Home > OSPF LSA Types Tutorial

OSPF LSA Types Tutorial

April 27th, 2014 in ROUTE Knowledge Go to comments

OSPF uses Link State Advertisement (LSA) to build up Link State Database (LSDB) so understanding how LSA works is the key point to grasp how OSPF operates.

OSPF_LSAs_Types.jpg

Quick review
In the topology above:
+ R3 and R4 only belong to Area 1. R1 only belongs to Area 0. R6 and R7 only belong to Area 2. These are known as Internal Routers.
+ R2 belongs to both Area 0 and Area 1. R5 belongs to both Area 0 and Area 2. These routers are known as Area Border Routers (ABRs).
+ Area 0 is known as Backbone Area. Every router which has an interface in Area 0 can be considered a Backbone Router. All other areas must have a connection to Area 0 (except using virtual-link). Without Area 0, routers can only function within that area.
+ When a change occurs in the network topology, the router experiencing the change creates a link-state advertisement (LSA) concerning that link.

OSPF has 11 LSA Types from 1 to 11 but some of them are not used like Type 6 (Multicast LSA), 8 (used for BGP), 9, 10, 11 (Opaque LSAs). In this tutorial we will learn more about other LSA Types (types 1 to 5 and 7).

Router link LSA (Type 1) – Each router generates a Type 1 LSA that lists its active interfaces, IP addresses, neighbors and the cost to each. LSA Type 1 is only flooded inside the router’s area, it does not cross ABR.

Therefore in the topology above, all routers sends LSA Type 1 to the neighbors within its area. The ABRs (R2 & R5) send LSAs to both areas it connects to. For example, R1 sends LSA Type 1 to R2 & R5 that contains the status of its links. R3 sends LSA Type 1 to R2 only while R2 (ABR) sends to R1, R3 & R4.

Network link LSA (Type 2) – is sent out by the designated router (DR) and lists all the routers on the segment it is adjacent to. Types 2 are flooded within its area only; does not cross ABR. Type 1 & type 2 are the basis of SPF path selection. For example, if R2 was elected the DR for the segment (R2-R3) & (R2-R4), it would generate a Type 2 LSA containing a list of all routers attached to it and flood inside Area 1.

Note: The broadcast and non-broadcast network types elect a DR/BDR.

Summary link LSA (Type 3) – ABRs generate this LSA to send between areas (so type 3 is called inter-area link). It gathers information it has learned on one of its attached areas and summarizes them before sending out to another area. LSAs Type 3 are injected by the ABR from the backbone area into other areas and from other areas into the backbone area. The picture below shows how LSAs Type 3 generated by R2 are propagated.

OSPF_LSAs_Types_3.jpg

Please notice that R2 sends LSAs Type 3 to both Area 0 & Area 1. But Type 3 LSAs sent into Area 0 will contain a list of networks within Area 1, including the cost to reach these networks. Type 3 LSAs sent into Area 1 will contain a list of networks within Area 0.

Although the name of LSA Type 3 is “Summary” Link but this word is very misleading because it does not summarize our subnets to a bigger one. The word “Summary” here should be understood as “LSA Type 3 takes information from LSA Type 1 and 2 and summarizes them into Type 3 LSA”. This summarization helps reduce the detailed topology information for other areas. We should consider Type 3 “Inter-Area” LSA. Type 3 LSAs are represented by O IA keyword in the routing tables.

Summary ASBR LSA (Type 4) – Generated by the ABR to describe an ASBR to routers in other areas so that routers in other areas know how to get to external routes through that ASBR. For example, suppose R8 is redistributing external route (EIGRP, RIP…) to R3. This makes R3 an Autonomous System Boundary Router (ASBR). When R2 (which is an ABR) receive this LSA Type 1 update, R2 will create LSA Type 4 and flood into Area 0 to inform them how to reach R3. When R5 receives this LSA it also floods into Area 2.

In the above example, the only ASBR belongs to area 1 so the two ABRs send LSA Type 4 to area 0 & area 2 (not vice versa). This is an indication of the existence of the ASBR in area 1.

OSPF_LSAs_Types_4.jpg

Note:
+ Type 4 LSAs contain the router ID of the ASBR.
+ There are no LSA Type 4 injected into Area 1 because every router inside area 1 knows how to reach R3. R3 only uses LSA Type 1 to inform R2 about R8 and inform R2 that R3 is an ASBR.

External Link LSA (LSA 5) – Generated by ASBR to describe routes redistributed into the area and point the destination for these external routes to the ASBR. These routes appear as O E1 or O E2 in the routing table. In the topology below, R3 generates LSAs Type 5 to describe the external routes redistributed from R8 and floods them to all other routers and tell them “hey, if you want to reach these external routes, send your packets to me!”. But other routers will ask “how can I reach you? You didn’t tell me where you are in your LSA Type 5!”. And that is what LSA Type 4 do – tell other routers in other areas where the ASBR is!

OSPF_LSAs_Types_5.jpg

Multicast LSA (Type 6) are specialized LSAs that are used in multicast OSPF applications. Cisco does not support it so we will not discuss it here.

NSSA External LSA (Type 7) – Generated by an ASBR inside a Not So Stubby Area (NSSA) to describe routes redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA. These routes appear as N1 or N2 in the routing table inside the NSSA. Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost upto the ASBR.

OSPF_LSAs_Types_7.jpg

We also have a GNS3 lab about OSPF LSA Types, you can read it here: http://www.digitaltut.com/ospf-lsa-types-lab.

Comments
  1. ndane
    May 6th, 2014

    good well explained with this i am sure of making it

  2. geoffrey
    May 16th, 2014

    pls, where do I find tutorial for each chapter in route 642-902

  3. digitaltut
    May 19th, 2014

    @geoffrey: We are sorry but currently this tutorial is the only one in digitaltut. We will add more tutorials in the future but this is a time-consuming work.

  4. Malik
    May 26th, 2014

    Great tutorial. minor mistake (i am sure is a typo). when describing LSA TYPE 1, second paragraph. you said “the ABRs (R2&R3)”. its supposed to be (R2&R5).

    Thanks for the Tutorial.

  5. Talha
    June 5th, 2014

    Kindy send me CCNP Route lectures at haroon.talha@gmail.com

  6. Ambro
    June 19th, 2014

    Great tutorial, clear, concise and delivers the important concept by the bullet ….

  7. digitaltut
    June 20th, 2014

    @Malik: Thanks for your detection. We fixed it!

  8. samit
    June 22nd, 2014

    Hello digtaltut

    I am your premium member can I get BGP tutorial like his tutorial.

    Thanks

  9. digitaltut
    June 22nd, 2014

    @samit: We are sorry but we don’t have BGP tutorial.

  10. Parti
    September 3rd, 2014

    Explained clearly!! Thanks a lot!!

  11. arthur
    September 12th, 2014

    small typo in router lsa description.

    For example, R1 sends LSA Type 1 to R2 & R5 that contains the status of its links. This LSA will be flooded to all other routers in Area 1

    this should say that to all other routers in Area 0 not 1 :) router lsa don’t pass from area to area.

    nice tutorial

  12. Anonymous
    September 15th, 2014

    download network tutorials files from bellow link

    http://uploadboy.com/users/wikinetwork

  13. Narendra V
    October 2nd, 2014

    Awesome tutorial Digitaltut..Thanks a ton..Much Appreciated!!

  14. Anonymous
    November 6th, 2014

    Excellent explanation!!! This is what I am looking for!

  15. clive
    November 8th, 2014

    Very good explanation ‘digitaltut’. However I noticed one smaller error. ASBR = Autonomous System Boundary Router and not an Autonomous System Border Router.

  16. SAM
    November 9th, 2014

    Hello everyone,

    i just cleared 642-902 ROUTE with full marks. i have yet to complete switch and Tshoot.

    my question is do i need to do those exams before they get expired on january 29th 2015 or i can sit for CCNP SWITCH 300-115 SWITCH and 300-135 TSHOOT even after the 29th of january??

    will there be an issue that my route is from expired one and the other 2 are from the new ?

    thanks a lot!

    Sam

  17. digitaltut
    November 10th, 2014

    @clive: Thanks for your detection. We have just fixed it.

    @SAM: You can do TSHOOT 300-135 & SWITCH 300-115 to get your CCNP cert after Jan-29. You can try the checking tool of Cisco here: http://www.cisco.com/web/learning/le3/le2/le37/le10/ccnp_exam_combo_tool.html

  18. SAM
    November 10th, 2014

    thank you digitaltut! i am gonna start switch now

  19. Piyal DG
    November 17th, 2014

    Very good to clear the concept ….

  20. eric
    November 26th, 2014

    Hello digitaltut!

    May I know, which type of LSA is advertised across area 1(transit area) between area 0(backbone) and area 3(let’s be eigrp as ospf area 3).

  21. mohammed
    January 13th, 2015

    @ all please what are the material for ccnp route ?

  22. LNK
    February 10th, 2015

    Awesome!

  23. omar
    June 2nd, 2015

    thank you fpr this effort :)

    ALLAH bless you

  24. omar
    June 2nd, 2015

    for*

  25. mbox23
    June 11th, 2015

    Awesome explanation!!!

  26. Chanaka
    September 15th, 2015

    Thank you very much this effort will make u as a genius

  27. Arvind Sahani
    October 13th, 2015

    Awesome explanation!!!

  28. Arvind Sahani
    October 13th, 2015

    This is good initiation to provide this type of material. This is informative material keep it up

  29. Ghulam
    November 4th, 2015

    Thanks!
    It was great and useful

  30. @bhi$hek
    December 9th, 2015

    Awesome explanation!!!

  31. anon
    December 10th, 2015

    Great explain

  32. mohammed
    December 27th, 2015

    Hello guys
    I want to clarify questions ccnp route 300-101 and how much their number

  33. Anonymous
    February 11th, 2016

    Shukla 11 feb,2016

    Great explanation

  34. Anonymous
    July 4th, 2016

    Thanks well explained now i understand LSAs better haha, very clear writing

  35. Ramesh
    August 2nd, 2016

    awesome explanation

  36. Ramesh
    August 2nd, 2016

    Requesting 2 two please give us BGP tutorial…..this is my humble request to you sir..

  37. eve
    September 6th, 2016

    where do i get study material for ccnp route?

  38. NARUTO
    November 15th, 2016

    very very clear explanation
    thank you so much

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