The Community Approach
The use of the community approach is very effective. Providers want to map certaincommunity values to corresponding local preference values (that is, RFC 1998). The providerwill automatically assign customers routing updates tagged with a specific community to acorresponding local preference value.To keep this scenario manageable, only routing and policy setting from ISP1's point of vieware addressed. An identical discussion would apply to ISP2. Traffic flow for the caseillustrated in Figure 7-21 can be divided into a minimum of three patterns.
Figure 7-21. Community Approach Solution
There can be more flow patterns, depending on how many connections a customer has to itsprovider, but the basic set of three illustrates required considerations.Flow patterns from ISP1's point of view can be summarized as follows:
• Pattern 1—
Routes originated by the customer AS1, or customer local routes.
• Pattern 2—
Routes transiting via AS1. These routes come from AS2 and consist of AS2's routesand all other routes that AS2 is receiving from ISP2. ISP1 uses this information toreach AS2 via AS1 as a backup in the event that AS2's link to ISP2 fails. This patternis referred to as customer transit routes.
• Pattern 3—
All other routes coming from ISP2, or ISP routes. These can include routes learnedfrom AS2.
Having divided the routes into different categories, ISP1 will assign a community value toeach pattern and will dynamically map it to the local preference. These are listed in Table 7-5.
ISP1 will inform all its customers and connected ISPs that its local preference values aredynamically set according to Table 7-5. Customers can then dynamically influence the ISP'sdecision by sending the corresponding community values. In Figure 7-21, AS1 will send itslocal routes with no community and the transit routes with community 400:40. ISP2 will sendits routes with community 400:60.According to the preferences summarized in Table 7-5, ISP1 prefers AS1's local routes via itsdirect link to AS1 (preference 100 is the highest). ISP1 prefers all other routes, including AS2routes, via ISP2 (preference 60 is higher than 40.)
Figure 7-21. Community Approach Solution
There can be more flow patterns, depending on how many connections a customer has to itsprovider, but the basic set of three illustrates required considerations.Flow patterns from ISP1's point of view can be summarized as follows:
• Pattern 1—
Routes originated by the customer AS1, or customer local routes.
• Pattern 2—
Routes transiting via AS1. These routes come from AS2 and consist of AS2's routesand all other routes that AS2 is receiving from ISP2. ISP1 uses this information toreach AS2 via AS1 as a backup in the event that AS2's link to ISP2 fails. This patternis referred to as customer transit routes.
• Pattern 3—
All other routes coming from ISP2, or ISP routes. These can include routes learnedfrom AS2.
Having divided the routes into different categories, ISP1 will assign a community value toeach pattern and will dynamically map it to the local preference. These are listed in Table 7-5.
ISP1 will inform all its customers and connected ISPs that its local preference values aredynamically set according to Table 7-5. Customers can then dynamically influence the ISP'sdecision by sending the corresponding community values. In Figure 7-21, AS1 will send itslocal routes with no community and the transit routes with community 400:40. ISP2 will sendits routes with community 400:60.According to the preferences summarized in Table 7-5, ISP1 prefers AS1's local routes via itsdirect link to AS1 (preference 100 is the highest). ISP1 prefers all other routes, including AS2routes, via ISP2 (preference 60 is higher than 40.)
