Pros and cons of BGP AS Over-ride

Unanswered Question
Giuseppe Larosa Thu, 03/18/2010 - 05:53

Hello Mohsin,

I see more advantages then disavantages in using as-override

The big advantage is that using it all CE node of a customer can use the same private BGP AS number, that is the BGP AS number can be associated to a customer and this is handy in troubleshooting and maintanance of the SP network.

Among possible drawbacks the interaction with AS path prepending done on customer side should be tested.

As override changes the private AS in the service provider AS, a remote CE sees a BGP route coming from another CE with an AS path

SP-ASN SP ASN

instead of

SP-ASN CE-ASN

this allows for use of same AS number in all sites without using neigh allowas-in on CE side

I would be interesting to check what happens if CE attempts to perform AS path prepending:

each CE ASN occurrence should be changed in a SP ASN in the AS path attribute

Hope to help

Giuseppe

ma4d Thu, 03/01/2012 - 23:28

As a customer of an ISP that uses as-override, we use prepend in production and it works very well as a way to make certain paths less desirable.  Giuseppe is correct in that each CE ASN occurrence in the as-path is changed to the SP ASN.

All of our sites use the same ASN, say 65001.  If the ISP uses ASN 99, then one site would see the path to another site as 99 99.  If we prepend 65001 on a particular path to make it less desirable, then another site would see that path as 99 99 99.

cadet alain Fri, 03/02/2012 - 01:46

Hi,

concerning the AS_PATH prepending it  could be a problem if you were using allowas-in feature but I don't think it is with as-override feature.

Regards.

Alain

milan.kulik Fri, 03/02/2012 - 01:54

Hi Giuseppe,

yes, it's easier from a provider point of view to use a single AS for all sites of one customer.

But from the customer point of view?

You never know which site has a particular prefix been originated from :-(

I'm currently redesigning an entrerprise network peering to several providers.

Some of them are using the customer single AS number concept with as-override, some of them don't.

(One is even using public AS numbers owned by somebody else in the backbone.)

And there is also local-as used on some sites.

So a total mess :-(

So IMHO, the provider should let the customer to chose if he wants to use a single or multiple AS numbers.

And agree on an AS numbering concept.

BR,

Milan

Peter Paluch Fri, 03/02/2012 - 03:22

Hi Giuseppe,

each CE ASN occurrence should be changed in a SP ASN in the AS path attribute

Hmmm - does the as-override indeed rewrite each CE ASN in the AS_PATH, even if interspersed with different AS numbers, or will it override only the head of the AS_PATH?

In other words, assuming that the CE ASN is 65000, how would the as-override rewrite this AS_PATH upon advertising the prefix to another CE, assuming that the PE ASN is 123?

123,65000,100,65000,65000

Thank you!

Best regards,

Peter

milan.kulik Fri, 03/02/2012 - 03:38

Hi Peter,

In other words, assuming that the CE ASN is 65000, how would the as-override rewrite this AS_PATH upon advertising the prefix to another CE, assuming that the PE ASN is 123?

123,65000,100,65000,65000

the result will be

  123,123,100,123,123

BR,

Milan

Peter Paluch Fri, 03/02/2012 - 08:08

Hello Milan,

So all occurences of the CE AS are replaced, regardless of their position in the AS_PATH. Thank you!

Best regards,

Peter

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