Re: OSPF LSA TYPE 3 OVER BACKDOOR LINK

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/configura
tion/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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