A router has a loopback address of 135.77.9.254. According to IS-IS addressing conventions, what is the correct IS-IS system ID derived from this loopback address?

Introduction / Context:
Intermediate System to Intermediate System is a link state routing protocol originally designed for connectionless network service. It uses Network Service Access Point style addresses, which include an area identifier, a system identifier, and a selector. On Cisco routers, a common method to derive the system ID for IS-IS from an Internet Protocol loopback address is used to maintain consistency and make configuration easier. This question tests your understanding of how to convert a loopback address into a valid IS-IS system ID according to typical Cisco conventions.

Given Data / Assumptions:

• The router loopback address is 135.77.9.254.
• We need to convert this IPv4 address into an IS-IS system ID.
• Cisco convention is to base the system ID on the loopback, using six bytes written in dotted notation with two digit groups.
• The question provides several candidate system IDs, one of which matches this convention correctly.

Concept / Approach:
Cisco often recommends constructing the system ID by writing the IPv4 loopback address in a dotted decimal format with three digits per octet and then regrouping the digits into six two digit groups. The goal is to create a six byte system ID suitable for IS-IS. For the given address, 135.77.9.254 is written as 135.077.009.254. This sequence has twelve digits: 1 3 5 0 7 7 0 0 9 2 5 4. These digits are then grouped into six pairs: 13, 50, 77, 00, 92, and 54, which are written as 13.50.77.00.92.54. This method ensures that the system ID is derived deterministically from the loopback address and remains unique within the network.

Step-by-Step Solution:
1. Start with the IPv4 loopback address: 135.77.9.254. 2. Convert each octet to a three digit decimal representation: - 135 remains 135. - 77 becomes 077. - 9 becomes 009. - 254 remains 254. 3. Write the address as 135.077.009.254 to make all octets three digits long. 4. Remove the dots and write the digits as a continuous sequence: 135077009254. 5. Group the digits into six two digit pairs: 13, 50, 77, 00, 92, 54. 6. Insert dots between these pairs to form the system ID: 13.50.77.00.92.54.

Verification / Alternative check:
Cisco documentation and many certification guides describe this process and often use almost identical examples to illustrate how to derive a system ID from a loopback address. The resulting system ID has six bytes, which matches the requirement for an IS-IS system ID, and is easy for engineers to map back to the original IPv4 loopback. In practice, you configure this system ID as part of the Network Service Access Point address along with an area ID, such as 49.0001.13.50.77.00.92.54.00, where the final selector byte is typically zero for routers.

Why Other Options Are Wrong:
1350.7700.9254.00: This format does not respect the six byte system ID requirement and groups digits incorrectly, resulting in more or fewer than six bytes.
135.077.009.254: This is just the IPv4 loopback address padded to three digits per octet, not the six byte system ID format used by IS-IS.
01.35.07.70.09.25: This sequence rearranges the digits arbitrarily and does not follow the standard dotted two digit grouping from the padded IPv4 address.
77.13.50.00.92.54: This again rearranges the digits and does not match the deterministic method used to derive the system ID from the original loopback.

Common Pitfalls:
A common mistake is to treat each IPv4 octet as a separate byte of the system ID without padding and regrouping, which leads to a four byte identifier instead of the required six bytes. Another pitfall is to attempt hexadecimal conversions unnecessarily, when the standard method used in many exam questions is purely decimal based. To avoid confusion, remember the sequence: pad each octet to three digits, concatenate, then split into six two digit groups. Applying this method consistently makes configuration and troubleshooting of IS-IS addressing much easier in complex networks.

Final Answer:
13.50.77.00.92.54