Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Get The TCP/IP Guide for your own computer.
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  TCP/IP Routing Protocols (Gateway Protocols)
                9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)
                     9  Open Shortest Path First (OSPF)

Previous Topic/Section
OSPF Basic Topology and the Link State Database (LSDB)
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
OSPF Route Determination Using SPF Trees
Next Topic/Section

OSPF Hierarchical Topology, Areas and Router Roles
(Page 2 of 3)

Router Roles in OSPF Hierarchical Topology

The topology described above is hierarchical because the routers in the AS are no longer all peers in a single group. The two-level hierarchy consists of the lower level containing individual areas, and the higher level that connects them together, which is called the backbone and is designated as “Area 0”. The routers are no longer all peers, but in fact play different roles depending on where they are located and how they are connected. There are three different labels applied to routers in this configuration:

  • Internal Routers: These are routers that are only connected to other routers or networks within a single area. They maintain an LSDB for only that area, and really have no knowledge of the topology of other areas.

  • Area Border Routers: These are routers that connect to routers or networks in more than one area. They maintain an LSDB for each area of which they are a part. They also participate in the backbone.

  • Backbone Routers: These are routers that are part of the OSPF backbone. By definition, this includes all area border routers, since those routers pass routing information between areas. However, a backbone router may also be a router that connects only to other backbone (or area border) routers, and is therefore not part of any area (other than Area 0).

    To summarize: an area border router is always also a backbone router, but a backbone router is not necessarily an area border router.

Note: The classifications above are independent of the designation of a router as being a boundary router or not, as described in the previous topic. A boundary router is one that talks to routers or networks outside the AS. Now a boundary router will also often be an area border router or a backbone router, but this is not necessarily the case—a boundary router could be an internal router in one area.


Okay, I bet you are now wondering… what is the point of all this? Well, the point is exactly the same as the point of using autonomous system architecture in the first place. The topology of each area matters only to the devices in that area. This means that changes in that topology only need to be propagated within the area. It also means that internal routers within Area 1 don't need to know about anything that goes on within Area 2, and don't need to maintain information about any area other than their own. Only the backbone routers (which include at least one area border router within each area) need to know the details of the entire autonomous system. These backbone routers condense information about the areas so that only a “summary” of each area's topology needs to be advertised on the backbone.

Routing in a hierarchical topology AS is performed in one of two ways, depending on the location of the devices. If the source and destination are in the same area, then routing occurs only over networks and routers in that area. If they are in a different area, then the datagram is routed from the source to an area border router in the source's area, over the backbone to an area border router in the destination's area, and then finally, delivered to the destination. Again, this is analogous to how routing works between ASes in the big-picture internetwork.


Previous Topic/Section
OSPF Basic Topology and the Link State Database (LSDB)
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
OSPF Route Determination Using SPF Trees
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.