RE: OSPF LSA TYPE 3 OVER BACKDOOR LINK

Before the backdoor link is up, R2 (PE) only learns of the 20.20.20.20/32 from 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
>
>
> Blogs and organic groups at http://www.ccie.net
>
>
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