Correct Answer is A. The two routers fail to form a neighbor relationship because they have different IS-IS area types.
Router Type L1 with L1/2 successfully form Adjacency if Area Id Matches, else no Adjacency
Let me explain something:
"The last two numbers of the NET are the NSEL; they are always 00 on a router. The next 12 numbers (separated into 3 groups of 4 numbers) are the system ID. On Cisco routers, the system ID is always this length—6 bytes. Anything to the left of the system ID is the area ID."
"System ID—This field is 6 octets long and immediately follows the area address. When the IS operates at Level 1, the system ID must be unique among all the Level-1 devices in the same area. When the IS operates at Level 2, the system ID must be unique among all devices in the domain."
https://www.ciscopress.com/articles/article.asp?p=101756
A is correct. B is incorrect.
https://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-is-is/200293-IS-IS-Adjacency-and-Area-Types.html
Seems even after 1 years this question is heavily debated. I can say with complete certainty that A is correct. From a lab I present the following debug output.
R1#debug isis adj-packets
IS-IS Adjacency related packets debugging is on for router process null
*Dec 10 20:50:30.367: ISIS-Adj: Rec L2 IIH from 5000.0004.0000 (GigabitEthernet0/0), cir type L1L2, cir id 1111.1111.1112.01, length 1497, ht(30)
*Dec 10 20:50:30.367: ISIS-Adj: is-type mismatch
*Dec 10 20:50:30.571: ISIS-Adj: Rec L1 IIH from 5000.0004.0000 (GigabitEthernet0/0), cir type L1L2, cir id 1111.1111.1112.01, length 1497, ht(30)
*Dec 10 20:50:30.571: ISIS-Adj: Area mismatch, level 1 IIH on GigabitEthernet0/0
*Dec 10 20:50:32.341: ISIS-Adj: Sending L1 LAN IIH on GigabitEthernet0/0, length 1497
One of the most debated questions.
at first glance option "A" seems correct, but the question says, different IS-IS area types.
Router Type L1 with L1/2 successfully form Adjacency if System ID area Matches.
option D says, system IDs are different.
R1 System ID area - 0012
R2 System ID area - 0022
The answer correct is D
I came to this conclusion, testing in a Lab.
Let me correct myself. System ID only has to be unique when they are in the same Area #. Since the area# is different, it doesn't even matter if system ID is the same or not.
OMG... before you say the answer is B based on Cisco documentation of L1L2 can form an adjacency with L1... the same rule as OSPF applies, you have to be in the same area... read the rest of the document before you just pull that bit of information. The CLNS ID or what they call the system ID says what area it belongs, they don't match 12 and 22 are not the same.
B is correct
And I quote the technical document from Cisco:
"A Level 1 router can become adjacent with the Level 1 and Level 1-2 (L1/L2) router. A Level 2 router can become adjacent with Level 2 or Level 1-2 (L1/L2) router. There is no adjacency between L1 only and L2 only router."
https://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-is-is/200293-IS-IS-Adjacency-and-Area-Types.html
I'm sorry, you are wrong. You may need to put some more research into this topic. R1 is configured to form an L1 adjacency only. As per the "technical documentation" you have provided "L1 Adjacency if Area ID Matches, else no Adjacency" you can quite clearly see here that the Area ID does not match between these routers (R1 Area - 0012, R2 Area - 0022).
If R1 had been configured 'is-type L1/L2' (which is the default) an L2 adjacency would form, but this is not the case... Therefor A is the answer, no adjacency will form.
You can NOT form a level 1 adjacency between two different IS-IS areas, this would require a Level 2 Backbone Adjacency for Inter-area Area. Level 1 Adjacency is only for Intra-area.
R1 is configured for Level 1 only and both routers have different Area IDs.
Hi! Sorry the correct answer is A, R1 & R2 will not be able to form an Adjacency as R1 is configured for Level 1 only and both routers have different Area IDs.
R1 Area ID - 0012
R2 Area ID - 0022
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