Using Private ASs

To conserve AS numbers, InterNIC generally does not assign a legal AS number to customers whose routing policies are an extension of the policies of their provider. Thus, in the situation where a customer is single-homed or multihomed to the same provider, the provider generally requests that the customer use an AS number taken from the private pool of ASs (64512-65535). As such, all BGP updates the provider receives from its customer contain private AS numbers.

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Troubleshooting:  
Example: Ch. 10, pp. 333-335. Using Private ASs

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Private AS numbers cannot be leaked to the Internet because they are not unique. For this reason, Cisco has implemented a feature to strip private AS numbers out of the AS_path list before the routes get propagated to the Internet. This is illustrated in figure 5-15.

Figure 5-15  Stripping private AS numbers.

In figure 5-15, AS1 is providing Internet connectivity to its customer AS65001. Because the customer has only this provider and no plans for having an additional provider in the near future, the customer has been allocated a private AS number. If the customer later needs to connect to another provider, a legal AS number should be assigned.

Prefixes originating from AS65001 have an AS_path of 65001. Note prefix 172.16.220.0/24 in figure 5-15 as it leaves AS65001. For AS1 to propagate the prefix to the Internet, it would have to strip the private AS number. When the prefix reaches the Internet, it would look like it has originated from the provider's AS. Note how prefix 172.16.220.0/24 has reached the NAP with AS_path 1.

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Notes:  
Chapter 1, "Evolution of the Internet," introduced the Network Access Points and their usage in interconnecting multiple providers. BGP connections to the NAP are usually done via a route server where multiple ASs peer via EBGP into a single system. The route server would have its own AS number. In figure 5-15, the NAP is represented by the route server RTE having AS number 7. Actually, the route server concept is not limited to the NAP; the NAP is a special case where the route server runs the RADB (Appendix A, "RIPE-181"). The route server concept would apply anytime multiple ASs rely on a single point for exchanging EBGP updates.

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BGP will strip private ASs only when propagating updates to the external peers. This means that the AS stripping would be configured on RTC as part of its neighbor connection to RTE.

Private ASs should only be connected to a single provider. If the AS_path contains a mixture of private and legal AS numbers, BGP will view this as an illegal design and will not strip the private AS numbers from the list, and the update will be treated as usual. Only AS_path lists that contain private AS numbers in the range 64512 to 65535 are stripped.

AS_Path and Route Aggregation Issues

Route aggregation involves summarizing ranges of routes into one or more aggregates or CIDR blocks to minimize the number of routes in the global routing tables. A drawback of route aggregation is the loss of granularity that existed in the specific routes that form the aggregate. The AS_path information that exists in multiple routes, for example, will be lost when these routes get summarized into one single advertisement. This would lead to potential routing loops because a route that has passed through an AS might be accepted by the same AS as a new route.

BGP defines another type of AS_path list called an AS-SET where the ASs are listed in an unordered set. The set includes all the ASs a route has traversed. Aggregates carrying the AS-SET information would have a collective set of the attributes that form the individual routes they summarize.

In figure 5-16, AS1 is advertising 192.213.1.0/24, and AS2 is advertising 192.213.2.0/24. AS3 is aggregating both routes into 192.213.0.0/16. An AS that advertises an aggregate considers itself the originator of that route, irrespective of where that route came from. When AS3 advertises the aggregate 192.213.0.0/16, the AS_path information would be just 3. This would cause a loss of information because the originators of the route AS1 and AS2 are no longer listed in the AS_path. In a situation where the aggregate is somehow advertised back to AS1 and AS2 by some other AS, AS1 and AS2 would accept the route that would potentially lead to routing loops.

Figure 5-16  Effects of the AS-SET.

With the notion of AS-SET, it is possible to have AS3 advertise the aggregate 192.213.0.0/16 while keeping information about the components of the aggregate. The set {1 2} indicates that the aggregate has come from both of these ASs in no particular order. The AS_path information of the aggregate with the AS-SET option would be 3 {1 2}.