I found the solution to this problem ! When we have a sham-link inside the same AS, there's no issues with routing loops with the sham-links because the external LSA will have the same route-tag.
When we have a sham-link between two AS's, and the sham-links are advertised by eBGP, there's no problem because the eBGP AD is lower than OSPF. But when we have a sham-link between two AS's and the sham-links are advertised by iBGP, there's a routing loop. The routers will prefer the OSPF learned route instead of the iBGP because of lower AD. So the solution is to use the same "domain-tag" under the OSPF process in both PE's. Regards, Antonio Soares, CCIE #18473 (R&S) [email protected] -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Antonio Soares Sent: segunda-feira, 8 de Junho de 2009 19:08 To: [email protected] Subject: [OSL | CCIE_SP] VOL2 - Section 1 - Task 8.2 My sham-link is flaping. As soon as the sham-link comes up, R1 and R8 start prefering the OSPF route instead of the iBGP route. I never saw this problem in regular MPLS VPNs inside one AS. In this because we have an Inter-AS MPLS VPN scenario ? +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ R1# 00:03:31: %OSPF-5-ADJCHG: Process 39, Nbr 8.8.8.8 on OSPF_SL0 from LOADING to FULL, Loading Done R1# R1#sh ip route vrf VPNB | inc 8.8.8.8 B 8.8.8.8 [200/0] via 6.7.8.8, 00:00:14 R1# R1# 00:03:47: %OSPF-5-ADJCHG: Process 39, Nbr 8.8.8.8 on OSPF_SL0 from FULL to DOWN, Neighbor Down: Interface down or detached R1# R1#sh ip route vrf VPNB | inc 8.8.8.8 O E2 8.8.8.8 [110/1] via 6.7.8.8, 00:00:00 R1# +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ R8# 00:06:28: %OSPF-5-ADJCHG: Process 39, Nbr 1.1.1.1 on OSPF_SL0 from LOADING to FULL, Loading Done R8# R8#sh ip route vrf VPNB | inc 1.1.1.1 B 1.1.1.1 [200/0] via 125.125.125.1, 00:00:13 R8# R8# 00:06:44: %OSPF-5-ADJCHG: Process 39, Nbr 1.1.1.1 on OSPF_SL0 from FULL to DOWN, Neighbor Down: Interface down or detached R8# R8#sh ip route vrf VPNB | inc 1.1.1.1 O E2 1.1.1.1 [110/1] via 125.125.125.1, 00:00:02 R8# +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ After a few hours trying to understand why this was happening, i was able to make it work tweaking the OSPF AD for the External routes in R1 and R8. Anyone saw this problem in this lab ? And why in the PG we don't see the sham-link interfaces in R3 and R9 ? Regards, Antonio Soares, CCIE #18473 (R&S) [email protected]
