When a BGP session is Active, it means the local router is trying to initiate a TCP connection. If the TCP handshake succeeds (port 179), the next step is BGP OPEN message exchange. A mismatched AS number in the OPEN message is a common cause for the session to be torn down immediately after the TCP connection, keeping the local state in Active as it retries. Hold timer mismatches typically allow the session to become Established first. Blocking UPDATEs or MTU issues would affect an already Established session.

For an OSPF virtual link to function, the router configured as one end of the link must be an ABR that connects the transit area (area 1 in this case) to the backbone area (area 0). This is a core rule in OSPF multi-area design. The virtual link is a tunnel through the transit area, and its endpoints must be ABRs to provide the logical connection back to area 0. The other options are incorrect: Router IDs must be unique, a full mesh in the transit area is not required, and virtual links are logical tunnels, not configured on physical area 0 interfaces.

In an EVPN-VXLAN control plane, MAC/IP reachability information (Type 2 routes) is distributed using Multiprotocol BGP (MP-BGP). The spine devices typically act as BGP route reflectors. For Leaf-1 to know the MAC address of a host on Leaf-4, it must receive a BGP EVPN route from Leaf-4, advertised via the spines. While an IGP is needed for underlay reachability, it does not carry MAC information. VXLAN encapsulation is the data plane mechanism, and LACP is a host-facing link aggregation protocol.

For Filter-Based Forwarding to work, the matching firewall filter must be applied as an input filter on the ingress interface (or in a loopback filter applied to the forwarding plane). This is the crucial step that intercepts the packet on arrival, performs the lookup in the custom routing table based on the filter action (`routing-instance`), and forwards it accordingly. Applying it as an output filter or manipulating routes between tables does not trigger the initial policy-based lookup for incoming packets.

The `keepalive-time` specifies the interval (in seconds) at which the primary Routing Engine sends keepalive or heartbeat messages to the backup Routing Engine. If the backup RE does not receive these messages within the `hold-time` interval (typically a multiple of the keepalive-time), it assumes the primary has failed and initiates a switchover. It is a core parameter for maintaining high availability between the REs, not related to BGP or RE-PFE communication.

On Junos, the `log` keyword is used within a firewall filter term’s action (like `accept`, `discard`, or `reject`) to generate a system log entry for packets matching that term. `then accept log;` is the correct action to permit the traffic and create a log entry. `count` is for counting packets, not logging details. `syslog` is not a valid filter action. `sample` is used for traffic sampling/mirroring (e.g., for cflowd), not for session logging.

The `bfdd` process is responsible for the Bidirectional Forwarding Detection protocol. BFD sessions run with very fast, sub-second timers to provide rapid link/neighbor failure detection. If BFD is enabled on a large number of interfaces or routing protocol sessions (like OSPF or BGP), the `bfdd` process must generate and process a high volume of control packets, leading to increased CPU load. The other options relate to different processes (`dfwd` for filters, `pimd` for multicast, `ntpd` for NTP).

IS-IS has a strict hierarchy: Level 1 (L1) and Level 2 (L2). By default, a default route originated with `default-information originate` is only advertised within the same level. If R2 is an L2 router originating the default, it will only be advertised to other L2 neighbors. If R1 is an L1/L2 router but its adjacency with R2 is only L1, it will not receive the L2-originated default route. The route must be explicitly leaked between levels if needed. IS-IS does support default routes, redistribution is not required, and while a metric can be set, its absence is not the primary issue here.

For a Link Aggregation Group (LAG) to form, all member links must have identical characteristics, including MTU (Maximum Transmission Unit). A mismatch in MTU is a fundamental parameter difference that will cause the aggregated interface to fail. LACP modes of ‘active’ and ‘passive’ are compatible and will form an aggregation. Different IP addresses are irrelevant for a Layer 2 LAG. Layer 2 encapsulation is typically uniform (like 802.1Q) across a trunk and not a per-link setting that would break LACP negotiation.

When a BGP session is Active, it means the local router is trying to initiate a TCP connection. If the TCP handshake succeeds (port 179), the next step is BGP OPEN message exchange. A mismatched AS number in the OPEN message is a common cause for the session to be torn down immediately after the TCP connection, keeping the local state in Active as it retries. Hold timer mismatches typically allow the session to become Established first. Blocking UPDATEs or MTU issues would affect an already Established session.0