OSPF LSA Type

There are 11 different LSA Types. As I mentioned in previously  POST :

Links State Advertisement : These messages are used by OSPF to share info about network and propagate it through all OSPF domain.

There are in general 11 types of LSA but most common and important one are LSA 1,2,3,4,5,7

Type 1 – Router LSA
Type 2 – Network LSA
Type 3 – Summary LSA
Type 4 – ASBR Summary LSA
Type 5 - External LSA
Type 6 – Group Membership - Multicast LSA
Type 7 – NSSA External Link LSA
Type 8 – External Attributes LSA for BGP
Type 9 – Opaque LSA (link-local scope) - used for Graceful Restart

Type10 – Opaque LSA (area-local scope) - used for Traffic Engineering
Type 11 - Opaque LSA (AS scope)


Type 1 – Router LSA

1. Router LSA are produced by every router within an area 
2. Flooded only within the area in which they are originated
3. Lists all of a router’s links, interfaces, state and outgoing costs of each link and any knows OSPF neighbors on the link 
4. It is identified by the Router ID or the originating router 
5. These LSA area represented in the routing table as “O” routes

 

 Type 1  - Router LSA propagation

Type 2 – Network LSA

1. Produced by the DR to flood LSAs to each router on a broadcast segment
2. Describes all the routers attached to the network. Includes networkID, subnet mask and also the list of attached routers in the transit. 
3. Lists all attached routers including the DR itself 
4.  No metric field – unlike the Router LSA 
5.  If no DR exists – e.g. point-to-point network there will be no Network LSA 
6.  Flooded only within the originating area 
7. These LSA area represented in the routing table as “O” routes

 

Type 2  - Network LSA propagation

Type 3 – Summary LSA 

1. Used for readvertising Router, Network or Network Summary LSAs to other areas (one type 3 LSA for each type 1, 2 and 3 LSAs). Cost is always recalculated from the originating ABR point of view.
2. Inter-are IA routing is like distance vector, we assume of knowledge of our neighbor – ABR in this case
3.  Originated by ABR 
4. This LSA tells the internal routers of an attached area what destinations the ABR can reach 
5. Advertises the destinations within its attached areas into the backbone 
6. When ABD originates Type 3 LSA it includes the cost from itself to the destination. If ABR knows of multiple routes to a destination within its own attached area it originates a single LSA3 into the backbone within the lowest cost. 
7. When intra router receives LSA3 from ABR it doesn’t run SPF algorithm to the destination. It simply adds the cost of the route to the ABR.This is very important point because when there is a fault in e.g area 0, routers in areas attached to backbone don’t have to run SPF. SPF algorithm runs only in area where there is a fault. When costs in LSA3 will be change because of fault routers in other area just simply perform 2 step calculation : cost in LSA3 + cost to ABR  
8. ABR doesn’t forward LSA1 and LSA2. 
9. ABRs in the same areas (non-backbone) ignore each other LSA 3 to avoid loops. 
10. These LSA area represented in the routing table as “OIA” routes

 

 

Type 3  - Summary LSA propagation

Type 4 – ASBR Summary LSA

1. ASBRs are identified by setting E bit in Router LSAs - they are unknown outside of their respective areas. ASBR Summary LSA is created by ABRs to advertise their location to other areas. Area flooding scope - thus rewritten by ABRs when crossing areas.
2. Originated by ABR – this is IP address of ASBR. 
3. The destination they advertise is IP address of ASBR 
4. Mask always is 0 
5. Link-State id is is the router ID of the ASBR 
6. LSA4 it is information about router, not network 
7. Created to support LSA5 E1/E2 Type metric calculation

Type 5 – External LSA

1. Used for advertising networks from outside of a OSPF domain. Domain flooding scope - by default External LSAs are reflooded to other areas untouched. Because of that, ASBR Summary LSA is required for routers in other areas to find the originating ASBR.
2. Originated by ASBRs
3. They advertise either a destination external to the OSPF AS or a default router extermal 
4. Are flooded throughout the OSPF domain except stubby areas.  
5. Router to install External LSA into RIB needs to see prefix as a intra-area or inter-area. That is why needs to know how to get to ASBR from LSA1 (if ASBR is in the same area) or from LSA4 (if ASBR is in different area) 

Type 4 and Type 5 - ASBR Summary and External LSA propagation

Type 7 - NSSA External Link LSA

1. Originated in NS(T)SA, advertise external networks like Type 5 LSAs, however only area scope, thus rewritten into type 5 LSAs when readvertised by ABRs.
2. Originated by ASBRsand flooded only within NSSA
3. Blocked by ABR and translated into LSA5. If many ABRs exist only the one with highest router-id does the translation.  
4. Describes routes ASBR is redistributing:  

          - Metric
          - Metric Type :
                        Type 1 = N1
                        Type 2 = N2 (default)
          - Forward Address
                        Who should I route towards to reach link (Usually ASBE itself)
          - Route tag 

 Type 7  - NSSA LSA propagation

All LSAs share a common header that contains:

• Link-State Age - time since LSA was first originated in the network. Measured in seconds, maximum of 3600sec. Junos by defaults refloods when the timer reaches 3000sec.
• Options - same as in Hello messages
• Link-State Type
• Link-State ID/Data - describes advertised link
• Advertising Router
• Link-State Sequence - each LSA update increases this counter. When local LSA has lower value than received LSA, LSR packet is generated to request the newer version.
• Checksum