12-12-2015 08:10 AM
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
12-17-2015 01:18 PM
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
12-18-2015 03:39 AM
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#
01-06-2016 08:32 AM
Hi Experts,
I would appreciate if anyone of you clarify my doubts here
-bava
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