OSPF and Higher sequence

[auosdavid2000]

I read (for BSCI exam)when each router receives the LSU and the entry already exists but the LSA have a higher sequence number the router adds the entry to its LSDB.

My question is why router adds this LSA that already exists (it was duplicated), why just update it with a new version.

BR,

Auos.

[Giuseppe Larosa]

Hello Auos,

each LSA is identified using the following fields:

LSA ID, advertising router Router-ID, sequence number and age.

An LSA leaves for 3600 seconds and it is refreshed by originator router every 1800 seconds (30 minutes)

When receiving a new LSA with same LSA ID, same advertising router and with a new sequence number this means this is the new version of this LSA and it is a replacement for the previous one.

So the new version of the LSA has a new sequence number and a new age.

Hope to help

Giuseppe

[auosdavid2000]

Many thanks for your great explanation, but the author said add LSA with higher sequence number not replacement, and that make me confuse

Br,

Auos.

[Giuseppe Larosa]

Hello Auos,

if you have access to a lab made of real devices or using dynamips you can see what happens by yourself

you need to use

sh ip ospf database with one of the option to see the detail of an LSA.

Or report the whole sentence here: there are 4 or 5 editions of BSCI books

Hope to help

Giuseppe

[auosdavid2000]

Many thanks for your great explanation, but the author said add LSA with higher sequence number not replacement, and that make me confuse

Br,

Auos.