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>wrote:

> Output :-)
>
>
> On Tue, Sep 24, 2013 at 3:45 PM, Tolulope Adeniji <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> 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
>> >
>> > 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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Received on Wed Sep 25 2013 - 09:53:55 ART