introduction to link state advertisements (lsa)
TRANSCRIPT
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?