OSPF LSA 4 and LSA 5

[richard.m.gilbert]

<p>Hello,</p>

<p>What is the difference between an OSPF LSA 4 and OSPF LSA 5?  Both are sent from an ASBR but I am not sure what makes them different.</p>

<p>Thanks,</p>

<p>Richard</p>

[Giuseppe Larosa · Accepted solution]

Hello Richard,

LSA type 4 are specialized summary LSAs that advertise ASBR nodes.

LSA type 4 are not domain wide so they are regenerated by ABRs.

There is a chance that even the first LSA type 4 is generated by the ABR between area 0 and the area where the ASBR node is connected as a reaction to a setting in the ASBR Router LSA header (E bit but I didn't check)  that tells it is an ASBR node.

For sure they are regenerated by ABRs at area boundaries.

In short, LSA type 4 tell what nodes are ASBR in an OSPF domain and provide routing information (path cost) to reach them using the OSPF inter-area methods.

An LSA type 5 is originated by an ASBR node and contain an IP prefix that is injected into the OSPF domain.

LSA type 5 are domain wide: they travel untouched through area 0 and all non stub areas.

LSA type 5 cannot be filtered like inter-area routes you can choice between accepting all of them or blocking all of them from entering a specific area (using the various stub concepts).

Hope to help

Giuseppe

[Giuseppe Larosa]

Hello Richard,

I've checked documentation and according to what I read (BSCI third edition and also Routing TCP/IP vol.I) I can confirm LSA type 4 are generated by ABR nodes only not only regenerated but also the original LSA should be created by the ABR(s) to which the ASBR node is connected to.

To be noted often ASBR are ABR too in real world.

Hope to help

Giuseppe

[richard.m.gilbert]

Giuseppe,

Thanks.  In the BSCI 4th edition exam cert guide it states that the AS external ASBR summary link LSA (type 4) is produced by the ASBR to advertise their presence.  But in the BSCI 3rd edition self study guide it states that Type 3 and 4 LSAs are generated by the ABR and the type 4 describes routes to ASBRs.  It goes on to state that when a ABR receives a type 1 LSA from a ASBR, it sends out a type 4 LSA to advertise the presense of the ASBR in other areas.

So ... If I understand that correctly ... the ASBR uses a type 1 LSA to let everyone in that are know about it (just like all the internal routers of that area) and the ABRs send the 3 and 4 LSAs to summarize all the external routes possible from that area.  The only LSA types that a ASBR sends is the type 1 (inside that area), the type 5 (summerize external routes), and the type 7 (used to transmit the external routes to a ABR to translate into a type 5).

I guess it is a typo in the first book I was using to study.

Richard