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OSPF LSA Types Lab

April 27th, 2014 in Basic Labs Go to comments

LSA Types 5 in Stub Area

In the next part we are going to configure Area 1 as a Stub Area. To configure a stub area we have to type below commands on all routers in that stub area:

R2, R3, R4:
router ospf 1
area 1 stub

A warning message “Warning: Router is currently an ASBR while having only one area which is a stub area” appears when we configure stub area on R3. Remember that R3 is currently an ASBR and redistributed routes are not accepted in stub areas.

OSPF_LSA_Types_After_Redistribute_R3_stub_area_ASBR.jpg

We can check to see Area 1 is now a stub area with the “show ip ospf” command:

OSPF_LSA_Types_After_Redistribute_R3_stub_area_1.jpg

LSA Type 5 is not allowed in stub area (only Type 1, 2, 3 allowed in Stub area) so we don’t see Type 5 in R4 OSPF database:

OSPF_LSA_Types_Stub_Area_R4_show_ip_ospf_database.jpg

As we can see in the output above, R4 has a Link ID of 0.0.0.0 advertised by R2. This is the default route to reach outside networks. It is injected into the area by the ABR (R2)

OSPF_LSA_Types_Stub_Area_R4_show_ip_route.jpg

LSA Types 4 & 5 are also removed from R7:

OSPF_LSA_Types_Stub_Area_R7_show_ip_ospf_database.jpg

In conclusion, LSA Type 5 is not supported in Stub area. But remember Stub area does support LSA Type 3.

LSA Type 5 in Not-So-Stubby-Area

Type 5 is not allowed in Stub area so we have to convert Area 1 into Not-So-Stubby-Area (NSSA)

Configure Area 1 as Not-So-Stubby Area:

R2, R3, R4:
router ospf 1
no area 1 stub //Before configuring Area 1 as NSSA we need to disable stub area first
area 1 nssa

When Area 1 becomes NSSA, ASBR is allowed to redistribute external routes but LSAs Type 5 are still not allowed in NSSA areas so the NSSA ASBR (R3) generates a type 7 LSA instead, which remains within the NSSA. This type 7 LSA gets translated back into a type 5 by the NSSA ABR (R2).

OSPF_LSAs_Types_7.jpgLet’s check the OSPF databases of some routers:

OSPF_LSA_Types_NSSA_R7_show_ip_ospf_database.jpg

On R7 only Type 5 LSA exists. R2 is the only router we can see both Type 5 and Type 7:

OSPF_LSA_Types_NSSA_R2_show_ip_ospf_database.jpg

Check R4, only Type 7 exists:

OSPF_LSA_Types_NSSA_R4_show_ip_ospf_database.jpg

Redistributed routes appear as O N2 in NSSA. For example on R4:

OSPF_LSA_Types_NSSA_R4_show_ip_route.jpg

while redistributed routes appear as O E2 on R7:

OSPF_LSA_Types_NSSA_R7_show_ip_route.jpg

Comments
  1. Lalit Sharma
    January 28th, 2017

    Fantabulous, never seen such a great explanation….
    One thing to add after changing Area 1 as NSSA the Link ID in LSA 4 has changed from 3.3.3.3 to 2.2.2.2 on router 7 and also forward address changed to 23.23.23.3, any guess ?

  2. TT
    February 14th, 2017

    Awesome explanation. I’ve simulated your configuration and was able to follow through.. This ‘s definitely the best Tutorial of LSA Type that I have seen so far.. Thank you so much.

  3. GP
    March 6th, 2017

    very good stuff, well thought out and demonstrated

  4. Red1
    April 21st, 2017

    Excellent explanation thanks.

  5. quest
    June 16th, 2017

    gracia!!!

  6. Anonymous
    June 20th, 2017

    Thank you !!!

  7. K
    July 3rd, 2017
  8. Ambroz
    July 31st, 2017

    Excellent work

  9. anonymous
    September 4th, 2017

    Great tutorial

  10. KO
    October 23rd, 2017

    Excellent, but I confused little about LSA type 2.

  11. Hazem
    October 28th, 2017

    Great tutorial!

  12. Anonymous
    November 21st, 2017

    good stuff

  13. PHX
    December 27th, 2017

    Nice Explanation,
    Question : Why does RS have both LSA-5 and LSA-7, while it’s configured as NSSA?

  14. PHX
    December 27th, 2017

    Nice Explanation,
    Question : Why does R2* have both LSA-5 and LSA-7, while it’s configured as NSSA?

  15. Przemek
    January 5th, 2018

    PHX, because it its also ABR, has special responsibility for distributing information between Areas and belongs to two areas.

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  18. airon
    April 1st, 2018

    Excelent!! Great explanation!! Thanks a lot. sometimes I was confused about lsa type 4 and 5

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