OSPFv3 Route Summarization, RFC1583 doesn't seem to be working?

[AK0]

I have the following topology:

IOU3 is the ABR, and is summarizing Area 0 prefixes as a summary route 2000::/16 into Area 1.

IOS version: 15.5.

RFC1583 is ENABLED on IOU3 (and everywhere else), which means that the cost of the summary route should be based on the specific subnet with the LOWEST cost, and from IOU3's perspective that would be a cost of 1 out its e0/1 interface to 2000:2323::/64.

But strangely enough, IOU3 is advertising the summary route with the HIGHEST cost, namely: 3 (cost of the 2000:1111-prefixes). That is strange, because RFC1583 is enabled.

IOU3's config:

IOU3#sh run | s router ospf
router ospfv3 1
 router-id 3.3.3.3
 !
 address-family ipv6 unicast
  area 0 range 2000::/16
 exit-address-family

IOU3#sh ospfv3
 OSPFv3 1 address-family ipv6
 Router ID 3.3.3.3
...
 Reference bandwidth unit is 100 mbps
 RFC1583 compatibility enabled
    Area BACKBONE(0)

IOU3's routing table:

IOU3#sh ipv6 route         
IPv6 Routing Table - default - 9 entries
...
O   2000::/16 [110/3]
     via Null0, directly connected
O   2000:1111:1::/64 [110/3]
     via FE80::A8BB:CCFF:FE00:700, Ethernet0/1
O   2000:1111:2::/64 [110/3]
     via FE80::A8BB:CCFF:FE00:700, Ethernet0/1
O   2000:1212::/64 [110/2]
     via FE80::A8BB:CCFF:FE00:700, Ethernet0/1
C   2000:2323::/64 [0/0]
     via Ethernet0/1, directly connected
L   2000:2323::3/128 [0/0]
     via Ethernet0/1, receive
C   2001:3434::/64 [0/0]
     via Ethernet0/0, directly connected
L   2001:3434::3/128 [0/0]
     via Ethernet0/0, receive
L   FF00::/8 [0/0]
     via Null0, receive

IOU3 is advertising the summary route to IOU4 in Area 1 with a cost of 3:

IOU4#sh ospfv3 database inter-area prefix

          OSPFv3 1 address-family ipv6 (router-id 4.4.4.4)

                Inter Area Prefix Link States (Area 1)

  LS age: 1675
  LS Type: Inter Area Prefix Links
  Link State ID: 0
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x39C2
  Length: 32
  Metric: 3 
  Prefix Address: 2000::
  Prefix Length: 16, Options: None

I don't understand why IOU3 is advertising the summary route with a cost of 3? RFC1583 is enabled, so it should be a cost of 1..?

[Martin L · Accepted solution]

 rfc1583 is ignored by ospf v3 IOS code; It is there as a sort of backward compatiblity of v2 and v3 (since u can run either one or both on the same router). 

RFC for v3 says " new code and it ignores it". See OSPFv3 RFC5340 " Furthermore, RFC1583 Compatibility is not an OSPF for IPv6 configuration parameter. Refer to Appendix C.1."

 Cisco explanation "Because of the introduction of RFC 2328, OSPF Version 2, the method used to calculate summary route costs has changed. Use the no compatible rfc1583 command to enable the calculation method used per RFC 2328."

Regards, ML
**Have fun labbing!!!***
***Please Rate All Helpful Responses ***

[Martin L]

see the testing lab of ospfv2 and v3 in CLN https://learningnetwork.cisco.com/s/feed/0D56e0000EBulguCQB

[paul driver]

Hello @Martin L 
Good catch mate , My initial response to the OP  now removed) was based on maybe the OP hadn't incorporated the metric cost towards the ABR, however upon looking further i was looking the wrong way - additionally after you posted this solution I had to test the compatibility out for myself for my own validation between OSPFv2 and OSPFv3 and  in v3  it does indeed seem to ignore rfc1583 inter-area summery rule of lowest metric cost of summarised routes, and default the the highest metric cost.

Gotta love OSPF RFCs to keep you on your toes!

[Harold Ritter]

Hi @AK0 ,

@Martin L is absolutely right about the expected behavior for the summary LSA metric.

I think your confusion came from the fact that you saw "RFC1583 compatibility enabled" in the "show ospfv3" output. The "RFC1583 compatibility enabled" in the ospfv3 context only refers to the following feature/behavior:

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/xe-3s/iro-xe-3s-book/ip6-route-ospfv3-extpath-xe.html