[j-nsp] Enhancement Request: detect as-path prepending

[Jeff S Wheeler]

I would like the ability to detect as-path prepending in routing policy,
and act on it by reducing my preference on those routes.  I believe this
would cause my policy to more closely follow the intent of the networks
announcing routes with as-path prepends.

I have discussed an enhancement request with a few friends, but I have
not noticed any suggestion to Juniper like this on "the Google."  Other
opinions would be welcome.

http://juniper.cluepon.net/index.php/ER_Detect_AS-PATH_prepends

I envision the feature would work something like this:
policy-options {
    as-path PREPEND1 ".* (.) $1 .*";
    as-path PREPEND2 ".* (.) $1 $1 .*";
    policy-statement TRANSIT-IMPORT {
        /* subtract 5 from local-preference if AS-PATH prepending is seen */
        term DETECT-PREPEND1 {
            from as-path PREPEND1;
            then local-preference subtract 5;
        }
        /* subtract 5 more if the same ASN is seen three times */
        term DETECT-PREPEND2 {
            from as-path PREPEND2;
            then local-preference subtract 5;
        }
    }
}

The regexp engine doesn't need to do additional work if backreferencing
is not used in a given as-path, which is known when the configuration is
committed.  Or a different statement than as-path could be created.

I also wish we had the ability to delete seen prepends, but if that were
possible and transit ASes started doing it to attract more customer
traffic, it would pretty much defeat the route originator's intent.  The
potential for loops would also seem to exist for some configurations.

-- 
Jeff S Wheeler <jsw at inconcepts.biz> +1-212-981-0607
Sr Network Operator  /  Innovative Network Concepts