350-501 · Question #6
Which two IS-IS parameters must match before two Level 2 peers can form an adjacency? (Choose two )
The correct answer is A. authentication settings D. MTU. Before two IS-IS Level 2 peers can form an adjacency, their authentication settings and the Maximum Transmission Unit (MTU) on their connecting interfaces must match.
Question
Which two IS-IS parameters must match before two Level 2 peers can form an adjacency? (Choose two )
Options
- Aauthentication settings
- Barea ID
- Csystem ID
- DMTU
- Ehello timer setting
How the community answered
(16 responses)- A88% (14)
- B6% (1)
- C6% (1)
Why each option
Before two IS-IS Level 2 peers can form an adjacency, their authentication settings and the Maximum Transmission Unit (MTU) on their connecting interfaces must match.
If authentication is configured, both IS-IS peers must have matching authentication types and keys to successfully exchange Hellos and prevent unauthorized routers from forming adjacencies.
Level 2 IS-IS routers, while part of the same routing domain, do not require matching Area IDs to form an adjacency, as Level 2 routing is inter-area.
The System ID uniquely identifies an IS-IS router and must be unique, not matching, between peers.
Similar to OSPF, IS-IS includes the interface MTU in its Hello packets, and a mismatch will prevent the adjacency from forming as it signals a potential issue with packet handling.
Mismatched hello timers can cause instability but do not inherently prevent the initial formation of an IS-IS adjacency.
Concept tested: IS-IS Level 2 adjacency requirements
Source: https://www.cisco.com/c/en/us/td/docs/ios/iproute_isis/configuration/guide/irs_isis_book/irs_config_isis.html#wp1026645
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