Note

Routing loops occur when two or more routers select the best path back to the other. This causes packets to ping pong back and forth until the packet's TTL times out and drops the packet.

BGP attributes such as Local Preference allow the ISP to control the flow of traffic between its network and other providers' networks. It does not allow them to effectively influence the best-path decisions of other networks. So, in one common form of asymmetrical routing, each ISP makes its own best-path decision. At this time, the tools for one ISP to affect the best-path decisions of another ISP are coarse and are not guaranteed to work. [16] Because each router, and perhaps each ISP, is making its own best-path decision and little can be done to effectively influence this on an Internet-wide scale, asymmetric traffic flow is the result. This affects where uRPF can be applied in the network.

When a router receives a packet from another router, it does not care which interface it came in on. The router examines the destination address, looks up the route in the FIB, and forwards the packet out another interface. With uRPF turned on, the router does need to care which interface the packet was received from. To make this work, the RIB needs to provide information to the FIB to allow uRPF to work. Working through an example is the best way to demonstrate how this applies. Figure 4-26 shows a multihomed custome r using a combination of split advertisement and advanced communities to traffic-engineer the network. This is an effective multihoming technique that provides good traffic engineering control to the customer while keeping the ISP's operational overhead for supporting the multihomed customer to a minimum. The RIB on Router A receives the advertisement from Router C for the two prefixes. The route maps used for RFC 1998-style community usage set the local preference for these prefixes in the ISP's network. T171 In this way, the local preference values are use to determine which of the multiple copies of the route—say, 169.21.0.0/17—is sent to the FIB.

Figure 4-26. BGP Local Preference Affects Traffic Flow with Multihomed

Customers

Figure 4-26. BGP Local Preference Affects Traffic Flow with Multihomed

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