Re: OSPF Down bit

Of course....what I just commented on is true in a certain configuration as
not to take away from Gary's response (which was true).

Assuming a route is introduced by one PE, the down bit will be set "UPWARD".
The route would traverse to a PE that is connected to 2 different CE routers
in 2 different areas. The route would be injected into both areas where the
down bit would be set to "DOWNWARD" as a result of redistribution. The route
is than reintroduced to the PE as an OSPF route with the down bit set but
would be ignored because the "routing bit" would not be set.

The PE would normally want to route the packet due to the lower AD of OSPF
instead of using the IBGP route. This is why the routing bit is not set when
the down bit is set. To help prevent loops as a result of this unique
problem.

Yes...it has happened!

Paul

-- 
Paul Negron
CCIE# 14856 CCSI# 22752
Senior Technical Instructor
www.micronicstraining.com
> From: garry baker <baker.garry_at_gmail.com>
> Reply-To: garry baker <baker.garry_at_gmail.com>
> Date: Wed, 4 May 2011 14:05:14 +0300
> To: Bilal Hansrod <bilal.hansrod_at_gmail.com>
> Cc: Cisco certification <ccielab_at_groupstudy.com>
> Subject: 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
>> 
>> 
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