Understanding Link State Advertisements

By Shawn Zandi

What is LSA?

• Link State Advertisement• Each OSPF update packet contains one or

more link state advertisement (LSA).• Updates are flooded into network with

different distances. Some are limited to the OSPF area and some can cross this boundary.

R2R6

R7

R3

R1

R5R4

Topology

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Backbone RouterArea Border Router

Stub Router

LSA Type 1

• Router LSA: Lists all router links

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 1Type1

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 1 Type1

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 1 Type1

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 1Type 1

Type 1

Type

1

Type

1

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

LSA Type 2

• Network LSA is advertised by Designated Router - DR

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

DR

BDR

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 2

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 2

LSA Type 3

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type3

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type

3

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type

3

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type

3

Type3

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type

3

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

External Routes

• Injection into protocol from other protocols:– Redistribution of Connected subnets– Redistribution of Static routes– Redistribution of Routing Protocols– Redistribution of other OSPF process

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Redistribution

ASBR

LSA Type 5

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 5

LSA Type 4

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 5

Type 5Type 4

Type 5Type 4

LSA5 in LSDB does not belong to any area, they are reachable across areas using LSA4LSA4 is re-generated like LSA3 for each area by corresponding ABR

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 5

Type 5Type 4

Type 5Type 4

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 5Type 4

Type 5Type 4

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 5Type 4

Type

5

Type

4

Type 5

NEW

NEWType 4When LSA5 moves across an area LSA4 will be generated by AREA ABR

Type 5

Type 5Type 4

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 5Type 4

Type

5

Type

4

Type 5

NEW

NEWType 4In the area which LSA5 is originated, there’s no need of LSA4

Type 5

Type 5Type 4

ASBR

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type 5Type 4

Type

5

Type

4

Type 5

NEW

NEWType 4When LSA5 moves across an area LSA4 will be generated by AREA ABR

Type 5

Type 5Type 4

ASBR

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type

5

Type

4

Type 5

NEW

NEWType 4

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Type

5

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

Redistribution

ASBR

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35

AREA 0(Backbone)

20+40 (4hops x10)

LSA4 Rules• IF ABR becomes also an ASBR :– No LSA4 is injected by that ABR to connected

areas because routers know that ABR with LSA1

• ABR of NSSA does not generate LSA4– Because its originator of LSA5 (converting LSA7)– Other ABRs will generate LSA4 for their areas

• LSA4 are created by ABRs, LSA5 remains intact

Stub Area

• No LSA4• No LSA5

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

Stub command must be configured on all routers in Stub area.

AREA 35 – Stub Area

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

Type 5Type 4

Type

5Ty

pe 4

LSA 4 & 5 are not permitted in Stub areas.

Type 5Type 4

Type

3Ty

pe 2

Type

1

Type 1,2,3

AREA 35 – Stub Area

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35 – Stub Area

AREA 0(Backbone)

Type 5Type 4

Stub area ABR injects a default route into the stub area, instead of sending LSA 4 & 5 into area.

Type 5Type 4 Type 1,2,3

TSA

• Totally Stubby Area– Does not need LSA type 3 (default route is

enough)– NO LSA 3– NO LSA 4– NO LSA 5

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

Type 5Type 4

Type

5Ty

pe 4

LSA 3, 4 & 5 are not permitted in totally stubby areas.

Type 5Type 4

Type

3Ty

pe 2

Type

1

Type 1,2

AREA 35 – Totally Stubby Area

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

Stub command must be configured on all routers in Stub area.

AREA 35 – Totally Stubby Area

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35 – Totally Stubby Area

AREA 0(Backbone)

Type 5Type 4

TSA ABR injects a default route into the stub area, instead of sending LSA 3, 4 & 5 into area.

Type 5Type 4

Type 1,2

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

Comparison:LSDB of TSA router (R4) versus a normal area router (R1)

Type 5Type 4

AREA 35 – Totally Stubby Area

NSSA

• Not So Stubby Area:– When there’s at least one ASBR inside Stub area– NSSA ABR does not generate default route by

default!!!

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35 – Not So Stubby Area

AREA 0(Backbone)

ASBR

RedistributionArea 35 is No longer stub area, there’s external

route. R5 is new ASBR.Stubby area which at least has an ASBR is called NSSA.

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

NSSA command must be configured on all routers in NSSA area.

AREA 35 – Not So Stubby Area

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 35 – Not So Stubby Area

AREA 0(Backbone)

ASBR

RedistributionLSA5 is not permitted in stub area.

For External route, NSSA ASBR uses LSA type 7

Type

7

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

Type 5

Type

5

LSA 4 & 5 are not permitted in NSSA.LSA 7 is substituted with LSA 5 by ABR.

Type

3Ty

pe 2

Type

1

Type

7

AREA 35 – Not So Stubby AreaASBR

Type 5

NEW

NEW

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

Type 5

Type

5

ABRs of non-directly connected areas generate LSA4 for LSA5 that is created by R3

Type

3Ty

pe 2

Type

1

Type

7

AREA 35 – Not So Stubby AreaASBR

Type 5

NEW

NEW

Type

4

NEWType

5

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

Type 5

NSSA external route is recognized by NFor other areas, it’s a regular E route.

Type

7

AREA 35 – Not So Stubby AreaASBR

NEW

Type

4

NEWType

5

T-NSSA

• Totally Not-So-Stubby Area• It’s NSSA without LSA type 3 permitted in.

(only a default route to ABR)• NO LSA3• NO LSA4• NO LSA5

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

Type 5

Type

5

LSA 3, 4 & 5 are not permitted in T-NSSA.LSA 7 is substituted with LSA 5 by ABR.

Type

3Ty

pe 2

Type

1

Type

7

AREA 35 – Totally Not So Stubby AreaASBR

Type 5

NEW

NEW

Type 1,2

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

ABR of T-NSSA generates default route for the area instead of allowing type 3, 4 and 5 in.

Type

7

AREA 35 – Totally Not So Stubby AreaASBR

Type 1,2

R2R6

R7

R3

R1

R5R4

AREA 67 AREA 31

AREA 0(Backbone)

ABR of T-NSSA generates default route for the area instead of allowing type 3, 4 and 5 in.

AREA 35 – Totally Not So Stubby AreaASBR

Thanks!

Questions?