OSPF route selection issue with 6PE/6VPE scenario

[KATTUBAVA E.S]

Hi Experts,

Using below topology for 6PE and 6VPE, Ipv6 prefix 60::6:6:6/128 on CE1 is advertised by PE1 using 6PE method on the MPLS backbone to PE2 and PE2 redistributes(OSPF) the same to CE3. On the other hand  PE1 advertises both 60::6:6:6/128 and 60::60:60:60/128 from CE2 using 6VPE method on MPLS VPN to PE3 and from there it redistributed(OSPF) to CE3. Both PE1 fa1/1 and PE3 fa1/0 are configured for same VRF with same RD/RT values. Now I could see both prefixes advertised by PE1 are reaching the respective egress PEs via BGP but on CE3  prefix 60::6:6:6/128 is installed as O E2 route (as its learned by 6PE method)  via PE2 instead of O IA route via PE3 (learned by 6VPE) 

By OSPF rule, O IA is better than O E2, then why CE3 chooses E2 route instead of IA for the same prefix? Can anyone explain this in detail?

Attaching all PE & CE routers config and routing/ospf database table.

R8-CE3#sh ipv6 route ospf
IPv6 Routing Table - default - 11 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, R - RIP, H - NHRP, I1 - ISIS L1
I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary, D - EIGRP
EX - EIGRP external, ND - ND Default, NDp - ND Prefix, DCE - Destination
NDr - Redirect, O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1
OE2 - OSPF ext 2, ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2, l - LISP

OE2 10::1:6:0/120 [110/1]
via FE80::C805:1EFF:FEF0:1C, FastEthernet0/0
OI 10::1:7:0/120 [110/2]
via FE80::C806:16FF:FEFC:1C, FastEthernet1/0
OE2 60::6:6:6/128 [110/1]
via FE80::C805:1EFF:FEF0:1C, FastEthernet0/0
OI 60::60:60:60/128 [110/2]
via FE80::C806:16FF:FEFC:1C, FastEthernet1/0

[Peter Paluch]

Hello,

It would appear that there is a race condition occurring in your scenario.

Your R8-CE3 is running two independent OSPFv3 processes. Each of them is learning about the 60::6:6:6/128 independently - one of them learns it through the 6PE as OE2, the other learns it as OIA through 6VPE. However, because neither of these two OSPFv3 processes sees the other alternative, they both consider their path to be the best, and they both offer it to the routing table with the same AD of 110 - and it seems that the process learning the route as OE2 is faster.

Remember - the path preference scenario (that O is better than O IA which is better than O E1/N1 which is better than O E2/N2) applies only when a single OSPF process has to choose between these variants. Two independent OSPF processes will not observe this preference rule because they do not know about each other.

What would happen if you shut down the link on R8-CE3 that goes to R4-PE2? Would the R8-CE3 learn the path as O IA?

Best regards,
Peter

[KATTUBAVA E.S]

Hi Peter,

Thanks for the reply. Actually I have tried with single ospfv3 process on R8-CE3 first, have seen the same issue, (i.e) route 60::6:6:6/128 (E2) received from PE4  chosen as best compared to route from PE5 for the same prefix with OIA. OSPF database is learning both summary and External route for the prefix but prefers O E2 instead of O IA.

R8-CE3#sh ospfv3 database

          OSPFv3 100 address-family ipv6 (router-id 8.8.8.8)

                Router Link States (Area 0)

ADV Router       Age         Seq#        Fragment ID  Link count  Bits
 4.4.4.4         1350        0x80000002  0            1           E
 5.5.5.5         1308        0x80000002  0            1           B E
 8.8.8.8         1307        0x80000003  0            2           None

                Net Link States (Area 0)

ADV Router       Age         Seq#        Link ID    Rtr count
 8.8.8.8         1353        0x80000001  2          2
 8.8.8.8         1307        0x80000001  3          2

                Inter Area Prefix Link States (Area 0)

ADV Router       Age         Seq#        Prefix
 5.5.5.5         605         0x80000001  10::1:7:0/120
 5.5.5.5         605         0x80000001  60::60:60:60/128

                Link (Type-8) Link States (Area 0)

ADV Router       Age         Seq#        Link ID    Interface
 5.5.5.5         1347        0x80000002  3          Fa1/0
 8.8.8.8         1376        0x80000001  3          Fa1/0
 4.4.4.4         1350        0x80000002  3          Fa0/0
 8.8.8.8         1513        0x80000002  2          Fa0/0

                Intra Area Prefix Link States (Area 0)

ADV Router       Age         Seq#        Link ID    Ref-lstype  Ref-LSID
 8.8.8.8         1307        0x80000008  0          0x2001      0
 8.8.8.8         1353        0x80000001  2048       0x2002      2
 8.8.8.8         1307        0x80000001  3072       0x2002      3

                Type-5 AS External Link States

ADV Router       Age         Seq#        Prefix
 4.4.4.4         605         0x80000001  10::1:6:0/120
 4.4.4.4         405         0x80000029  60::6:6:6/128
R8-CE3#
R8-CE3#sh ipv6 route 60::6:6:6/128
Routing entry for 60::6:6:6/128
  Known via "ospf 100", distance 110, metric 1, type extern 2
  Route count is 1/1, share count 0
  Routing paths:
    FE80::C805:1EFF:FEF0:1C, FastEthernet0/0
      Last updated 00:06:57 ago

-------------------------------------------------------------------------------------------------------------------------

I could see O IA route getting installed in routing table of R8-CE3 after shutdown the link between R8-CE3 and R4-PE3.

R8-CE3#sh ipv6 route 60::6:6:6/128
Routing entry for 60::6:6:6/128
  Known via "ospf 100", distance 110, metric 1, type extern 2
  Route count is 1/1, share count 0
  Routing paths:
    FE80::C805:1EFF:FEF0:1C, FastEthernet0/0
      Last updated 00:08:25 ago

R8-CE3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R8-CE3(config)#int fa0/0
R8-CE3(config-if)#shu
R8-CE3(config-if)#shutdown
R8-CE3(config-if)#end
R8-CE3#
*Dec 18 17:04:25.111: %OSPFv3-5-ADJCHG: Process 100, IPv6, Nbr 4.4.4.4 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Dec 18 17:04:26.051: %SYS-5-CONFIG_I: Configured from console by console
R8-CE3#
R8-CE3#
*Dec 18 17:04:27.123: %LINK-5-CHANGED: Interface FastEthernet0/0, changed state to administratively down
*Dec 18 17:04:28.123: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to down
R8-CE3#
R8-CE3#sh ipv6 route 60::6:6:6/128
Routing entry for 60::6:6:6/128
  Known via "ospf 100", distance 110, metric 2, type inter area
  Route count is 1/1, share count 0
  Routing paths:
    FE80::C806:16FF:FEFC:1C, FastEthernet1/0
      Last updated 00:00:00 ago

R8-CE3#

[KATTUBAVA E.S]

Hi Experts,

I would appreciate if anyone of you clarify my doubts here

-bava