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!

The whole site in one document for easy reference!
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 Exterior Gateway/Routing Protocols (BGP and EGP)
                     9  TCP/IP Border Gateway Protocol (BGP/BGP-4)
                          9  BGP Fundamentals and General Operation

Previous Topic/Section
BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)
Previous Page
Pages in Current Topic/Section
1
23
Next Page
BGP Route Determination and the BGP Decision Process
Next Topic/Section

BGP Path Attributes and Algorithm Overview
(Page 1 of 3)

Routing protocols using a distance-vector algorithm such as RIP are relatively simple in large part because the information each device stores about each route is itself simple. Each router only knows that it can reach a network at a specific cost through a particular next-hop router. It doesn't have knowledge of the route that datagrams will take to reach any of these networks. This level of knowledge is simply insufficient for the needs of a protocol like BGP.

BGP's Path-Vector Routing Algorithm and Path Attributes

In order to handle the calculation of efficient, non-looping routes in an arbitrary topology of autonomous systems, we need to know not just “to get to network N7 send to router R4”; we need to have some understanding of the characteristics of the entire path between ourselves and network N7. By storing this additional information, it is possible to make decisions about how to compute and change routes, using knowledge of the entire path between a router and a network.

Thus, in BGP, instead of advertising networks in terms of a destination and the distance to that destination, BGP devices advertise networks as destination addresses and path descriptions to reach those destinations. This means BGP uses, instead of a distance-vector algorithm, a path-vector algorithm. Each communication of a reachable network provides considerable information about the entire sequence of routers to a destination. Due to this inclusion of topology information, path-vector protocols are sometimes described as a combination of distance-vector and link-state algorithms. This doesn't really do them justice, however, since they do not function in the same way as either of those algorithm types.

On The Web: If you are interested in additional general information about path-vector algorithms, you can find some in RFC 1322, A Unified Approach to Inter-Domain Routing. Warning: do not read before operating heavy machinery. J


The information about the path to each route is stored in the Routing Information Base (RIB) of each BGP speaker in the form of BGP path attributes. These attributes are used to advertise routes to networks when BGP devices send out Update messages. The storing, processing, sending and receiving of path attributes is “the” method by which routers decide how to create routes, so understanding them is obviously quite important.

There are several different path attributes, each of which describes a particular characteristic of a route. Attributes are divided into different categories based on their level of importance and specific rules designed to manage their propagation. The most important path attributes are called well-known attributes; every BGP speaker must recognize and process these, but only some are required to be sent with every route. Other attributes are optional and may or not be implemented. These are further differentiated based on how they are handled when received by a device that does not recognize them.


Previous Topic/Section
BGP Route Storage and Advertisement, and BGP Routing Information Bases (RIBs)
Previous Page
Pages in Current Topic/Section
1
23
Next Page
BGP Route Determination and the BGP Decision Process
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.