The Backup Designated Router exists specifically to provide immediate failover redundancy for the Designated Router role on a multi-access broadcast or NBMA segment: the BDR maintains full adjacencies with every other router on the segment concurrently with the DR, precisely so that if the DR ever fails or is withdrawn, the BDR can be promoted directly to DR essentially instantaneously, without needing to wait through a fresh, full DR/BDR election process, and a new BDR election then takes place separately among the remaining DROther routers to fill the now-vacant backup role. This immediate promotion behavior confirms the first statement as correct. The Designated Router's other core responsibility on the segment is originating the Type 2 Network LSA, which describes the multi-access network itself as a pseudonode, listing every router attached to that segment; this LSA type exists specifically to avoid the full-mesh explosion of Router LSA adjacency listings that would otherwise be needed to describe a shared broadcast segment, and only the DR — never the BDR, and never any DROther — is responsible for generating and maintaining this particular LSA under normal, stable conditions, which confirms the third statement while directly ruling out the fourth. There is no such thing as the DR 'electing' an area border router; ABR status is instead a role a router acquires organically by having interfaces in more than one OSPF area, entirely independent of any DR/BDR election process on any individual segment. Reference topics: Junos Enterprise Routing – OSPF, DR/BDR Roles and Type 2 Network LSA Origination.

The entire purpose of route aggregation is summarization — replacing a set of more-specific contributing routes with a single, broader prefix advertised outward — and this summarization inherently hides internal routing instability from external peers: if one of the underlying contributing /24 blocks within a summarized /16 flaps up and down due to a local link issue, external peers who only see the stable, unchanging /16 aggregate are completely insulated from that churn, since the aggregate itself remains active and advertised as long as at least one contributing route is present, regardless of how much internal fluctuation occurs among the individual contributors. This stability-hiding characteristic is one of the primary operational benefits that motivates using aggregate routes in the first place, confirming that statement as correct. Regarding next-hop behavior, official Juniper documentation is explicit and unambiguous: when an aggregate route is installed in the routing table, Junos assigns it a reject next hop by default, meaning traffic matching only the aggregate (and no more-specific contributor) is dropped and an ICMP unreachable message is returned to the source; a discard next hop is available only as an optional, explicitly configured alternative when silent dropping without ICMP notification is preferred, but it is not the default. Aggregate routes categorically do not advertise their individual contributing routes; suppressing those specific, more-granular prefixes from onward advertisement in favor of the single summary is the aggregate's defining function, directly contradicting the first statement. Reference topics: Junos Enterprise Routing – Protocol Independent Routing, Default Reject Next Hop and Route Summarization Benefits.

Configuring interface-type p2p under an OSPF interface tells Junos to treat that interface as a point-to-point network type regardless of the underlying physical medium, which removes the interface from the standard broadcast-network state machine entirely. On a true broadcast network, two routers must first complete a Designated Router and Backup Designated Router election, wait through the associated timers, and only then proceed through 2-Way, ExStart, Exchange, and Loading before reaching Full — a multi-step process. A point-to-point interface skips DR/BDR election altogether and proceeds directly from 2-Way into ExStart, so adjacencies form measurably faster, which is the first correct benefit. Because there is no DR on a point-to-point interface, Junos also does not generate a Type 2 Network LSA to describe the segment; the connected subnet is instead advertised as a simple stub-network entry inside the Router LSA itself, eliminating an entire LSA type from the area's link-state database and reducing its overall size, which is the second correct benefit. Point-to-point configuration is entirely incompatible with DR election, so a router configured this way cannot 'win' a DR election — there is no DR to win. Advertisement of the connected network happens on any operational OSPF interface irrespective of network type, so that option describes normal OSPF behavior rather than a benefit specific to the point-to-point setting. Reference topics: Junos Enterprise Routing – OSPF Network Types, Configuring OSPF Interface Type.

The described symptom — excessive packet drops specifically attributed to exceeding the MTU on a GRE tunnel carrying full-size video streaming traffic across default-MTU gigabit Ethernet interfaces — is a direct consequence of the 24 bytes of GRE encapsulation overhead pushing packets that were already at or near the standard 1500-byte Ethernet MTU past the physical interface's transmission limit once the outer GRE and IP headers are added. The most durable fix is to proactively increase the physical member interfaces' MTU beyond the 1500-byte default — to 1524 bytes or higher — which provides sufficient headroom to absorb the full GRE encapsulation overhead without ever causing the fully encapsulated packet to exceed the interface's transmission capability, allowing full-size 1500-byte payloads to pass through the tunnel intact and without fragmentation or drops. As a complementary measure, configuring the clear-dont-fragment-bit option on the tunnel or member interfaces instructs Junos to clear the DF (don't fragment) bit on oversized packets rather than dropping them outright when they do exceed the available MTU, allowing the Packet Forwarding Engine to fragment and later reassemble the traffic instead of silently discarding it, which directly addresses the reported packet loss even under transient conditions. Simply raising the MTU only to 1500 bytes fails to account for the 24-byte GRE overhead and would not resolve the underlying oversize condition. Routing the tunnel over a higher-bandwidth link addresses potential congestion but does nothing to correct an MTU-based packet-size violation, which is the root cause explicitly identified in this scenario. Reference topics: Junos Enterprise Routing – Tunneling, GRE MTU Sizing and Fragmentation Handling for High-Throughput Applications.

MACsec (IEEE 802.1AE) secures a point-to-point Ethernet link independently of how the logical unit above it is configured for VLAN tagging, so neither trunk mode nor access mode has any bearing on whether the MACsec Key Agreement (MKA) protocol can establish a secure channel. What does determine success is the pre-shared key material: when static CAK security mode is used, the Connectivity Association Key (CAK) and the Connectivity Association Key Name (CKN) must be configured identically on both ends of the link. If they do not match, MKA never completes the exchange, no secure channel is created, and all traffic on that interface is dropped rather than merely left unencrypted. Equally important, and frequently overlooked, is that MACsec adds up to 32 bytes of overhead per frame for the security tag (SecTAG) and integrity check value (ICV); Juniper's official guidance is to ensure the difference between the interface's physical MTU and the protocol MTU is large enough to absorb this overhead, or frames will be silently discarded once encryption is active, producing symptoms that look identical to a failed session. Both the CAK/CKN and MTU checks are documented first-line troubleshooting steps for MACsec on EX Series switches. Reference topics: Junos Enterprise Switching – Layer 2 Security, Configuring and Troubleshooting MACsec on EX Series Switches.

OSPF relies on two IANA-reserved multicast addresses to distribute its control-plane traffic across broadcast and NBMA network types, and Link State Update (LSU) packets, which carry the actual LSAs being flooded, are addressed to one or the other depending on the role of the sending router. The address 224.0.0.5, designated AllSPFRouters, is used when any OSPF router — including a router that is itself the Designated Router or Backup Designated Router — needs to flood an LSU to every OSPF-speaking router on the segment; this is the address used for Hello packets universally and for LSU flooding directed at the entire segment. The address 224.0.0.6, designated AllDRouters, is used specifically by non-DR, non-BDR routers (DROthers) when they need to send an LSU or other update destined for the DR and BDR only, since only the DR and BDR actually listen on this address; the DR then takes responsibility for reflooding that information to the rest of the segment using 224.0.0.5. This two-address design is central to how OSPF achieves flooding efficiency on multi-access networks by funneling updates through the DR rather than requiring full-mesh flooding among every router pair. The addresses 224.0.0.8 and 224.0.0.9 are not used by OSPF at all; 224.0.0.9 is in fact reserved for RIPv2 multicast updates, making it a plausible-looking but incorrect distractor. Reference topics: Junos Enterprise Routing – OSPF, Multicast Addressing for Hello and Link State Update Packets.

IS-IS (Intermediate System to Intermediate System) was designed by ISO as a link-state Interior Gateway Protocol for routing within a single autonomous system, functioning on a hierarchical two-level model conceptually similar to OSPF's areas, which makes the first statement unambiguously correct. Within that hierarchy, Level 1 routers maintain detailed topology information only for their own local area and use a default route toward the nearest Level 2 or Level 1/Level 2 router to reach destinations outside the area; they explicitly do not carry inter-area topology information and therefore cannot route traffic between areas on their own. Routers that participate in both levels simultaneously — Level 1/Level 2 (L1/L2) routers — sit at the area boundary, maintaining a Level 1 database for their local area and a Level 2 database for the backbone, and it is specifically these dual-level routers that stitch inter-area traffic together, forwarding packets from the local area into the Level 2 backbone and vice versa, which confirms the second correct statement. Regarding route preference, Junos assigns distinct default preference (administrative distance) values of 15 for IS-IS Level 1 routes and 18 for IS-IS Level 2 routes — neither of which is 20 — so the preference-value statement is factually incorrect and included as a distractor testing precise knowledge of Junos default protocol preferences. Reference topics: Junos Enterprise Routing – IS-IS, Level 1, Level 2, and L1/L2 Router Roles; Default Route Preference Values.