Re: OSPF Down bit

Hi Bilal.
To see how it works you better have one of the CE connected to 2 PEs.

DN bit should be seen in show ip ospf database for particular LSA in
"Options" line as "Downward"

HTH
A.

On 5/4/2011 9:31 PM, Bilal Hansrod wrote:
> Thank you Dr.Fleming& Gary, I just wanted to understand the traffic flow
> and when is LSA is marked as DN. I read Cisco documentation and INE blog,
> but unable to get my head around.
>
> For example:
>
> CE_A1 {OSPF} PE1{iBGP} -> P -> {iBGP) PE2 {OSPF} CE_A2 (1.1.1.0/24)
>
> CE_A1 is connected to PE1 via OSPF
> PE1 is connected to PE2 via iBPG
> PE2 is connected to CE_A2 via OSPF
>
>
> So, if CE_A2 advertises 1.1.1.0/24 routes via OSPF to PE2. PE2 will
> redistribute OSPF into MBGP and advertises to iBGP neighbour (PE1). The DN
> bit will be set here after MBGP is redistributed into OSPF. Is this the
> place when Down bit is set, to avoid loop. If somehow CE1 leaks to any PE
> and if the down bit is enabled, than route will be discarded.
>
> I apologise if the above scenario is not very clear. I will also lab it up,
> but not sure if I can see Down bit via any show command or need to capture
> via wireshark.
>
> Thanks again guys-
>
> Bilal Hansrod
>
>
> On Wed, May 4, 2011 at 9:05 PM, garry baker<baker.garry_at_gmail.com> wrote:
>
>> 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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Received on Wed May 04 2011 - 22:19:40 ART