Working Example of uRPF Multihomed Customers and Asymmetrical Routing

In this example, depicted in Figure 4-28, the enterprise customer of the ISP is multihomed into two different routers. BGP is used with a private ASN assigned by the ISP. The enterprise's IP address block would be allocated from the ISP or from an IP registry. As the route is advertised into the ISP's routers, an internal BGP weight is applied. This ensures that if there is a tie between two identical prefixes, the one directly from Router C will be preferred on the local router and entered into the FIB.

Figure 4-28. Multihomed Leased-Line Customer and uRPF

Figure 4-28. Multihomed Leased-Line Customer and uRPF

In Example 4-5, the customer divides its allocated /16 address block into two /17s, allowing for some level of traffic engineering. The customer advertises the /16 and two /17s out both links (required to have uRPF work on the ISP's ingress). Preference is communicated to the ISP through BGP communities. The ISP uses the communities advertised from the customer to set the BGP local preference of the /17, making one preferred over the other. T191 Both /17s have the BGP community no-export to ensure that they stay inside their upstream ISPs.

Some basic restrictions govern applying uRPF to these multihomed customers:

• Customers should not be multihomed to the same router. This is common sense—it breaks the purpose of building a redundant service for the customer. If the same router is used, the circuits should be configured for parallel paths (using something along the lines of CEF load balancing). uRPF works with parallel path circuits between two routers.

• Customers need to ensure that the packets flowing up the link (out to the Internet) match the prefixes advertised out the link. Otherwise, uRPF will filter those packets. Advertising the same routes out both links is the best way to make sure that this will happen.

• The traffic-engineering trick of splitting the IP address space in half, with each half advertising one link, cannot be used. For example, the enterprise customer cannot advertise 169.23.0.0/17 and 169.23.0.0/16 out one upstream link and 169.23.128.0/17 and 169.23.0.0/16 out the other link. This would break uRPF—unless an ACL is applied to uRPF to allow for this case. The recommended technique is highlighted in Example 4-5 using BGP communities to set local preference. T201

Continue reading here: Example 45 Multihomed uRPF

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