what is the defference between LSA type 4 & LSA type 5

Answered Question
Dec 22nd, 2009

dear experts, hello

i'd like to know the defference between the LSA type 4 and LSA type 5 in the ospf LSA types because i read it in BSCI ebook (cisco press)

but unfortunatily didn't understand it, thanks alot for your response and help

Labib Makar

I have this problem too.
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Correct Answer by Muhammad Anser Khan about 6 years 11 months ago

Dear Labib,

LSA Type 4 :

# LSA Type 4 is the ASBR Summary LSA. This LSA informs other OSPF speaking routers about the existence of an ASBR. It's needed for reachability to external routes.

# LSAType 4 Sent by ASBR, but only internally. This describes to the others how to get to the ASBR itself

LSA Type 5:

# LSA Type 5 is the External LSA. This LSA informs other OSPF speaking routers about externet routes(outside from the OSPF domain)

# Used to describe external routes to internal areas.

Regards,

Anser

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Correct Answer
Muhammad Anser Khan Tue, 12/22/2009 - 07:59

Dear Labib,

LSA Type 4 :

# LSA Type 4 is the ASBR Summary LSA. This LSA informs other OSPF speaking routers about the existence of an ASBR. It's needed for reachability to external routes.

# LSAType 4 Sent by ASBR, but only internally. This describes to the others how to get to the ASBR itself

LSA Type 5:

# LSA Type 5 is the External LSA. This LSA informs other OSPF speaking routers about externet routes(outside from the OSPF domain)

# Used to describe external routes to internal areas.

Regards,

Anser

Peter Paluch Tue, 12/22/2009 - 11:50

Hello Anser,

Nice answer. Just a single correction. You wrote:

LSAType 4 Sent by ASBR, but only internally. This describes to the others how to get to the ASBR itself

A LSA4 is generated and flooded by an ABR, not by an ASBR. It is very easy to show - the routers in the same area with an ASBR do not have any LSA4 describing the ASBR. Only when the LSA5 are flooded by an ABR to a different area, the ABR also creates a LSA4 that describes the ASBR that generated the flooded LSA5.

Best regards,

Peter

Giuseppe Larosa Tue, 12/22/2009 - 12:25

Hello Anser, Peter,

just to add a little to Peter's note:

the LSA4 is not needed within the area where the ASBR is located because the node advertises its ASBR status by setting the E flag bit in its own router LSA header  that is flooded in all the area. From this information the ABRs of the area generate the LSA4 to propagate the existance of this ASBR in other areas.

Hope to help

Giuseppe

Peter Paluch Tue, 12/22/2009 - 12:41

Giuseppe,

Thank you. I sincerely value your clarifications and I am thankful for them.

Best regards,

Peter

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