ospf network type question

Unanswered Question
Sep 24th, 2007

How much of a published text can I cut and paste in this forum to gain insight into something that makes no sense?

I've used three different sets of materials for CCNP BSCI and I am still baffled by the OSPF network types. It would be easier to ask my question for clarification purposes if I could post a paragraph from the text I'm currently working with. Is that allowable?

I have this problem too.
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cyphur353 Mon, 09/24/2007 - 12:39

Are you referring to point-to-point, NBMA, broadcast multi-access, and point-to-multipoint, in relation to how they form adjancies and how they handle updates and hellos?


Network Types

OSPF defines five network types:

Point-to-point networks, Broadcast networks, Nonbroadcast Multiaccess (NBMA) networks, Point-to-multipoint networks, Virtual links

Point-to-point networks, such as a T1, DS-3, or SONET link, connect a single pair of routers. Valid neighbors on point-to-point networks will always become adjacent. The destination address of OSPF packets on these networks will always be the reserved class D address, known as AllSPFRouters.[5]

Broadcast networks, such as Ethernet, Token Ring, and FDDI, might be better defined as broadcast multi-access networks to distinguish them from NBMA networks. Broadcast networks are multi-access in that they are capable of connecting more than two devices, and they are broadcast in that all attached devices can receive a single transmitted packet. OSPF routers on broadcast networks will elect a DR and a BDR, as described in the next section, "Designated Routers and Backup Designated Routers." Hello packets are multicast with the AllSPFRouters destination address, as are all OSPF packets originated by the DR and BDR. The destination Media Access Control (MAC) identifier of the frames carrying these packets is 0100.5E00.0005. All other routers will multicast link-state update and link-state acknowledgment packets (described later) to the reserved class D address, known as AllDRouters. The destination MAC identifier of the frames carrying these packets is 0100.5E00.0006.

NBMA networks, such as X.25, Frame Relay, and ATM, are capable of connecting more than two routers but have no broadcast capability. A packet sent by one of the attached routers would not be received by all other attached routers. As a result, extra configuration might be necessary for routers on these networks to acquire their neighbors. OSPF routers on NBMA networks elect a DR and BDR, and all OSPF packets are unicast.

Point-to-multipoint networks are a special configuration of NBMA networks in which the networks are treated as a collection of point-to-point links. Routers on these networks do not elect a DR and BDR, and the OSPF packets are unicast to each known neighbor.

Virtual links, described in a later section, are special configurations that are interpreted by the router as unnumbered point-to-point networks. OSPF packets are unicast over virtual links.

In addition to these five network types, it should be noted that all networks fall into one of two more-general types:

Transit networks have two or more attached routers. They might carry packets that are "just passing through"packets that were originated on and are destined for a network other than the transit network.

Stub networks have only a single attached router.[6] Packets on a stub network always have either a source or a destination address belonging to that network. That is, all packets were either originated by a device on the network or are destined for a device on the network. OSPF advertises host routes (routes with a mask of as stub networks. Loopback interfaces are also considered stub networks and are advertised as host routes.


- Excerpt from Routing TCP/IP, Volume 1, Second Edition.

Jeff Doyle, #1919, Jennifer Carroll, # 1402

CCIE Professional Development Series.

I hope I've done an adequate job of giving credit where credit is due, as I could not paraphrase them adequately without reducing the clarity which the authors achieved.

suelange Mon, 09/24/2007 - 17:10

Thank you for this post. That is the best treatment of this very confusing subject that I have read so far. I've seen other excerpts from Mr. Doyle and he's a very good author. Kudos to him and Ms. Carroll and thank you for sharing this.

Kevin Dorrell Mon, 09/24/2007 - 22:45

suelange, I can endorse that book as well (or both of them, 'cos the second volume is good as well, dealing with BGP and multicast). It is one of my favorites. It is one CCIE level book that can be read easily by a CCNA candidate. Somehow they manage to make the intricate detail exciting enough to make you want to read to the end of each chapter. And it seems to be reasonably error-free, which is a rare attribute these days.

Kevin Dorrell



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