A good tip to know when one is studying OSPF for either their CCNP or CCIE.

R10#sh ip ospf data

            OSPF Router with ID (150.1.10.10) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
150.1.1.1       150.1.1.1       1095        0x80000004 0x00E2BE 5
150.1.2.2       150.1.2.2       1096        0x80000002 0x00C144 4
150.1.3.3       150.1.3.3       1093        0x80000002 0x000FD5 6
150.1.4.4       150.1.4.4       1094        0x80000004 0x00A1A2 5
150.1.5.5       150.1.5.5       1051        0x80000004 0x008442 5
150.1.6.6       150.1.6.6       1094        0x80000004 0x007862 3
150.1.7.7       150.1.7.7       1094        0x80000002 0x001AD5 5
150.1.8.8       150.1.8.8       1124        0x80000006 0x00E37F 4
150.1.9.9       150.1.9.9       1095        0x80000002 0x00155C 3
150.1.10.10     150.1.10.10     1132        0x80000003 0x00909C 3

 

You may have an ouput like this when you do a #sh ip ospf data

You might also notice you see a field called LINK COUNT. This only applies for Type 1 Router LSAs. What this link count means is not the amount of adjacency or neighbors you may have, but the links you’ve learned from this router.

For example, above I see I learned 5 links from 150.1.1.1. But which links? To dig deeper, do a #sh ip ospf data 150.1.1.1 router

R10#sh ip ospf data router 150.1.1.1

            OSPF Router with ID (150.1.10.10) (Process ID 1)

                Router Link States (Area 0)

  LS age: 1490
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 150.1.1.1
  Advertising Router: 150.1.1.1
  LS Seq Number: 80000004
  Checksum: 0xE2BE
  Length: 84
  Number of Links: 5

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 150.1.1.1
     (Link Data) Network Mask: 255.255.255.255
      Number of MTID metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 155.1.0.0
     (Link Data) Network Mask: 255.255.255.0
      Number of MTID metrics: 0
       TOS 0 Metrics: 1000

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 169.254.100.5
     (Link Data) Router Interface address: 169.254.100.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 10

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 155.1.13.3
     (Link Data) Router Interface address: 155.1.13.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 10

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 155.1.146.6
     (Link Data) Router Interface address: 155.1.146.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 10

Notice here we see 5 links that this router knows about.

If we add/remove links from this router, we’ll see them appear/disappear from the Link Count.

William Zambrano

William Zambrano

NYC networkers is run by William Zambrano, a passionate network engineer who has been in the IT industry for eight years who posts up blog articles, YouTube videos, and holds meetup.com events in the NYC area. He lives in Queens, New York and has consulted in various different companies in the NY area. Previously William worked as a Cisco Certified Systems Instructor (CCSI) but now currently works for Arista Networks serving as a Systems Engineer. William can be reached by email at willzambrano@gmail.com

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