Gday Antonio,
I've tried to re-create the issue you described below with no luck:
10.10.18.1/32 is the R1 loopback interface for the Sham
10.10.18.8/32 is the R8 loopback interface for the Sham
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
R1#sh ip ospf neighbor 98.9.8.8 | inc Neighbor|interface
Neighbor 98.9.8.8, interface address 10.10.18.8
In the area 0 via interface OSPF_SL0
Neighbor priority is 0, State is FULL, 6 state changes
Neighbor is up for 00:21:00
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
R1#sh ip ospf database | beg Type-5
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
4.0.0.4 31.3.1.1 1325 0x80000001 0x009E9A 3489661053
10.10.18.1 98.9.8.8 1282 0x80000001 0x004D4A 3489661606
10.10.18.8 31.3.1.1 1279 0x80000001 0x00E82A 3489661053
11.0.0.11 31.3.1.1 1279 0x80000001 0x00FC2E 3489661053
54.5.4.0 31.3.1.1 1325 0x80000001 0x00D130 3489661053
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
R1#sh ip route vrf VPNB 10.10.18.8
Routing entry for 10.10.18.8/32
Known via "bgp 125", distance 200, metric 0
Tag 678, type internal
Redistributing via ospf 125
Advertised by ospf 125 subnets
Last update from 6.7.8.8 00:21:34 ago
Routing Descriptor Blocks:
* 6.7.8.8 (Default-IP-Routing-Table), from 125.125.125.2, 00:21:34 ago
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 678
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
R8#
R8#sh ip ospf neighbor 31.3.1.1 | inc Neigh|inter
Neighbor 31.3.1.1, interface address 10.10.18.1
In the area 0 via interface OSPF_SL0
Neighbor priority is 0, State is FULL, 6 state changes
Neighbor is up for 00:22:02
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
R8#sh ip ospf data | beg Type-5
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
4.0.0.4 31.3.1.1 1386 0x80000001 0x009E9A 3489661053
10.10.18.1 98.9.8.8 1338 0x80000001 0x004D4A 3489661606
10.10.18.8 31.3.1.1 1341 0x80000001 0x00E82A 3489661053
11.0.0.11 31.3.1.1 1 (DNA) 0x80000001 0x00FC2E 3489661053
54.5.4.0 31.3.1.1 1386 0x80000001 0x00D130 3489661053
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
R8#sh ip route vrf VPNB 10.10.18.1
Routing entry for 10.10.18.1/32
Known via "bgp 678", distance 200, metric 0
Tag 125, type internal
Redistributing via ospf 678
Advertised by ospf 678 subnets
Last update from 125.125.125.1 00:22:34 ago
Routing Descriptor Blocks:
* 125.125.125.1 (Default-IP-Routing-Table), from 6.7.8.7, 00:22:34 ago
Route metric is 0, traffic share count is 1
AS Hops 1
Route tag 125
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
As you can see the Sham-Link is stable, however I have not manipulated
the Domain-Tag as shown in the OSPF database.
Tag 3489661606 = 11010000000000000000001010100110
1010100110 = AS 678
Tag 3489661053 = 11010000000000000000000001111101
1111101 = AS 125
I tried this with 12.0S and 12.4Mainline code.
Cheers,
Con.
>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]
>
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