OSPF_LSA_Deep_Dive

So we are back to OSPF, as I never really went into a deep dive with LSA types, or the LSA DataBase from different routers to show their perspective of the OSPF network caused by these Link State Advertisements.

I’ve only configured base OSPF and RIP configurations in this lab so far, the above Topology does NOT have NSSA / Stub / Redistribution configred at this point (though we will get there and examine some different routers LSA DB’s).

So first, I will start with a description of each LSA type of the 7 of them:

  • LSA Type 1 “Router” – “Router Link States” will be its header in the LSA DB, and the name is self explanatory, these LSA’s are generated by each router with updates on its local Link States, all router types generate and flood this LSA Type.
  • LSA Type 2 “Network” – “Net Link States” are only generated and sent by DR’s and BDR’s to routers in the Same Area, that are also on the same multi-access network type, LSA type stays within its own Area, only seen in NON-Point-to-Point network types
  • LSA Type 3 “Summary” – “Summary Net Link States” has nothing to do with summarization, but floods its summary of networks from one Area into others except for the Area it is part of – Not flooded into Total-Stub’d Areas (Stub or NSSA)
  • LSA Type 4 “Summary ASB” – “Summary ASB Link States” LSA type is only created by ABR’s back to the ASBR, so when redistribution is configured on the ASBR Router it flips a bit in its “Router LSA” (Type 1!), and the ABR(s) then create LSA type 4’s to pass along throughout the network giving OSPF neighbors the path back to the ASBR – Not flooded into Stub Areas.
  • LSA Type 5 “Autonomous System External Link State” – or “AS External Link States” in the OSPF LSA DB, these are your “O E1” and “O E2”  Redistributed routes, generated from the ASBR itself OUTSIDE an NSSA Area – Not flooded into Stub Areas.
  • LSA Type 6 – Not needed for the CCNP ROUTE, but it is for Multicast Extensions of OSPF (MOSPF), but again is not referenced in the the ROUTE exam, just wanted to mention for the sake of thoroughness
  • LSA Type 7 “NSSA LSA’s” – This type of LSA is generated by the ASBR INSIDE an NSSA Area does Redistribution, as Type 5 Redistribution LSA’s cannot enter an NSSA Area

Phew. So to cover what type of routers create which type of LSA’s ONE MORE TIME:

  • Type 1 – All Routers
  • Type 2 – All DR’s
  • Type 3, 4 – All ABR’s
  • Type 5 – ASBR’s OUTSIDE the NSSA Areas (NSSA’s don’t allow LSA type 5)
  • Type 6 – Reserved for MOSPF
  • Type 7 – ASBR’s INSIDE the NSSA Areas (Type 7 LSA’s [N1, N2 in route table])

It is VERY important to know which routers create what types of LSA’s, as this is such a heavy topic (up there with BGP), there is a high chance to see these topics on exam day!

The thing to watch for, is not necessarily in the question, but also in the answer, as the exam may present what seems like a correct answer but the behavior is incorrect, example:

You are looking at a topology above and the question asks “Which of the following commands will allow you to create an OSPF Summary Route for R1 that contain the links connected to R5?”

Then it will give you the correct configurations to summarize a route, but you cannot summarize OSPF routes on a router that is not directly connected to them, so all answers are wrong in that case – This is one example of Cisco’s “gotchas” that requires this deeper dive into understanding the behaviors so we don’t get got homie 🙂

So to kick this party off, lets take a look at R2’s LSA Database since it is smallest:

R2#sh ip ospf database

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         350         0x80000002 0x00DCA2 1
2.2.2.2         2.2.2.2         350         0x80000007 0x00E65D 2
3.3.3.3         3.3.3.3         351         0x80000007 0x001D10 2

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
172.12.123.1    1.1.1.1         346         0x80000004 0x0023BE

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         1.1.1.1         351         0x80000004 0x0041EF
2.2.2.2         2.2.2.2         1406        0x80000003 0x00F633
3.3.3.3         3.3.3.3         1373        0x80000003 0x00AA77
172.12.15.0     1.1.1.1         351         0x80000005 0x0072F9
172.12.34.0     3.3.3.3         1373        0x80000005 0x0064EC

                Router Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         1406        0x80000003 0x00A571 1

                Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum
1.1.1.1         2.2.2.2         340         0x80000001 0x00AB44
3.3.3.3         2.2.2.2         340         0x80000001 0x004F98
22.2.2.2        2.2.2.2         1164        0x80000002 0x00F323
33.3.3.3        2.2.2.2         340         0x80000001 0x00C702
172.12.15.0     2.2.2.2         340         0x80000001 0x00DE4D
172.12.34.0     2.2.2.2         340         0x80000001 0x000D0C
172.12.123.0    2.2.2.2         340         0x80000001 0x002C94
R2#

As can be seen it will show all connected OSPF Area’s LSA’s, and starting from the top:

  • Type 1: Router Link States, showing the “links” (RID’s) of all routers with an interface participating in Area 0, as can be see under Router Link States for Area 2 is only itself
  • Type 2: Network Link States, generated only on DR’s on non-point-to-point links, the DR finds these Multi-Access routers links and provides the DR physical interface IP along with the RID
  • Type 3: Summary Net Link States – These are our “O IA” routes, this LSA type is created by the BDR and flooded into Areas that is not its own, and given this confused me for quite awhile here so I want to explain Area 0’s and Area 2’s output in two different pieces

Area 0’s Summary Net Link States will only advertise routes to Areas it is not participating in, that is why you DON’T see 172.12.123.0 or any of the 11.x.x.x / 22.x.x.x / etc in the Database for Area 0.

Area 2’s Summary Net Link appears to contain all the routes so far, except for one, and that would be it’s own routes of 2.2.2.2 because it will not include routes from the Area that it is part of, however it will show every other route (including the main NBMA 172.12.123.0 route) within this Area of the OSPF DB.

So to this point, OSPF works in harmony with each other as LSA 1’s monitor all their connected links states and share them across to other OSPF neighbors, LSA 2’s generated by the DR make sure all Multi-Access routers are included and get updates, and our ABR’s as can be seen are flooding routes / networks not in their own Area into other Area’s.

That is about as thorough as I can get tonight, as the next post will be covering the rest of them, that just took an extremely long time to get absolutely accurate information regarding these!

I will continue on to clearly demonstrate and understand the upcoming Redistribution, and NSSA LSA types, I am hoping to get that into one post on my next write up!