Phil,

Thank you for your reply.

Regards,
Jane D. Jiang

Philip Chen <pchenENNOVATENETWORKS.COM>DISCUSS.MICROSOFT.COM> on
05/07/2001 12:16:43 PM

Please respond to Mailing List <OSPFDISCUSS.MICROSOFT.COM>

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To: OSPFDISCUSS.MICROSOFT.COM
cc:
Subject: Re: # of IGPs in an AS

Jane,

  I do not think RFC2328's "AS" maps to "Administrative Domain--AD" concept
strictly on an one-to-one basis (--like BGP does). In each AD, there could
be
multiple "Routing Domains--RD". Each RD runs one IGP. OSPF AS is more like
a
RD in this sense. There is no restriction on how many RDs can have within a
AD
regardless if a given AD is private (virtual or not virtual) or public.
That
is to say, regardless a give AD represents a VPN or a carrier backbone
(could
itself be a private network), one can always run OSPF over one set of
networks
and run RIP over another set of networks. Some of the routers in a AD may
participate in multiple RDs. Those routers are the RD boundary routers
(RDBR)--"ASBR" in OSPF term. The ASBRs are responsible for leak routes from
one RD to another RD, so all networks can be reached from any where within
the
AD. It is perfect for a VPN customer to have one site runs RIP and another
site runs OSPF between CE and PE routers. In between PEs, it may run OSPF
or
RIP (virtual router appraoch) or BGP (RFC2475 overlay approach). In this
case,
PEs will act as RDBR.

--Phil

"Jane D. Jiang" wrote:

> Hi All,
>
> On page of RFC 2328, it states that:
> "Interior Gateway Protocol
> The routing protocol spoken by the routers belonging to an
> Autonomous system. Abbreviated as IGP. Each Autonomous
> System has a single IGP. Separate Autonomous Systems may be
> running different IGPs."
>
> When it says that "Each Autonomous System has a single IGP.", does it
mean
> that for each VPN, we are limited to one IGP (say, OSPF or RIP)? Or,
> should the selection of an IGP routing protocol be specified at the VPN
> level or at the interface level of of a VPN?
>
> Thanks a lot.
>
> Jane D. Jiang

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