Which of the following descriptions of IS-IS Hello messages are correct? (Multiple choice)

Question

Options

• AP2P IH does not have a Priority field
• BLevel-1 LAN IHHT contains System ID
• CLevel-2 LAN IH contains systemid
• DP2P IH and LAN IH are exactly the same

• A
  70% (16)
• B
  9% (2)
• D
  22% (5)

Explanation

A is correct because Point-to-Point IS-IS Hellos (P2P IIH, PDU type 17) are sent on links where only two routers exist, so there is no Designated Intermediate System (DIS) election - and therefore no Priority field is needed. The Priority field only appears in LAN IIH PDUs (types 15 and 16) where a DIS must be elected.

C is correct because the Level-2 LAN IIH (PDU type 16) does include a System ID in its Source ID field, which identifies the originating router - this is standard for both Level-1 and Level-2 LAN Hello messages.

B is wrong because the PDU type is incorrectly named "IHHT," which is not a valid IS-IS PDU type - the correct term is "IIH" (IS-IS Hello). This makes the statement factually invalid regardless of the underlying claim, and serves as a classic distractor that tests whether you know the correct IS-IS terminology.

D is wrong because P2P IIH and LAN IIH differ structurally in several ways: P2P IIH lacks a Priority field and a LAN ID field, while LAN IIH includes both; they also carry different PDU type codes (17 for P2P vs. 15/16 for LAN Level-1/Level-2).

Memory tip: Think "P2P = no election, no Priority" - since there are only two routers on a point-to-point link, there's no need to vote for a DIS, so the Priority field is simply absent.

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