Multi-Area OSPF LSA Types
I started off my studies today with some review of multi-area OSPF before I dive back into IS-IS, which is definitely proving to be a challenging topic. One of the toughest things for me to grasp about OSPF so far as been the different LSA types that routers use, which routers use them and which ones stay within a specific area. Below is a table to try to help make this a bit clearer in my own mind:
LSA Type 1 - Sent by All OSPF Routers - Stays within one area - Also called Router Link States
LSA Type 2 - Sent by BDR’s Only - Stays within one area - Also called Net Link States
LSA Type 3, 4 - Sent by ABR’s Only - travels outside areas - Type 3 is also called a Summary Net Link State and is a Inter-Area Summary Route - Type 4 is also called a Summary ASB Link State and is a route to the ASBR
LSA Type 5,7 - Sent by ASBR’s Only - Travels outside areas - Type 5 is also called External Link State and is a route that has been redistributed into OSPF - Type 7 only exists within a NSSA and is translated into a type 5 LSA when it leaves the NSSA, otherwise it is the same as a type 5.
LSA Type 6 - This is reserved for Multicast OSPF, which is not current supported by Cisco
Let me just write out some of those acronyms…
ABR - Area Border Router - Connects OSPF areas and has at least 1 interface in each area. Route summarization happens here.
ASBR - Autonomous System Border Router - Connects 1 routing domain to another, often with a different routing protocol. This router does redistribution of external routes into the OSPF domain.
NSSA - Not-So-Stubby-Area - Essentially, this is the same as a Stubby Area, except that it contains an ASBR and a link to an external network. Otherwise, it has only one path into the rest of the OSPF network.
LSA - Link State Advertisement - the messages that OSPF routers send each other to update their topology tables.
