Large enterprises have multiple site offices spanning geographical locations and are connected via MPLS service providers. As a standard practice, all the location office connect via BGP using the same AS Number assigned to customer (Site-A and Site-B). As a standard behaviour of BGP remote site (using same AS Number) prefix received by other site of same customer will drop the route since it sees its own AS Number in the received packet. Below is sample scenario to illustrate BGP default behaviour on receiving route having its own AS number in its AS Path.
As shown above, Router “CE-A” at site A advertises network 220.127.116.11/32. Service provider receives the route having AS path as “200 I” . Now “PE2” Router advertised the same route towards Site-B Router “CE-B”. However on receiving the BGP route, it verifies the AS path attribute and finds its own AS Number of “200”. As a standard behavior of BGP, Router CE-B drops the route.
The methodology to circumvent or mitigate default behavior of BGP is to use the BGP features of “AS-Override” or else “Allowas-in”. While former is used by PE to modify the AS Number in AS Path so that prefix is not dropped, latter is implemented in CE device to introduce an exception in BGP AS path loop prevention mechanism.
More information on “As-Override” is available in this Link.
In a few days , “Allowas-in” related link will be made available.
Let’s further discuss on how “AS-Override” and “Allowas-in” differ from each other.One significant difference being that “As Override” needs to be provisioned at the Service provider end while “Allowas-in” will be configured at the CE device at customer site. If customer requires to keep minimal configuration at CE side and let provider perform the BGP routing control, best approach will be to use “As-Override“.
Below table enumerates the comparison between “As-Override” and “Allowas-in” –
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