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OSPF "routing information sources" clarification question

I have the following OSPF network:

network diagram.png

OSPF has been configured on all routers except for R3 and R4. R5 lists R1, R2 and R6 as neighbours. But it only lists R2 and R6 as Routing Information Sources in the show ip protocols output:

R5(config-if)#do show ip protocols

Routing Protocol is "ospf 1"

  Outgoing update filter list for all interfaces is not set

  Incoming update filter list for all interfaces is not set

  Router ID 5.5.5.5

  It is an area border router

  Number of areas in this router is 2. 2 normal 0 stub 0 nssa

  Maximum path: 4

  Routing for Networks:

    10.15.0.0 0.0.0.3 area 0

    10.25.0.0 0.0.0.3 area 0

  Routing on Interfaces Configured Explicitly (Area 9):

    Serial0/0

Reference bandwidth unit is 100 mbps

  Passive Interface(s):

    Serial0/1

    VoIP-Null0

  Routing Information Sources:

    Gateway         Distance      Last Update

    6.6.6.6              110      00:00:05

    2.2.2.2              110      00:00:05

    9.9.9.9              110      00:05:59

  Distance: (default is 110)

Why is R1 (with RID 1.1.1.1 as you might expect) not listed as a routing source? I'm not 100% clear on the term Routing Information Source.

I figured that it might mean the best routing paths are through R2, but even if I kill the neighbourship to R2 by disabling the connected interface, 2.2.2.2 remains as the Routing Source and 1.1.1.1 does not appear.

Can anyone clarify what this means?

Everyone's tags (5)
4 REPLIES
Cisco Employee

OSPF "routing information sources" clarification question

Hi Steven,

I do have an idea but it's unwarranted at this point. However, can you please post the output of the show ip ospf database router and show ip ospf database network commands on R5? The listing will be fairly large but I believe the clue is hidden there. I will try to comment on it as soon as possible.

Thank you!

Best regards,

Peter

Community Member

OSPF "routing information sources" clarification question

Hi Peter. Thanks for the interest in my query

Here is the requested output

R5#show ip ospf database router

            OSPF Router with ID (5.5.5.5) (Process ID 1)

                Router Link States (Area 0)

  LS age: 1460
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 1.1.1.1
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000008
  Checksum: 0xDD79
  Length: 60
  Number of Links: 3

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.16.0.2
     (Link Data) Router Interface address: 10.16.0.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.15.0.1
     (Link Data) Router Interface address: 10.15.0.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.12.0.2
     (Link Data) Router Interface address: 10.12.0.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 10


  LS age: 498
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 2.2.2.2
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000010
  Checksum: 0xD349
  Length: 60
  Number of Links: 3

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.26.0.2
     (Link Data) Router Interface address: 10.26.0.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.25.0.2
     (Link Data) Router Interface address: 10.25.0.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.12.0.2
     (Link Data) Router Interface address: 10.12.0.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 10


  LS age: 471
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 5.5.5.5
  Advertising Router: 5.5.5.5
  LS Seq Number: 80000015
  Checksum: 0x2C31
  Length: 48
  Area Border Router
  Number of Links: 2

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.25.0.2
     (Link Data) Router Interface address: 10.25.0.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.15.0.1
     (Link Data) Router Interface address: 10.15.0.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 10


  Routing Bit Set on this LSA
  LS age: 1526
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 6.6.6.6
  Advertising Router: 6.6.6.6
  LS Seq Number: 80000005
  Checksum: 0xE976
  Length: 48
  Area Border Router
  Number of Links: 2

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.26.0.2
     (Link Data) Router Interface address: 10.26.0.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.16.0.2
     (Link Data) Router Interface address: 10.16.0.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

                Router Link States (Area 9)

  LS age: 197
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 5.5.5.5
  Advertising Router: 5.5.5.5
  LS Seq Number: 8000000D
  Checksum: 0x3B87
  Length: 48
  Area Border Router
  Number of Links: 2

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 9.9.9.9
     (Link Data) Router Interface address: 10.59.0.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 64

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 10.59.0.0
     (Link Data) Network Mask: 255.255.255.252
      Number of TOS metrics: 0
       TOS 0 Metrics: 64


  LS age: 199
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 9.9.9.9
  Advertising Router: 9.9.9.9
  LS Seq Number: 8000000F
  Checksum: 0x58CB
  Length: 60
  Number of Links: 3

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 5.5.5.5
     (Link Data) Router Interface address: 10.59.0.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 64

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 10.59.0.0
     (Link Data) Network Mask: 255.255.255.252
      Number of TOS metrics: 0
       TOS 0 Metrics: 64

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 192.168.9.0
     (Link Data) Network Mask: 255.255.255.0
      Number of TOS metrics: 0
       TOS 0 Metrics: 10


R5#show ip ospf database network

            OSPF Router with ID (5.5.5.5) (Process ID 1)

                Net Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 1483
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 10.12.0.2 (address of Designated Router)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000004
  Checksum: 0x9A73
  Length: 32
  Network Mask: /30
        Attached Router: 2.2.2.2
        Attached Router: 1.1.1.1

  Routing Bit Set on this LSA
  LS age: 1920
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 10.15.0.1 (address of Designated Router)
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000003
  Checksum: 0x47BD
  Length: 32
  Network Mask: /30
        Attached Router: 1.1.1.1
        Attached Router: 5.5.5.5

  Routing Bit Set on this LSA
  LS age: 1470
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 10.16.0.2 (address of Designated Router)
  Advertising Router: 6.6.6.6
  LS Seq Number: 80000004
  Checksum: 0x7A6F
  Length: 32
  Network Mask: /30
        Attached Router: 6.6.6.6
        Attached Router: 1.1.1.1

  Routing Bit Set on this LSA
  LS age: 419
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 10.25.0.2 (address of Designated Router)
  Advertising Router: 5.5.5.5
  LS Seq Number: 80000003
  Checksum: 0x3EA7
  Length: 32
  Network Mask: /30
        Attached Router: 5.5.5.5
        Attached Router: 2.2.2.2

  Routing Bit Set on this LSA
  LS age: 1473
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 10.26.0.2 (address of Designated Router)
  Advertising Router: 6.6.6.6
  LS Seq Number: 80000004
  Checksum: 0x34A7
  Length: 32
  Network Mask: /30
        Attached Router: 6.6.6.6
        Attached Router: 2.2.2.2

Cisco Employee

OSPF "routing information sources" clarification question

Hello Steven,

Thank you for posting the information!

I believe I now see the reason. Check the information about router R1: its LSA-1 (show ip ospf database router) only contains three references (i.e. links) to multiaccess networks - check the "Link connected to: a Transit Network" entries, basically referencing the LSA-2 generated by the appropriate DR whose IP address is indicated in these entries. In other words, this LSA-1 basically describes only the topological details regarding R1's placement in the network but it does not hold any network address information itself. IP addresses of these multiaccess networks are to be found in corresponding LSA-2 including their netmasks. Furthermore, there is no other LSA-2/LSA-3/LSA-5/LSA-7 originated by R1.

Indeed, then, R1 does not source any routing information, as it does not emit any addressing information in its LSAs. That is why the show ip protocols does not list R1 as a routing information source.

Please note that the only entries in LSA-1 that can hold addressing information are Stub Network entries (Link ID type 3). These are used to express network addresses on point-to-point or point-to-multipoint interfaces and on loopbacks. All other entry types in LSA-1 (point-to-point connection to another router, connection to a transit network, virtual link to another router) are topological relations, not network addressing information. Addressing information is further placed into LSA-2 (for Transit multiaccess networks), LSA-3 (inter-area prefixes) and LSA-5/7 (external networks). If a router does not source LSA-2, LSA-3, LSA-5/7, and its LSA-1 does not contain any stub network entries, it is not considered a routing information source by the show ip protocols because indeed, it does not advertise any network address information itself.

These link-state protocol intricacies are funny indeed

Best regards,

Peter

Cisco Employee

OSPF "routing information sources" clarification question

Hi Steven,

One slight correction/addition to my explanation above.

I said: "the only entries in LSA-1 that can hold addressing information are Stub  Network entries (Link ID type 3). These are used to express network  addresses on point-to-point or point-to-multipoint interfaces and on  loopbacks."

I forgot to write that also networks from other interfaces (multiaccess, nonbroadcast multiaccess) are indicated in LSA-1 as stub networks if there is no other OSPF detected on such an interface. If it is, then DR/BDR elections take place and the addressing information would be put into LSA-2 generated by the DR. So even Ethernet segments may be indicated directly in LSA-1 if there is no other router present in those segments (i.e. if they are not transit segments).

The main point remains, however: a router is considered a routing information source if any of these conditions are met:

  • The router emits LSA-1 containing Stub Network (Link ID type 3) entries
  • The router emits LSA-2
  • The router emits LSA-3
  • The router emits LSA-5 or LSA-7

Best regards,

Peter

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