28.06.2014 Views

Discussion

Discussion

Discussion

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Chapter<br />

CHAPTER<br />

13<br />

13<br />

BGP 13<br />

13.0 Introduction<br />

The IGPs OSPF, IS-IS, and RIP maintain the mapping for the topology within a single<br />

administrative domain or AS, along with the set of best paths between systems<br />

within the domain. Each AS uses one or more common IGPs and common metrics to<br />

determine how to route packets within the AS. The administration of an AS appears<br />

to other ASs to have a single coherent interior routing scheme and presents a consistent<br />

picture of what destinations are reachable through it.<br />

To handle inter-AS routing, IGPs use an EGP. EGPs keep track of how routing<br />

domains are connected to each other and the sequence of domains that must be traversed<br />

to reach a particular destination. Although a number of EGPs were developed<br />

in the late 1980s, the Border Gateway Protocol (BGP) is the only one currently being<br />

used on IP networks and the Internet. Version 1 of BGP was introduced in 1989, and<br />

the current iteration, Version 4, is defined in RFC 1771 and has been in use since<br />

1995. A number of additional RFCs define extensions to the base BGP protocol (see<br />

http://www.bgp4.as/rfc).<br />

BGP is the routing protocol that holds the Internet together, providing the mesh-like<br />

connectivity of Internet service provider (ISP) networks that forms what we call the<br />

Internet. ISPs use BGP to connect to each other, forming the virtual backbone of the<br />

Internet. Large enterprises also sometimes use BGP to connect to their ISPs, as well<br />

as to connect portions of their internal corporate network.<br />

BGP uses a path vector algorithm to determine network topology and paths to destinations.<br />

This algorithm defines a route as a pairing between a destination and the<br />

attributes of the path to that destination. It considers multiple attributes of the path<br />

in order to choose the best route to the destination. In comparison, a distance-vector<br />

protocol uses a single distance metric to choose the best route. BGP routing updates<br />

carry path information, which is a full list of the transit ASs that must be traversed<br />

between the AS receiving the update and the AS that can deliver the packet using its<br />

IGP. BGP uses this list to eliminate loops in the path because a router can check the<br />

418<br />

This is the Title of the Book, eMatter Edition<br />

Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!