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In legacy hierarchical network design, three key layers are used to create a scalable and structured network:

Step-by-Step Breakdown:

Access Layer:

The access layer is where end devices, such as computers and IP phones, connect to the network. It typically involves switches that provide connectivity for devices at the edge of the network.

Aggregation Layer (Distribution Layer):

The aggregation layer (also called the distribution layer) aggregates traffic from multiple access layer devices and applies policies such as filtering and QoS. It also provides redundancy and load balancing.

Core Layer:

The core layer provides high-speed connectivity between aggregation layer devices and facilitates traffic within the data center or between different network segments.

Juniper Reference:

Legacy Hierarchical Design: Juniper networks often follow the traditional three-layer design (Access, Aggregation, and Core) to ensure scalability and high performance.

In Junos BGP configuration, export policy can be applied at multiple levels, including the BGP group level and the individual neighbor level. When an export policy is configured directly under a specific neighbor, that policy is explicitly associated with advertisements sent to that neighbor and is therefore the policy that applies to that neighbor. In the exhibit, neighbor 172.16.1.101 has an export statement configured within the neighbor stanza, and that statement references the advertise-ospf policy. This is the most specific policy attachment in the configuration because it is tied to a single peer rather than to the entire group or the whole BGP process.

Group-level export policies, such as advertise-aggregate configured under the group, apply broadly to all neighbors in that group unless a neighbor has additional neighbor-specific policy. A BGP-level export policy configured outside the group applies as a more general export policy scope. However, the question asks which export policy applies to the specific neighbor, and the correct selection is the neighbor-specific export policy that is explicitly configured for that peer.

Operationally, this allows data center engineers to advertise different route sets to different peers, for example exporting OSPF-derived infrastructure routes to one neighbor while exporting aggregates or static routes to another, all while keeping a consistent group template for shared settings such as peer AS and session type.

An access port is a Layer 2 switch interface mode intended for an endpoint that belongs to a single VLAN. Traffic on an access port is associated with exactly one VLAN, and frames are typically transmitted and received untagged on the wire. The switch internally maps that untagged traffic into the configured access VLAN, placing the endpoint into the correct broadcast domain. This behavior is widely used for server access, management ports, out of band devices, and any endpoint that does not tag VLANs.

By contrast, trunk ports are designed to carry multiple VLANs simultaneously, usually with 802.1Q tagging, and are typically used between switches, to routers, to virtualization hosts, or to appliances that handle multiple VLANs. That is why assigning multiple VLANs to an access port is not the standard access mode behavior.

An access port does not have to connect to a router or a firewall. It can connect directly to any Ethernet endpoint. Routing between VLANs is provided by a Layer 3 interface such as an IRB interface on the switch or an external routed device, but that is independent of whether the endpoint connects on an access port.

Verification sources from Juniper documentation

https://www.juniper.net/documentation/us/en/software/junos/multicast-l2/topics/topic-map/bridging-and-vlans.html

https://www.juniper.net/documentation/us/en/software/junos/multicast-l2/topics/task/interfaces-configuring-ethernet-switching-access.html

To intentionally drop traffic to a specific destination while also informing the sender that the destination is unreachable, Junos provides a routing action specifically meant for this behavior: a static route with a next hop of reject. A reject route installs a forwarding entry that causes matching packets to be dropped, and it also triggers generation of an ICMP unreachable message back to the traffic source. This is useful in data center routing for controlled blackholing with feedback, for example when you want to signal misrouted traffic, invalid destinations, or to enforce policy with explicit notification.

A discard route also drops packets, but it does so silently without sending ICMP unreachable messages. That makes discard appropriate for traffic-sink use cases such as safe summarization or DDoS mitigation where you do not want to generate return traffic. Adding addresses to martians is intended to block invalid special-use prefixes from being treated as routable, not to create a targeted unreachable response for an arbitrary destination. Using inet.3 is related to MPLS label-switched forwarding and does not address the requirement. A firewall filter that discards traffic will typically drop silently unless explicitly configured to send rejects in a security context, and it is not the standard routing-table-based method when the requirement is an ICMP unreachable response for a destination prefix.

Therefore, a reject static route is the correct solution.

In a Virtual Chassis (VC) configuration, multiple Juniper switches are interconnected to form a single logical device. Each member switch in the Virtual Chassis plays a specific role.

Step-by-Step Breakdown:

1.     Line Card Role:

o   Member switches acting as line cards provide additional ports for traffic forwarding but do not perform control or routing functions. These switches depend on the routing engine to handle control-plane tasks.

2.     Routing Engine Role:

o   A switch in the routing-engine role is responsible for control-plane operations such as routing protocol management and control of the Virtual Chassis.

3.     Virtual Chassis Roles:

o   Master Routing Engine: Handles control-plane functions and manages the entire Virtual Chassis.

o   Backup Routing Engine: Takes over if the master fails.

o   Line Card: Provides additional ports and handles data-plane operations.

Juniper Reference:

·        Virtual Chassis: In a five-member Virtual Chassis, multiple switches act as line cards, while one or more switches are designated as the routing engines (master and backup).

On Junos-based Ethernet switching platforms used in data centers, Layer 2 forwarding is handled primarily in the forwarding plane. When a switch receives a frame, it first performs source MAC learning. If the source MAC address is not already present in the Ethernet switching table for that VLAN or bridge domain, the switch creates a new entry that maps the source MAC to the ingress interface and the associated VLAN context. This learning step is fundamental to building the MAC table dynamically and enables efficient forwarding for subsequent frames destined back to that source.

Next, the switch attempts to forward the frame based on the destination MAC lookup in the same VLAN or bridge domain. Because the destination MAC is also not in the Ethernet switching table, the frame is treated as an unknown unicast. The default behavior for unknown unicast in a Layer 2 broadcast domain is to flood the frame out all other interfaces that belong to that VLAN or bridge domain, excluding the ingress interface. Flooding ensures the frame has the best chance of reaching the correct destination host. When the destination responds, the switch then learns that MAC address as a source on the return traffic, allowing future traffic to be forwarded as known unicast instead of flooded.

The switch does not drop the frame by default, and it does not forward the frame to the Routing Engine because this is normal Layer 2 bridging behavior, not a control-plane routing decision.

BGP uses a small set of well-defined message types to form and maintain peerings and to exchange routing information. The Open message is used during session establishment after the TCP connection is up. It communicates the parameters required to form the BGP session, such as the BGP version, the autonomous system number, the negotiated hold time, the BGP identifier, and optional capabilities. Capabilities are especially important in data center designs because they enable features such as 4 byte ASNs, route refresh, and EVPN signaling when applicable.

The Update message is the core mechanism BGP uses to advertise reachability and to withdraw routes that are no longer valid. In a data center underlay using EBGP, Update messages carry the prefixes that represent loopbacks and point-to-point links, enabling leaf and spine reachability. In an EVPN control plane, Update messages carry EVPN Network Layer Reachability Information to distribute MAC and IP reachability and multihoming information across the fabric.

Hello is not a BGP message type. Hello is commonly associated with protocols like OSPF, IS-IS, and some discovery mechanisms. LSA is not a BGP message type either; Link State Advertisements are specific to OSPF.

An IP fabric underlay is the routed foundation of a modern leaf-spine data center. Its purpose is to provide scalable, deterministic Layer 3 reachability between all fabric nodes, typically using point-to-point routed links between leaves and spines. In this design, EBGP is commonly used as an underlay routing protocol because it scales well, supports clear policy boundaries, and enables fast convergence and operational simplicity. Each leaf forms EBGP sessions to each spine, advertising loopback addresses and link subnets so that overlay endpoints and control plane services can reach one another reliably.

RSTP is a Layer 2 spanning tree mechanism and is not the standard protocol for a routed underlay. EVPN is an overlay control plane used to distribute tenant reachability and multihoming information; it is not the underlay routing protocol itself. VXLAN is a data plane encapsulation used by the overlay to transport Layer 2 segments across a Layer 3 fabric; it also is not the underlay routing protocol.

In Juniper data center architectures, the underlay is intentionally kept simple and purely routed, while overlays such as EVPN VXLAN deliver multi-tenant Layer 2 and Layer 3 services on top of that underlay. EBGP fits the underlay requirement among the provided options.

To confirm that your router is sending a specific route to a particular BGP neighbor, you must inspect the outbound advertisements toward that neighbor. In Junos, the command that shows routes being advertised to a peer is show route advertising-protocol bgp with the neighbor address specified. Adding 0.0.0.0 to the command filters the output to the default route, making it the most direct way to validate that the default route is actually being exported to peer 10.100.25.6. This is especially important in data center deployments where default route advertisement is often controlled by policy, conditional origination, or specific export terms, and where you want to verify the real operational result rather than just configuration intent.

The receive-protocol variant shows what you are learning from that neighbor, not what you are sending to it. The show route protocol bgp 0.0.0.0 command only confirms that the default route exists in your local routing table as a BGP-learned route, which does not prove it is being exported to the neighbor. The show route protocol static 0.0.0.0 command would confirm the presence of a static default route locally, but again it does not confirm that it is being advertised over BGP. Therefore, the outbound advertisement command is the correct verification method.

The exhibit shows a Folded IP Clos Architecture, which is also referred to as a 3-stage Clos network design. This architecture typically consists of two layers of switches:

Spine Layer: The top row of switches.

Leaf Layer: The bottom row of switches.

Step-by-Step Breakdown:

Clos Architecture:A 3-stage Clos network has two types of devices: spine and leaf. In this design, each leaf switch connects to every spine switch, providing a high level of redundancy and load balancing.

Stage Explanation:

Stage 1: The first set of leaf switches.

Stage 2: The spine switches.

Stage 3: The second set of leaf switches.

The Folded Clos architecture shown here effectively "folds" the 3-stage design by combining the ingress and egress leaf layers into one, reducing it to two visible layers, but still maintaining the overall 3-stage architecture.

Juniper Reference:

IP Clos Architecture: The 3-stage Clos design is commonly used in modern data centers for high availability, redundancy, and scalability.