Re: OSPF Down bit

http://www.ietf.org/rfc/rfc4577.txt

OSPF as the Provider/Customer Edge Protocol for BGP/MPLS IP Virtual Private
Networks (VPNs)
4.1.5. Prevention of Loops

   If a route sent from a PE router to a CE router could then be
   received by another PE router from one of its own CE routers, it
   would be possible for routing loops to occur. To prevent this, a PE
   sets the DN bit [OSPF-DN] in any LSA that it sends to a CE, and a PE
   ignores any LSA received from a CE that already has the DN bit sent.
   Older implementations may use an OSPF Route Tag instead of the DN
   bit, in some cases. See Sections 4.2.5.1 and 4.2.5.2

4.2.5. Loop Prevention

4.2.5.1. The DN Bit

   When a type 3 LSA is sent from a PE router to a CE router, the DN bit
   [OSPF-DN] in the LSA Options field MUST be set. This is used to
   ensure that if any CE router sends this type 3 LSA to a PE router,
   the PE router will not redistribute it further.

   When a PE router needs to distribute to a CE router a route that
   comes from a site outside the latter's OSPF domain, the PE router
   presents itself as an ASBR (Autonomous System Border Router), and
   distributes the route in a type 5 LSA. The DN bit [OSPF-DN] MUST be
   set in these LSAs to ensure that they will be ignored by any other PE
   routers that receive them.

   There are deployed implementations that do not set the DN bit, but
   instead use OSPF route tagging to ensure that a type 5 LSA generated
   by a PE router will be ignored by any other PE router that may
   receive it. A special OSPF route tag, which we will call the VPN
   Route Tag (see Section 4.2.5.2), is used for this purpose. To ensure
   backward compatibility, all implementations adhering to this
   specification MUST by default support the VPN Route Tag procedures
   specified in Sections 4.2.5.2, 4.2.8.1, and 4.2.8.2. When it is no
   longer necessary to use the VPN Route Tag in a particular deployment,
   its use (both sending and receiving) may be disabled by
   configuration.

 --
Garry L. Baker

"With sufficient thrust, pigs fly just fine..." - RFC 1925

On Wed, May 4, 2011 at 4:12 AM, Bilal Hansrod <bilal.hansrod_at_gmail.com>wrote:

> Hello Everyone,
>
> Can anyone kind enough to clarify my theoretical doubts about OSPF Down
> bit.
> This is what I believe; please correct me if I am wrong
> CE1 advertise OSPF routes to PE1
>
> PE1 redistributes OSPF routes into MBGP
>
> PE2 redistributes MBPG routes into OSPF and set Down bit and advertises to
> CE2
>
> CE2 advertises back same route to PE1 via additional connection and it is
> discarded as Down bit is set to prevent loop.
>
>
> The Down bit can only be set for LSA 2,3,7. LSA 5 doesn t support Down
> bit,
> so we use route tag to perform the same operation.
>
> Thanks,
>
> Bilal
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html

Blogs and organic groups at http://www.ccie.net
Received on Wed May 04 2011 - 14:05:14 ART