Refer to the exhibit. The four routers in this network have formed adjacencies. Router R2 is an IS- IS level 1-2 router that connects to other areas, and the attach bit is set in the R2 level 1 LSP ad

Question

Exhibit

Options

• AUse the is-type level-1-2 backdoor command to unset the attach bit on R1 and use a prefix list to
• BUse the is-type level-1-2 backdoor command to unset the attach bit on R2 and configure a
• CSet the attach bit on R1 and use a prefix list to filter prefixes from R2.
• DConfigure R2 as a level 1 router and use a prefix list to filter prefixes to R1

• A
  9% (6)
• B
  84% (59)
• C
  3% (2)
• D
  4% (3)

Explanation

Option B is correct because the attach bit, when set on a Level 1-2 router like R2, causes Level 1 routers (R1) to install a default route pointing toward R2 - not specific Level 2 prefixes. To achieve the desired behavior of pushing specific routes, you must unset the attach bit on R2 (the router generating the LSP) to stop advertising the default, then use a prefix list to selectively leak Level 2 routes into the Level 1 domain so R1 receives and installs those specific prefixes.

Why the distractors fail:

• A is wrong because it targets R1, but R1 is a Level 1-only router - the attach bit originates in R2's LSP, so R1 cannot unset what R2 is advertising.
• C is wrong because R1, as a Level 1 router, has no mechanism to set the attach bit (only L1/L2 routers do), and the underlying default-route problem would remain.
• D is wrong because downgrading R2 to Level 1 severs its Level 2 connectivity entirely, destroying the inter-area design - R2 must remain L1/L2.

Memory tip: The attach bit is R2's way of saying "use me as your exit - here's a default route." To stop that and deliver specific routes instead, you must silence R2's broadcast (unset attach bit at source) and hand-deliver the mail manually (route leaking via prefix list). Always act at the source of the LSP, not the receiver.

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