TAILIEUCHUNG - The Illustrated Network- P43

The Illustrated Network- P43:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 15 Border Gateway Protocol 389 But more ASs generate more and longer path information. RFC 1774 in 1995 estimated that 100 000 routes generated by 3000 ASs would have paths about 20 ASs long. There was a concern about router memory and processor requirements to store and maintain all of this information especially in smaller routers. Several mechanisms are built into BGP to address this. ISPs would not usually accept a BGP route advertisement with a mask more than 19 bits long 19 . This was called the universally reachable address level. The price for compact routing tables and maintenance was a loss of routing accuracy and many ISPs relaxed this policy. Most today accept 24 prefixes although they can accept more specific addresses from their own customers of course . The other BGP mechanisms to cut down on routing table size and maintenance complexity are route reflectors confederations also called subconfederations and route damping or dampening . All of these are beyond the scope of this chapter but should be mentioned. IBPG AND EBGP BGP is an EGP that runs between individual routing domains or ASs. When BGP speakers the term for routers configured to peer with BGP neighbors are in different ASs the routers use an exterior BGP EBPG session to exchange information. When BGP peers are within the same AS the routers use interior BGP IBGP . These terms often appear as E-BPG I-BGP or eBGP iBGP IBGP is not some IGP version of BGP. It is used to allow BGP routers to exchange BGP routing information inside the same AS. IBGP sessions are usually only required when an AS is multihomed or has multiple links to other ASs. However we used them on the Illustrated Network anyway and that s fine too. An AS with only a single link to one other AS need only run EBGP on the border router and relies on the IGP to distribute routes learned by EBPG to the other routers. In the case where there is only one exit point for the entire AS a single static default route to the .

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