Question about ISIS NSAP addressing

[ngkin2010]

Dear Community,

I am taking over a network with ISIS as underlay IGP, which mainly for the connectivity between routers' loopback address.  It's working fine, but I found their NSAP's AFI is not 49 (private addressing). Instead, the AFI currently in-use is 10.

For example, the NSAP assigned to router is:

router isis
 net 10.0123.9999.9999.9999.00

I know that the NSAP format is based on AFI :

 

For example, we usually assign AFI=49 for private addressing, and such that the length of IDI=0, while the length of HO-DSP=12

AFI	Length of IDI (octets)	Length of HO-DSP (octets)	Description
49	0	12 (area number)	Private Addressing
39	2 (country code)	10 (area number)	ISO 3166
45	8 (phone number)	4 (area number)	ITU-T E.164
47	2 (Organization code)	10 (area number)	ISO 6523

My question is, how router will treat the NSAP not fall into neither category listed in the above?

For example, given an NSAP =  49.0123.9999.9999.9999.00 

Then I know, AFI is 49;  Area number is 0123; System-id is 9999.9999.9999; and SEL is 00.

But what if  NSAP =  10.0123.9999.9999.9999.00 ?

Any documentation defined the "default rule" for parsing the NSAP?

Thanks!!

[Georg Pauwen]

Hello,

not really sure what you mean by 'the "default rule" for parsing the NSAP', but is this IS-IS network somehow connected to the 'outside' ? If not, that is, if it is strictly private, the chosen area ID (10.0123 in your case) is completely arbitrary (similar to IPv4 RFC 1918 addresses), and you can chose whatever you want.

[ngkin2010]

Dear Georg,

Thanks for your reply. The IS-IS network is not connected to outside.

You mentioned there is public range and private range for IS-IS addressing, glad you could share more information about it. 

I was confused because I wrongly expect that all valid NSAP should either starts with 39 / 45 / 47 / 49: 

AFI	Length of IDI (octets)	Length of HO-DSP (octets)	Description
49	0	12 (area number)	Private Addressing
39	2 (country code)	10 (area number)	ISO 3166
45	8 (phone number)	4 (area number)	ITU-T E.164
47	2 (Organization code)	10 (area number)	ISO 6523

The "default rule" I mentioned mean if the NSAP is not starting with 39 / 45 / 47 / 49, then how router determine the IDI / HO-DSP / Area-id from the NSAP.

But seems, router doesn't care about the IDI / HO-DSP, the first  3 octets of NSAP (10.0123.9999.9999.9999.00) will always be the area id. 

The next 6 octets of NSAP will always be the system-id (10.0123.9999.9999.9999.00)

The last octet of NSAP will always be the SEL (10.0123.9999.9999.9999.00)

Am I correct..?

[Georg Pauwen · Accepted solution]

Hello,

as far as I recall, the proposed area IDs are just recommendations, just like RFC 1918 IPv4 addresses. As long as everything stays private, you can use whatever addressing you want. In IPv4, you could use eg. 1.1.1.0.24, or 163.17.0.0/28. it doesn't matter.

The proof is actually that your IS-IS networks works just fine, even with area ID 10.0123.

You are correct about the format of the address. The format needs to conform to the octet length, the numbers you use do not matter.

[ngkin2010]

hi Georg,

Got it, thanks a lot