Re: OSPF LSA TYPE 3 OVER BACKDOOR LINK

whao.......thanx for your time and the detailed explanation. the mind
bugling is over.

On Thu, Sep 26, 2013 at 4:52 PM, Brian Dennis <bdennis_at_ine.com> wrote:

> Before the backdoor link is up, R2 (PE) only learns of the 20.20.20.20/32from iBGP across the MPLS VPN but once the backdoor link comes up, R2 will
> then have the 20.20.20.20/32 learned via iBGP and OSPF. Since OSPF has a
> lower AD it will be the one installed in the RIB for the VRF. This means
> the iBGP route will not be in the RIB on R2 and in turn will not be
> redistributed into OSPF. As long as the backdoor link between R1 and XR2
> is up, R2 will not have the 20.20.20.20/32 in its RIB for the VRF learned
> via iBGP.
>
> If you want R2 to redistribute its iBGP route learned across the MPLS VPN
> you can filter off the 20.20.20.20/32 from being installed into the RIB
> via OSPF (distributed-list in) which will then mean the iBGP route will be
> installed. You could also change the AD as needed for R2 to install the
> 20.20.20.20/32 via iBGP.
>
> Lastly your output below of the summary LSAs for 20.20.20.20/32 confirms
> you are only learning one. The first summary LSA is learned from
> 20.20.20.20 (Advertising Router) and the second summary LSA is generating
> by R1 itself since it is an ABR.
>
> --
>
> Brian Dennis, CCIEx5 #2210 (R&S/ISP-Dial/Security/SP/Voice)
> bdennis_at_ine.com
>
> INE, Inc.
> http://www.INE.com
>
> ________________________________________
> From: Tolulope Adeniji [toluadeniji_at_gmail.com]
> Sent: Wednesday, September 25, 2013 1:53 AM
> To: Marko Milivojevic
> Cc: Brian Dennis; Cisco certification
> Subject: Re: OSPF LSA TYPE 3 OVER BACKDOOR LINK
>
> Ok Guys, see below brief logical diagram and the summary routes on the CE
> router
>
> lo0--(Area 1)---->R1 (CE1) ----(Area 0)---->XR2 (CE2)-----(Area 120)--->lo0
> I I
> (Area 0) (Area 0)
> I I
> R2 (PE1)-----(MPLS)----->XR1 (PE2)
>
> Note : OSPF COST 100 APPLIED ACROSS THE BACKDOOR LINK
>
>
>
> R1#sh ip ospf database summ 20.20.20.20
>
> OSPF Router with ID (10.1.1.1) (Process ID 100)
>
> Summary Net Link States (Area 0)
>
> Routing Bit Set on this LSA in topology Base with MTID 0
> LS age: 391
> Options: (No TOS-capability, DC, Upward)
> LS Type: Summary Links(Network)
> Link State ID: 20.20.20.20 (summary Network Number)
> Advertising Router: 20.20.20.20
> LS Seq Number: 80000001
> Checksum: 0x9DFD
> Length: 28
> Network Mask: /32
> MTID: 0 Metric: 1
>
>
> Summary Net Link States (Area 1)
>
> LS age: 375
> Options: (No TOS-capability, DC, Upward)
> LS Type: Summary Links(Network)
> Link State ID: 20.20.20.20 (summary Network Number)
> Advertising Router: 10.1.1.1
> LS Seq Number: 80000002
> Checksum: 0x7207
> Length: 28
> Network Mask: /32
> MTID: 0 Metric: 101
>
>
>
>
>
> On Wed, Sep 25, 2013 at 12:38 AM, Marko Milivojevic <markom_at_ipexpert.com
> <mailto:markom_at_ipexpert.com>> wrote:
> Output :-)
>
>
> On Tue, Sep 24, 2013 at 3:45 PM, Tolulope Adeniji <toluadeniji_at_gmail.com
> <mailto:toluadeniji_at_gmail.com>> wrote:
> Thanks Brian. Quite aware of the down bit but this is not a VRF lite CE. I
> was actually working on INE SP lab 4.12 (MPLS L3 VPN and OSPF Domain-ID)
> but modified the backdoor link area to be in AREA 0 while every other thing
> remain same.
>
> Also i don't think Sham link would help out here b'cos the original route
> is advertised into OSPF as OIA routes (i.e the loopback 0 on R1 and XR2).
>
> Would appreciate further inputs on this.
>
>
> On Tue, Sep 24, 2013 at 9:12 PM, Brian Dennis <bdennis_at_ine.com<mailto:
> bdennis_at_ine.com>> wrote:
>
> > If the CE's OSPF process is within a VRF then it will check for the down
> > bit on type 3 LSAs as a loop prevention mechanism (see the first link
> below
> > for more detail). If the PE is setting the down bit (this happens when
> > MBGP is redistributed into the PE-CE OSPF process for type 3 LSAs the PE
> > generates) then the CE will not use the type 3 LSA that has the down bit
> > set by default. If you want the CE to ignore the down bit and use the
> type
> > 3 LSA from the PE that has the down bit set, then the capability vrf-lite
> > command under the OSPF VRF process on the CE should be enabled.
> >
> > A SHAM link will also resolve the issue. With a SHAM link from PE to PE
> > across the MPLS VPN the down bit will not be set by the PEs any longer
> for
> > the type 3 LSAs since the loop prevention mechanisms of the down bit and
> > domain tag are not needed any longer when a SHAM link is used.
> >
> > Inter-Area Loop Prevention in OSPF
> > http://ine.co/0yauq
> >
> > Configuring VRF-lite - Cisco
> >
> >
> http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/25ew/configuration/guide/vrf.html
> >
> > --
> > Brian Dennis, CCIEx5 #2210 (R&S/ISP-Dial/Security/SP/Voice)
> bdennis_at_ine.com<mailto:bdennis_at_ine.com>
> >
> > INE, Inc. http://www.INE.com
> >
> >
> >
> > On 09/24/2013 08:00 AM, Tolulope Adeniji wrote:
> >
> > Hello
> >
> > Why would a CE prefer a OSPF type 3 lsa learnt over backdoor link to same
> > type 3 lsa advertised by connecting PE despite the higher metric across
> the
> > backdoor link.
> >
> > Regards
> >
> >
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> >
> >
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Received on Thu Sep 26 2013 - 17:23:36 ART