JN0-650 Exam Dumps : Enterprise Routing and Switching - Professional (JNCIP-ENT)

In Junos OS 24.4, Captive Portal is used as a web-based authentication method for Layer 2 network access control, often in environments where 802.1X is not feasible for all users.

Fallback Mechanism (Option D): On EX Series switches, Juniper supports a flexible authentication order. By default, the switch attempts authentication in the order of 802.1X, then MAC RADIUS, and finally Captive Portal. If a client fails both 802.1X and MAC RADIUS, the switch can fall back to Captive Portal to redirect the user to a login page.

MAC Allowlist (Option B): Captive Portal relies on intercepting HTTP/HTTPS traffic to redirect users. However, "headless" devices like printers or cameras lack web browsers and cannot interact with the portal. To accommodate these, Junos allows administrators to configure an authentication allowlist (or whitelist), which identifies these devices by their MAC addresses and permits them to bypass the portal entirely.

Precedence (Option A): This is incorrect because Captive Portal is generally the last method in the default sequence, not the first.

Layer 2 Participation (Option C): While Captive Portal requires a Layer 3 interface (RVI/IRB) for the redirection process, it is explicitly used to control Layer 2 access on EX Series switches.

VSTP (VLAN Spanning Tree Protocol) is Juniper's implementation that provides a separate spanning tree instance for each VLAN, ensuring compatibility with Cisco's PVST+. However, it has significant scaling limitations:

Instance Limits: On many Juniper EX and QFX series switches, VSTP is restricted to a maximum of 253 or 510 VLAN instances depending on the software version (ELS vs. non-ELS). In your scenario, having 450 VLANs exceeds the standard 253-instance limit found on many platforms.

The Solution (Option B): When the number of VLANs exceeds the VSTP capacity, the recommended best practice is to enable RSTP (Rapid Spanning Tree Protocol). Unlike VSTP, RSTP runs a single spanning tree instance for the entire switch, regardless of how many VLANs are configured. This ensures that all 450 VLANs are protected from loops without hitting the hardware's instance-count limit.

Other Options: Option A (force-version) only affects the BPDU format for compatibility but doesn't solve the instance limit. Option C and Option D are parameter tuning actions that do not address the architectural limitation of the number of running instances.

Junos OS uses specific routing tables for handling multicast traffic to ensure loop prevention through Reverse Path Forwarding (RPF) checks:

Multicast RPF (Option C): The inet.0 table is the default unicast routing table. In most standard multicast deployments, Protocol Independent Multicast (PIM) performs its RPF checks against this table to determine the upstream interface toward the multicast source.

Topology Differences (Option A): The inet.2 table is specifically designed for multicast RPF lookups when the multicast forwarding topology differs from the unicast forwarding topology. This is common in environments using Multiprotocol BGP (MBGP) where you might want multicast traffic to follow a different path than standard data traffic.

inet.1 Role (Option B): The inet.1 table is the multicast forwarding cache. It stores (S,G) and (*,G) entries for actual packet forwarding. Routing protocols like MBGP or IS-IS do not place routes directly into inet.1; instead, PIM uses information from inet.0 or inet.2 to populate inet.1.

IS-IS and OSPFv3 (Option D): These protocols place their IPv4/IPv6 unicast routes into inet.0 or inet6.0. They do not place routes into inet.2 "directly" unless specific RIB-group configurations or address families are enabled to share those routes for multicast RPF purposes.