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type1 always send within area so if it wants to send other area in ospf we need type3 summary lsa. type1 lsa travel to abr router from there type3 lsa gen
LSA is a link state advertisement the type3 summarizes the networks behind the ABR including the external routes while type4 doesn't allow the external routes
Hi,
I see that folks have already answered this. I will just try to simplify it further.
Type3 LSA is generated by ABR, and flooded it into other areas. So if this is the scenario:
A------B-----C,
A is in area1
C is in area2
B is in area0 so it acts as an ABR.
Now, B will generate type3 LSA for A, and send it to C, and vice versa. In this way A will reach C via B, and vice versa.
Type4 LSA is also generated by ABR.
Lets extend the above topology, and add one more router, say D.
A-----B-----C----D
Suppose that, we are running eigrp between C and D. On C, we have redistributed eigrp into ospf and vice versa. In this scenario, B will generate a type4 LSA to reach C, and give this information to A.
But, wait a minute, isnt it the same thing that type3 LSA was doing? :-) A can reach C via LSA type3, so now an interesting question arises, why do we need LSA type4 when we already have a type3 LSA to reach C from A? :-)
The reason is LSA type4 is route to the "router-id". And router-id isn't necessarily always advertised in ospf. So, to play safe, creators of OSPF introduced LSA type4 which is a /32 router-id.
To summarize, type3 LSA is the actual route, and type4 LSA is reachability to the router-id (of ASBR).
Hi,
· Type3 – Summary LSA: The summary LSA is created by the ABR and flooded into other areas. The ABR is in charged to convert the intra-area LSA types to this type of LSA in order to differentiate the intra-area routes from the inter-are routes. There is one type-3 LSA per route.
· Type4 – Summary ASBR LSA: This basically identifies the ASBR of a specific area (note that inside the area where the ASBR is this LSA will not exist, instead the type-1 LSA will be used in order to announce the ASBR of the area). Who is the ASBR of the area? The type-4 LSA is there in order to answer this question, so this is injected by the ABR in order to announce the presence of the ASBR in a particular area.
Also check "https://learningnetwork.cisco.com/docs/DOC-7924" for reference.
Source: https://learningnetwork.cisco.com
LSA3 is generated by ABR to check the Reachbility information of ASBR
LSA4 is generated by ASBR for redistribution or distribution of external routes
