what is the difference btw OSPF area authentication VS OSPF interface authentication

please check this article please ..

http://www.ciscopress.com/articles/article.asp?p=1728836


my question when i configured ospf authentication per one interface like this

|---lan----router A-s0---------------------s1-- router B------ lan ----|
i make here a interface authentication...


if i have like this scenario

|---lan----router A-s0---------------------s1-- router B-s0---s1-router C--------- lan ----|
all 3 router in the same area !
if i make a area authentication i should go in each router and enable the password in only one interface ???? + add area 0 authenticationn


like on router B

i make
  Router B(config)#int s0
  Router B(config-inf)#ip ospf authentication message-digest
  Router B(config-int)#ip ospf message-digest-key 12 md5 "password"

  Router B(config-router)#area 0 authentication message-digest

no need to make for the second interface s1 for router B ..
??
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AFIF JABADOAsked:
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BigPapaGottiCommented:
The different between the two is that for interface authentication you only enable authentication for one link rather than for the whole area. Additionally if you wanted to only do one type of authentication for certain links and then use a different type (plain text or MD5) then you could use interface authentication.

Configuring Area authentication will enable authentication for the entire area. The area authentication is a quicker way to enable authentication to save you some key strokes.

Keep in mind that anything configured at the interface level will override the area configuration command.
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AFIF JABADOAuthor Commented:
mmmm in the middle router when i choose interface authentication in the s0 interface for example and than i apply area authentication  --- > no need to make authentication in the second interface of the middle router B ???

Area authentication mean in all UP interfaces in the router ?
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BigPapaGottiCommented:
No, you will still need to go underneath the other interface of router B and specify what key that interface should use.
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AFIF JABADOAuthor Commented:
so what i need to make authentication  for area ?
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BigPapaGottiCommented:
Assuming you want to use MD5 authentication between two routers connected back-to-back on S0 using area 0 authentication below are the steps you would perform to enable authentication.

R1#config t
R1(config)#int s0
R1(config-if)# ip address 192.168.1.1 255.255.255.0
R1(config-if)# ip ospf message-digest-key 1 md5 password
R1(config-if)#exit
R1(config)#router ospf 0
R1(config-router)#area 0 authentication message-digest

R2#config t
R2(config)#int s0
R2(config-if)# ip address 192.168.1.2 255.255.255.0
R2(config-if)# ip ospf message-digest-key 1 md5 password
R2(config-if)#exit
R2(config)#router ospf 0
R2(config-router)#area 0 authentication message-digest

Please note that the "area 0 authentication message-digest" command will enable authentication for all of the interfaces in area 0. So say for instance Serial0/1 was in area 0 on both of these routers you would then need to run the "ip ospf message-digest-key 1 md5 password" command on that interface and it would not be necessary to run the command "ip ospf authentication message-digest" command as mentioned below.

Alternatively you could omit the "area authentication message-digest" command and use the "ip ospf authentication message-digest" command underneath the interface instead. Again, if for instance you had Serial0/1 on both areas that were in area 0 you would need to use the "ip ospf authentication message-digest" command underneath that interface. Below would be the steps to accomplish this.

R1#config t
R1(config)#int s0
R1(config-if)# ip address 192.168.1.1 255.255.255.0
R1(config-if)# ip ospf message-digest-key 1 md5 password
R1(config-if)# ip ospf authentication message-digest

R2#config t
R2(config)#int s0
R2(config-if)# ip address 192.168.1.2 255.255.255.0
R2(config-if)# ip ospf message-digest-key 1 md5 password
R2(config-if)# ip ospf authentication message-digest
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