In Junos, the default export policy for OSPF is to reject all routes from being exported.

Step-by-Step Breakdown:

Default Export Policy : By default, OSPF in Junos does not export any routes to other routing protocols or neighbors. This is a safety mechanism to prevent unintended route advertisements.

Custom Export Policies :

If you need to export routes, you must create a custom export policy that explicitly defines which routes to advertise.

Example: You can create an export policy to redistribute static or connected routes into OSPF.

Juniper Reference:

OSPF Export Behavior : In Juniper devices, the default policy for OSPF is to reject route advertisements unless explicitly configured otherwise through custom policies.

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.

In a leaf spine IP fabric, spines form the high-speed transit layer and leaves provide the attachment points for servers, services, and edge connectivity. The defining physical topology rule is that every spine connects directly to every leaf. This design creates consistent one-hop transit through a spine for traffic between any two leaves, which keeps latency predictable and simplifies capacity planning. It also enables equal-cost multipath routing across all available spine links, allowing the fabric to use bandwidth efficiently and recover quickly from failures by shifting traffic to remaining paths.

Spine-to-spine connections are not required in a classic two-tier leaf spine fabric. Adding spine-to-spine links can create unnecessary complexity and does not improve the standard forwarding model, because spines are intended to provide transit between leaves, not to act as an additional meshed layer. Likewise, there is no inherent requirement that each spine must have two or more physical links to each leaf. Many fabrics start with one link per spine-leaf pair and scale capacity by adding more spines or adding additional parallel links as demand grows. The redundancy objective is achieved primarily by multiple spines and multiple available routed paths, not by mandating multiple links between every spine and every leaf from the outset.

A modern IP fabric in a leaf spine topology is built to deliver predictable latency, high bandwidth, and horizontal scale. The defining characteristic is that every leaf switch connects upstream to every spine switch. This full mesh between leaf and spine layers creates multiple equal cost paths between any two endpoints connected anywhere in the fabric. With this design, east west traffic between servers attached to different leaves can traverse the fabric using one spine hop, keeping path length consistent and enabling efficient load sharing across links.

Leaf to leaf direct connections are not used in a standard leaf spine IP fabric because they create irregular topologies, complicate troubleshooting, and undermine the uniform multi path behavior that makes fabrics scalable. Similarly, a leaf does not inherently require two or more physical connections to each spine. Many designs start with a single link per leaf spine pair and increase capacity by adding additional parallel links as needed. Redundancy is achieved by having multiple spines and multiple equal cost paths, not by mandating multiple links to every spine from day one.

Server connectivity requirements also vary. Some servers use single uplinks, others use dual homing for redundancy. That decision is independent of the fundamental requirement that each leaf should connect to every spine.

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).

In OSPF, routers exchange link-state information in different stages to establish full adjacency. The MTU size is checked during the Exchange state.

Step-by-Step Breakdown:

1.      OSPF Adjacency Process :

o    OSPF routers go through multiple stages when forming an adjacency: Down , Init , 2-Way , ExStart , Exchange , Loading , and Full .

2.      Exchange State :

o    During the Exchange state, OSPF routers exchange Database Description (DBD) packets to describe their link-state databases. The MTU size is checked at this stage to ensure both routers can successfully exchange these packets without fragmentation.

o    If there is an MTU mismatch , the routers may fail to proceed past the Exchange state.

Juniper Reference:

·         MTU Checking in OSPF : Junos uses the Exchange state to check for MTU mismatches, ensuring that routers can properly exchange database information without packet fragmentation issues.

Aggregate routes and generated routes are used to create summarized routes in Junos, but they behave differently in terms of forwarding.

Step-by-Step Breakdown:

Aggregate Routes :

An aggregate route summarizes a set of more specific routes, but it does not have a direct forwarding next hop. Instead, it points to the more specific routes for actual packet forwarding.

Generated Routes :

A generated route also summarizes specific routes, but it has a forwarding next hop that is determined based on the availability of contributing routes. The generated route can be used to directly forward traffic.

Juniper Reference:

Aggregate and Generated Routes : In Junos, aggregate routes rely on more specific routes for forwarding, while generated routes can forward traffic directly based on their next-hop information.

The configuration shown is under protocols bgp and defines a BGP group named service-pod with the statement type internal. In Junos, the BGP group type determines whether the sessions formed by neighbors in that group are treated as internal BGP or external BGP. When the group is set to type internal, the router establishes an iBGP session to the configured neighbor address. That means both peers are expected to be in the same autonomous system, and the session is used to exchange routes within that single AS.

The local-address statement sets the source address that the router will use when initiating the TCP connection for BGP. In data center designs, this is commonly a loopback address so the session can be resilient and independent of any single physical interface. The neighbor statement identifies the remote peer address. With type internal, the session is iBGP even if the neighbor is directly connected, because the relationship is defined by AS scope rather than topology.

OSPF and IS-IS are different routing protocols entirely and are not established through BGP configuration. EBGP would require type external and typically involves different AS numbers across the peering. Therefore, the only correct answer is IBGP.

EBGP is defined as BGP peering between different autonomous systems, which makes statement A correct. In data center IP fabrics, it is common to assign different private AS numbers to leaves and spines or to use a structured AS design so that every leaf forms EBGP sessions to each spine. This provides clear policy boundaries, straightforward troubleshooting, and predictable route propagation without needing an additional interior gateway protocol to carry underlay reachability.

Statement C is also correct because EBGP can be deployed without a supporting IGP. BGP itself can distribute the underlay routes needed for fabric reachability, such as loopback addresses and point to point link prefixes. This is a widely used approach for leaf spine fabrics because it reduces protocol complexity and avoids running multiple control planes for the underlay. Convergence can be improved using multipath, rapid failure detection mechanisms, and consistent routing policy.

Statement B is incorrect because BGP within a single AS is IBGP, not EBGP. Statement D is incorrect because while some designs may choose to run an IGP and use BGP only for certain functions, EBGP does not inherently require an IGP to operate or to provide underlay connectivity in a fabric design.

A data center underlay IP fabric is a routed leaf-spine network designed to provide scalable Layer 3 connectivity between all fabric nodes. A key property of these fabrics is that there are multiple equal-cost paths between any two endpoints, typically across multiple spine devices. Equal-Cost Multi-Path load balancing is used to distribute traffic across those parallel paths. The routing table installs multiple next hops for the same destination prefix, and the forwarding plane selects an egress link per flow using a hash, which keeps packets in-order within a flow while using the fabric’s aggregate bandwidth. This makes statement B correct.

Because the underlay is routed, loop avoidance is handled by the routing protocol and the fundamental properties of IP forwarding, not by spanning tree. Routing protocols compute a loop-free forwarding topology and use mechanisms like shortest-path calculation and next-hop selection so that even when multiple paths exist, traffic is forwarded along valid loop-free routes. This makes statement C correct. Statement D is incorrect because spanning tree is a Layer 2 loop prevention mechanism and is not required or desired in a routed underlay fabric. Statement A is also incorrect because traffic distribution depends on ECMP and forwarding behavior, not on all devices being the same hardware model. Mixed platforms can interoperate as long as the design accounts for capacity, features, and consistent routing behavior.