2006 (UTC)

[edit] A good introduction article

I think this guy has done a good job on introducing the topic. Its a little Cisco oriented, but unlike the corporate literature, his writting style is human, or what Art Kleiner call "native" in his book The Age of Heretics.

One thing that I found confusing though, what is the explicit message behind this paragraph? Does he imply that OSPF can not scale? I am betting its communication problem as the guy seems to know his stuff really well

"The U.S. Post Office is putting in 38,000 Cisco routers. Note that if we use our rules of thumb, 38,000 divided by 100 routers says we'd need 380 areas. Alternatives include multiple OSPF AS's, and other protocols (like EIGRP). I'd very definitely want healthy route summarization in such a network. Note that since OSPF only allows us to summarize at ABR's, we can only have one level of summarization. With 38,000 routers, we might well want to summarize at the regional, state, and portion of state levels. That can't be done with pure OSPF." [1]

I know Pete Welcher, the author of that link, and have discussed the project with him. Let me put in my own opinion. First, OSPF probably could not scale to 38,000 routers and probably at least times that in subnets. Within a single OSPF process, you only have one level of summarization. Do you create one area for each of the three-digit regional prefixes of ZIP codes? What is an administrative hierarchy that makes sense?
For this sort of network, I'd establish a number of OSPF domains (i.e., an area 0.0.0.0 and some number of nonzero areas), and interconnect them with a backbone-of-backbones running BGP. That is about as scalable a technique as we know. Yes, OSPF is faster converging with a reasonable number of routers and subnets, but making the whole service one relatively flat domain makes you extremely vulnerable to anything going into oscillation.Hcberkowitz 21:07, 14 June 2007 (UTC)

I reverted my unvandalisation, removing the IPstack template, because it's the template which has been vandalised. I don't know how to remove the link to 'ANUS' from it.

[edit] MD5 to authenticate ?

I dont know much about OSPF or Networking, but what I know is MD5 is used for verifying Integrity and not for authentication. May be the line " OSPF Protocol can operate (communicate with other routers about "best-path" routes to save in their LSDBs) securely, using MD5 to authenticate peers before forming adjacencies" shall be considered for correction.

--Empit 21:54, 13 April 2007 (UTC)

OSPF routers can transmit MD5-hashed password in Hello packets to authenticate adjacent routers. MD5 hashing is a basic protection against sniffing the password.
OSPF also supports plaintext passwords. I've had situations where we were not especially concerned with sniffing as a danger, but were in the process of combining OSPF networks of two merged companies. As we converted, we used one password for each of the old networks and another password for the merged network. This kept us from accidentally picking up routers that weren't converted to the new conventions. With our particular router vendor, there were easy-to-code but also easy-to-pick-up-garbage configuration options that could easily bring in things we didn't want in the merged system. If the passwords didn't match, that ended the possibility of a mixed environment.Hcberkowitz 21:11, 14 June 2007 (UTC)

[edit] Full Disclosure and All That

In the section I added on OSPF Applications, I did cite one of my peer-reviewed NANOG presentations, along with one by my esteemed colleague, Dave Katz. One of the reasons that most IS-IS implementations tend to work together is that Dave wrote most of them. There are assorted authors of OSPF implementations. Howard C. Berkowitz 23:28, 19 September 2007 (UTC)

[edit] Request for further information

I've just tidied up the article, trying to remove duplication and some citations that don't seem to work (not sure if this was a previous way of doing references?)

Anyway, the thing I don't get from reading the article is how OSPF actually works. It would be really helpful if someone who understands this (and I don't) could write a section on how routes are actually exchanged between routers, and how the optimum routes are actually calculated. Thanks in advance.--Phil Holmes 09:43, 3 October 2007 (UTC)

Once I get some books out of storage, or maybe without (I've been moving), I can get down to that level in OSPF. Some feedback about how detailed an explanation is appropriate here would be useful, as this easily gets to be book-length if all is covered.
Let make some brief comments, just to give a flavor. Some of these are already covered, to some extent, my question is how much more detail is appropriate. It's purely my opinion, but I tend to think packet functios, but not formats, are appropriate for an encyclopedia. OSPF has a standard header on packets it sends out, which should be understood do not use any transport protocol.
When reliable delivery is important, OSPF does its own retransmission. Are the mechanics of that retransmission relevant, especially because it has fault-tolerance mechanisms to allow the Backup Designated Router to take over if the DR goes down?
Standard header discussion?
I should note that things are reasonably straightforward on broadcast (e.g., LAN) and point-to-point media. Things get significantly more complicated on nonbroadcast multiaccess (NBMA, such as Frame Relay) or demand (e.g., dialup).

There are a reasonably small number of packet types; the real complexity is in the Link State Announcements (LSA) that most of them carry:

  1. Hello
  2. Database Description packet
  3. The Link State Request packet
  4. Link State Update packet
  5. The Link State Acknowledgment packet
All but the Hello packet carry one or more LSAs, either in summary or full form. There is what I'd call a basic set of LSAs, and then a newer group, typically used for optional capabilities such as traffic engineering, that need considerably more explanation. Here's the basic set, ignoring two obsolete types:
  1. Router
  2. Network
  3. Summary (two subtypes)
  4. External (two major subtypes, and subcategories of those)
My question to you and all other people concerned: is this level of list, with narrative explanations, adequate for an encyclopedia? Obviously, I haven't gotten into things like how the initialization of new routers' databases takes place, how routes are added and deleted within an area, what packets go in or out of areas, how the basic intra-area computation works (with several versions, some vendor-dependent), how inter-area and externals work if the area type permits them. That's a start. Howard C. Berkowitz 11:09, 3 October 2007 (UTC)
For me, I think I could summarise what I see as important by asking "how does the forwarding/routing table get populated?". So I think the details of packet headers, transmission mechanisms, transmission reliability, etc., are not needed for an encyclopedia article. But how the routers learn routes from each other - that seems to me to be the fundamental function of OSPF, and therefore the fundamental "how to" need.
I hope that's helpful. --Phil Holmes 14:58, 3 October 2007 (UTC)