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Examples: Configuring a Local AS

You can include the local-as statement to configure a router to use a different AS number than the one for which the router is configured. The local AS is used in all BGP protocol exchanges with the routers that are configured for simulating a virtual AS.

Note: If you configure the local AS values for any BGP group, the detection of routing loops is performed using both the AS and the local AS values for all BGP groups.

Use the local-as statement when ISPs merge and want to preserve a customer’s configuration, particularly the AS the customer is configured to peer with. Use the local-as statement to simulate the AS number already in place in customer routers, even if the ISP’s router is in a different AS now.

Figure 10: Local AS Configuration

Image g017007.gif

In Figure 10, Router 1 and Router 2 are in AS 65500, Router 4 is in AS 64513, and Router 3 is in AS 64512. Router 2 uses AS 65001 as its local AS.

Router 2 adds AS 65001 when announcing Router 1’s routes to Router 3. Router 3 sees an AS path of 65001 65500 64512 for the prefix 10/8. To prevent Router 2 from adding the virtual AS number in its announcements to other peers, use the local-as autonomous-system private statement. The local-as autonomous-system private statement configures Router 2 to not include the virtual AS number configured in local-as when announcing Router 1’s routes to Router 3. In this case, Router 3 sees an AS path of 65500 64512 for the prefix 10/8.

The configuration for each router follows.

On Router 1:
routing-options {
autonomous-system 65500;
}
protocols {
bgp {
group internal-AS65500 {
type internal;
local-address 10.1.1.1;
neighbor 10.1.1.2;
}
}
}
On Router 2:
routing-options {
autonomous-system 65500;
}
protocols {
bgp {
group internal-AS65500 {
type internal;
local-address 10.1.1.2;
neighbor 10.1.1.1;
}
group external-AS64513 {
type external;
peer-as 64513;
neighbor 192.168.1.2;
}
group external-AS64512 {
type external;
peer-as 64512;
neighbor 192.168.10.2;
}
}
}
On Router 3:
routing-options {
autonomous-system 64512;
}
protocols {
bgp {
group external-AS65001 {
type external;
peer-as 65001;
neighbor 192.168.10.1;
}
}
}
On Router 4:
routing-options {
autonomous-system 64513;
}
protocols {
bgp {
group external-65500 {
peer-as 65500;
neighbor 192.168.1.1;
}
}
}
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