OSPF LSA Types

Type 1 LSA - it build the graph for intra-area SPF

1. Describe their connected links
2. What are the cost of the links
3. What are the neighbors that are adjacent on the links

TO VERIFY : sho ip ospf database router [id]

Type 2 LSA - Generated by DR on broadcast and non broadcast network type

1. Not flooded outside the area they originated in :
2. The LSA will not move to the ABR

TO VERIFY : sho ip ospf database NETWORK [link ID] for DR


Type 3 LSA - Generated by ABR to summarize the topology to move traffic from area 0 to area 1 with out running SPF

example :
ABR can reach link 1 via SPF w/ the cost of 50
I can reach the ABR via SPF of 10 in my area
to reach link 1 via SPF in cost of 50+10 . this why inter-area is called DV (DISTANCE VICTOR)

TO VERIFY : sho ip ospf database summary [link ID]

Type 4 LSA - ASBR summary - Generated by ABR and describes the ABR reach-ability to ASBR, it does include cost but hides ABR's as actual path to destination

1. ABR can reach ASBR via SPF w/ the cost of 50
2. I can reach the ABR via SPF of 10 in my area
3. I can reach ASBR via SPF in cost of 50+10 this why intra-area is called DV (DISTANCE VICTOR)

TO VERIFY : sho ip ospf database asbr-summary [link ID]

Type 5 LSA - Generated by ASBR and describe routes the ASBR is redistributing

example: R1 run SPF cost of 5 to reach ABR, ABR runs SPF of 10 to reach ASBR, ASBR run cost of 15 to reach external route
R1's one cost to reach the external is 5+10+15

so if you look @ the topology LSA type 4 describe how to reach the ASBR and type 5 LSA provide the cost to the outside network



TO VERIFY : sho ip ospf database external [link ID]

DR / BDR / DROther

• Desinated Router ( DR )

1. used on broadcast links to minimize adjacencies and LSA replication

• Backup Designated Router  ( BDR )

1. Used for redundancy of DR

• DROthers

1. For all other routers on the network
2. form adjacency w/ DR & BDR
3. Stop at 2-way adjacency with each other

• The DR and BDR are chosen through election process  

• DROther send LSUs to DR/BDR via multicast 224.0.0.6 
• DR forwards LSUs to DROthers via multicast 224.0.0.5 
• Prevents repeated forwarding of unneeded LSAs on the network
• BDR does not forward LSUs , it only waits for DR to fail
  

OSPF Network Types

• Broadcast ==> Ethernet
• Non Broadcast ===> ATM or Frame-relay
• Point - to - Point ===> HDLC or PPP
• Point-to-Multipoint ===> work around for ATM and Frame-relay design
• Point-to-Multipoint Non Broadcast ===> work around for ATM and Frame-relay design 
• loopback ==> used for software loopbacks