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The skinny on CLNS vs CLNP


Alright, I'm being driven crazy by this. Even though I definitely know more than enough about IS-IS operation to be comfy on the BSCI exam, I'm vexed trying to get a better understanding anyway of the OSI protocols.

Which I notice are sometimes also called the ISO protocols.

Which one is an actual layer 3 protocol? And, how would you classify the other one?

One BSCI textbook tells me IS-IS uses CLNS to transmit routing data, even if it's IP routes it's transmitting.

Another textbook tells me IS-IS itself is a Layer 3 specification; that it sits right on top of the data-link layer and uses neither IP nor CLNS to send info, but rather its own Layer 3 format.

I know I don't need to know these things inside and out to pass the exam, but I like knowing things anyway.

I tried looking at the official ISO documents on CLNS and CLNP but it seems you have to pay money to look at those (not likely in my case).

Please shed the light for me....

7 Replies 7

Giuseppe Larosa
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Hello Suey,

IS-IS is directly over L2 and it is not transported inside a CLNP packet as happens with OSPF with IPv4.

On an Ethernet the frame should have the following aspect:


it uses 802.2 LLC (0xFE|0xFE|0x03) the last byte makes the difference between IS-IS (0x83) and CLNP (0x81) so formally they are two distinct L3 protocols.

Hope to help


Grazie, Giuseppe.

If CLNP is a layer 3 specification on its own, I guess what I really want to understand then is what is CLNS?

The Cisco Internetworking Handbook chapter on the OSI protocols points us to an ISO spec, I think it was 8473; but as I said I can't find a free copy of that document. Are there RFC's for these things as well?

(P.S. Did you really mean OSPF info is carried in CLNP packets, rather than IPv4 packets?)

Hello Suey,

here is the nice part

CLNP = is the connectionless protocol

CLNS = is the connectionless service specifications. ISO naming is a little too articulated.

Then in the common use CLNS is used to reference the protocol too.

I took most of this info from an older edition of BSCI course (actually a pdf update when IS-IS had been added to the exam).


I was meaning that OSPF is clearly carried inside IPv4 packets so it cannot be seen as L3 on its own

Hope to help



So CLNP is the L3 protocol specified by the ISO; CLNS is result of the ISO having an extra glass of chianti and running their mouths.


Hey Giuseppe, where do you get the info on which actual values in the Ethernet frame indicate which protocols?

0x83 = IS-IS


0x81 = CLNP

is very helpful, but where can I look up all the possible values (AppleTalk, Apollo, DECnet...) ?

I've checked the IEEE website but as usual they want me to pay money to look at the full 802.3 specification.

Would the values be the same for 802.11 and for 802.5 token-ring frame headers?



Hello Stuey,

Happy New Year

Ciscopress had published a supplementary chapter for IS-IS when in it was introduced in BSCI course (actually the course before was named BSCN).

See the attachment file.

the values should be the same also because IS-IS uses LLC encapsulation that is above the MAC layer.

so are three bytes

FE FE 83 is IS-IS

FE FE 81 is CLNP

sure for 802.11, 802.5 should have an LLC encapsulation too so it is 99% correct.

Hope to help


Mille grazie again Giuseppe.

I remember when the course was named BSCN - I've had my eye on this exam since it was named ACRC!!!

I'll pass it before February's over, I betcha...

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