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Figure 91 EGP model
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Basic BGP and the Customer Side of Exterior Routing
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Figure 92 1987-vintage BGP
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VPNs, are being implemented with BGP due to its flexibility, but many of these requirements go against the grain of the fundamental architectural assumptions of BGP There is a growing consensus that we can continue to tune BGP and related operational practices for 5 to 7 years, but eventually we will need to look at new paradigms for exterior routing [Doria 2002, Huston 2001a] Scaling BGP is a constant battle of trade-offs between such things as stability and fast convergence, more fine-grained control and more resources required for routing, and so on We do not truly know what governs BGP convergence at the level of the Internet as a whole, at the level of an individual AS, or at the level of individual routers There is considerable research on these issues, especially the first [Labovits] and third [Berkowitz 2001a, d, e] The current BGP specification [RFC 1771] is badly out of date with respect to current operational practice, and is under active revision [Rekhter 2001b] With luck, a new BGP RFC may be available at, or soon after, the publication of this book We are monitoring growth patterns in the number of prefixes (Figure 93) and AS numbers (Figure 94) in the Internet The growth in prefixes seems due to multihoming and traffic engineering The growth in AS numbers, if broken into end versus transit ASs, suggests that end user multihoming is driving the increase It is uncertain whether this is the true cause, and experimentation is under way to obtain better information [Berkowitz 2001e] Today s Internet topology is, to put it gently, chaotic (Figure 95) It is flattening with respect to hierarchy, which jeopardizes the scalability assumption of prefix aggregation
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140,000 AS 1221 AS 286 AS 6447
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120,000 Active BGP entries (FIB)
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80,000
60,000
40,000 20,000
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Date
Figure 93 Prefix growth
BGP, iBGP, and eBGP
There is only one BGP, although you will see the terms internal BGP (iBGP) and external BGP (eBGP) used extensively The same protocol is used in both, but iBGP runs between BGP speakers in your AS, while eBGP connects your AS to an AS with a different AS number (see Figure 96) As discussed in 4, routing policies control the information that BGP advertises and accepts Think of them in these terms Any routing update that includes a reachable route, is, in the words of noted routing engineer Avi Friedman, a promise to carry traffic [to that route] In Figure 97, AS1 promises AS2 that it will carry traffic to blocks A and C AS1, however, only offers block A traffic to AS3 and AS4, and does not offer connectivity to block B to any outside AS
Figure 94 AS growth
Basic BGP and the Customer Side of Exterior Routing
Tier 1 Provider
Tier 1 Provider
Tier 1 Provider
Regional Provider
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User AS
User AS
User AS
User AS
User AS
User AS
Figure 95 2001-vintage Internet
eBGP Speaker eBGP Speaker
IGP IGP eBGP Speaker eBGP Speaker
eBGP Speaker eBGP Speaker
Importing (extreme care)
Our AS
eBGP iBGP IGP
Figure 96 eBGP and iBGP
9
Announces A
Announces A, C
Announces A (B in Future)
Prefix Block A
Prefix Block B
Prefix Block C
Figure 97 Don t ask, don t tell: BGP routing policies
The simple way you indicate to other ASs that you do not want to transport traffic to some specific destination is not to tell them about it You can be selective in telling or not telling different ASs different things Advertising policies specify what promises you will make Acceptance policies specify promises from others to which you will agree Don t confuse BGP policy with BGP protocol flow Figure 98 shows several ways in which policy information flows over different media between pairs of AS You can specify routing policy down to the router interface level, but it is not required The three configurations in Figure 98, while using different physical connectivity, would use the same routing policy In general, policy is defined between ASs, not IP addresses A single physical connection to a single ISP, as shown in Figure 99, is certainly one you may create in the lab, but is rarely needed in actual practice It would be quite unusual to justify running BGP to an ISP if you have a single connection to it Some ISPs may expect the customer site to run BGP simply as a keepalive so they know that the site is up In general, however, ISPs prefer to have customers avoid running BGP unless it is actually necessary In fact, ISPs may control a BGP router at the customer s premises, because errors in BGP can have wide-ranging effects on the entire Internet