In an IS-IS network, a Level 1 (L1) router maintains a link-state database only for its own area. To route packets to destinations outside this area, it requires a default route. This default route points to the nearest Level 1-Level 2 (L1/L2) router, which acts as a gateway to the Level 2 backbone. An L1/L2 router advertises its capability to act as this gateway by setting the Attach (ATT) bit in its L1 Link State PDUs (LSPs). When other L1 routers in the area receive an LSP with the ATT bit set, they identify the sender as a path out of the area and install a default route pointing to it.

A. options bit: This is a generic term. The specific mechanism in IS-IS for identifying an L2 gateway is the Attach bit, not a general "options bit."

B. status bit: This is not a standard term used in the IS-IS protocol for identifying an L1/L2 router's gateway function.

D. overload bit: The overload (OL) bit signals that a router should not be used for transit traffic, typically during maintenance, which is unrelated to identifying an area gateway.

1. Internet Engineering Task Force (IETF) RFC 1195

"Use of OSI IS-IS for routing in TCP/IP and dual environments

" Section 3.10.2

"Generation of L1 LSPs by L1/L2 routers." The document states: "An L1/L2 router MUST set the 'attached' bit in its L1 LSPs. This bit indicates that the L1/L2 router is attached to the L2 backbone

and can be used as a default route by L1 routers in its area."

2. Cisco Systems Official Documentation

"IS-IS Design Guide

" Section: "IS-IS Areas and Levels." This guide explains: "The L1/L2 router sets a special bit in its L1 LSP called the attached bit. This bit tells the L1 routers that it is attached to an L2 router and can be used to reach other areas."

3. Alwayn

V. (2002). Advanced MPLS Design and Implementation. Cisco Press. Chapter 2

"IS-IS Protocol Basics

" under the section "IS-IS Hierarchical Routing

" explains that the ATT bit is set by the L1/L2 router to attract L1 routers to send it inter-area traffic.