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Routing & Switching · Advanced

Attached Bit

A flag set in an L1/L2 router's Level 1 LSP signalling it has Level 2 connectivity, prompting L1 routers to install a default route.

Definition

L1 routers install a default route toward the nearest L1/L2 router with the ATT bit set, which may produce suboptimal paths to other areas. Route leaking or suppressing the ATT bit can influence this behavior.

Example

set-attached-bit route-map CHECK-L2

Exam tip

Suboptimal inter-area routing in IS-IS is fixed by leaking L2 routes into L1.

Related terms