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The incremental cost of adding an ISP2 link to garlic and an ISP1 link to oregano (dashed lines in Figure 929) will probably be small, and doing so will protect against a simultaneous failure of a link to one router and a crash of the second router If you do this, try very hard to get diverse local loop facilities to the POPs of the two ISPs My client was in a suburban office park, which, as is increasingly common in new construction, was on a dual SONET ring that gave redundant communica-
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Figure 929 Let BGP sort it out
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tions to the two POPs If this had not been the case, and if the ILEC did not have the park homed to multiple central offices, I would next have looked into business-grade CATV as a backup, then into wireless connectivity to a separate CO
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In this example, my client connected to two large ISPs and had the router capacity to accept full routing A different approach, which is especially common when routers have limited memory, is to treat ISP1 as your primary carrier and ISP2 as a backup, but with the exception that you want to use ISP1 as the primary carrier to its direct customers (that is, in its address space) or to those ASs that are directly connected to ISP1
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accept: from AS1 default pref=2 from AS2 default pref=3 ^+ pref=1
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This policy will take the primary default from AS1 and use the AS2 default only if you lose default from AS1 Indeed, you don t even need to take any routing from AS1 other than the default
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It is far more difficult to influence the path inbound traffic takes to you than it is to control how traffic leaves your AS The fundamental issue here is economic: Service providers with which you have no economic relationship have
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no incentive to follow your preferences They will probably use hot potato routing that ignores your preferences
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You are more likely to be able to influence inbound traffic from adjacent ASs than from distant ones For most enterprise connections, most adjacent ASs will be service providers you are paying for transit Your money makes them listen to your preferences Enterprises with BGP connections to other enterprises may not directly pay one another, but the fact that they have the connection indicates they have some economic motivation for talking to one another It is reasonable to assume that the peering arrangement can be negotiated to create a win-win topology for both sides It s far less likely that tier 1 providers will care very much about the preferences of other tier 1 providers customers When there is a need for controlled quality of service between major providers, the best strategy is usually to contract with one provider for a VPN and make that provider responsible for the interprovider economic incentives to make other providers care about your QoS requirement
AS Path Prepending and Its Limits
I find it ironic that the most popular way to make a route less desirable for incoming traffic depends on a route preference factor that is not part of the BGP specification: AS path length AS path prepending is a technique in which you insert your AS number more than once into the AS path (Figure 930) In principle, the longer the AS path, the less desirable the route The basic limitation of AS path prepending is that you have no control over the update as it moves through multiple ASs By the time your updates arrive at the destination, the less preferred path may have a shorter AS path length than the more preferred path There have been periodic proposals to introduce a BGP attribute (destination path attribute) that conveys an absolute preference of the originator for which incoming path to take Unfortunately for its proponents, there are simply no economic motivations for its adoption Since you do not control the path your update takes beyond your directly connected ASs, as in Figure 930, your less desirable path actually is more desirable at the destination This will lead to asymmetric routing