OSPF Network LSA: Type 2

Welcome back to my LSA re-review. No need for a long winded intro here. Let’s get at it.



The OSPF Type 2 LSA, also known as the Network LSA, is used to describe all transit networks, both broadcast or non-broadcast. A transit network is defined as any link that has two or more routers attached to it. The Type 2 LSA is originated by the Designated Router (DR) of the link, and the Type 2 LSA lists all the routers that are attached to it, including the DR itself.

Section 14.4.2 of RFC 2328 has the detailed explanation of all things Type 2.

Cisco IOS displays Type 2 LSA’s like so:

  Routing Bit Set on this LSA
  LS age: 55
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 10.0.123.1 (address of Designated Router)
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000002
  Checksum: 0xF998
  Length: 36
  Network Mask: /24
    Attached Router: 1.1.1.1
    Attached Router: 2.2.2.2
    Attached Router: 3.3.3.3

And just like last time, line by line.

  Routing Bit Set on this LSA
  LS age: 55
  Options: (No TOS-capability, DC)

We’ve been over this before. This isn’t anything new.

  LS Type: Network Links

This is new. Same field, new value. As I’m sure you know, we’re looking at at Type 2 LSA. This confirms that.

  Link State ID: 10.0.123.1 (address of Designated Router)

We’ve seen the LSID field before, but this time the field carries…. Wait for it… The address of the DR. See, the way this works is that the DR acts as a representative of the link on behalf of all the attached routers. The DR still sends out a Type 1 LSA to identify itself, and now it also sends out a Type 2 to represent the transit link. You can think of it as if the DR IS the link itself.

  Advertising Router: 1.1.1.1

This is also the same as last time. This is the RID of the DR that originated this LSA.

  LS Seq Number: 80000002
  Checksum: 0xF998
  Length: 36

This is also all the same as a Type 1.

  Network Mask: /24

Since this LSA describes a link, and links are fairly synonymous with networks, we need the prefix length to define the network boundaries (We already know the prefix from the LSID field).

    Attached Router: 1.1.1.1
    Attached Router: 2.2.2.2
    Attached Router: 3.3.3.3

And finally the LSA lists the RID’s of all the attached routers.

That’s about all there is to a Type 2 LSA. This one is pretty straight forward.

The complete list of OSPF LSA breakdowns:

http://blog.brokennetwork.ca/2012/10/ospf-router-lsa-type-1.html
http://blog.brokennetwork.ca/2012/10/ospf-network-lsa-type-2.html
http://blog.brokennetwork.ca/2012/10/ospf-summary-lsas-type-3-4.html
http://blog.brokennetwork.ca/2012/11/ospf-external-lsa-type-5.html
http://blog.brokennetwork.ca/2012/11/ospf-nssa-external-lsa-type-7.html