A MAC address of FF-FF-FF-FF-FF-FF is the Ethernet broadcast address. When a switch receives a frame with this destination MAC address, it is required to forward the frame to all interfaces except the one it was received on.

Step-by-Step Breakdown:

Broadcast Frame Handling:

When a frame with the broadcast MAC address is received, the switch will flood it out of all active ports that belong to the same VLAN as the incoming frame. The broadcast frame is not sent back out of the ingress interface (the interface where the frame was originally received).

Purpose of Flooding:

Broadcasting is used to ensure that the frame reaches all devices within the broadcast domain (all devices within the same VLAN), which may not have a specific entry for the MAC address in their MAC address table.

Juniper Reference:

Layer 2 Frame Forwarding: Juniper switches flood broadcast frames to all ports in the same VLAN, except the port the frame was received on.

In the exhibit, R2 receives the same OSPF update from both R1 and R3. OSPF has mechanisms to prevent unnecessary processing of duplicate LSAs (Link-State Advertisements).

Step-by-Step Breakdown:

OSPF LSA Processing:

OSPF uses LSAs to exchange link-state information between routers. When a router receives an LSA, it checks if it already has a copy of the LSA in its Link-State Database (LSDB).

Duplicate LSAs:

If R2 has already received and processed the update from R1, it will ignore the update from R3 because it already has the same LSA in its database. OSPF uses the concept of flooding, but it does not reprocess LSAs that it already knows about.

R2 Behavior:

R2 will keep the update from R1 (the first one it received) and will ignore the same LSA from R3, as it is already in the LSDB.

Juniper Reference:

OSPF LSA Processing: Junos adheres to OSPF standards, ensuring that duplicate LSAs are not processed multiple times to avoid unnecessary recalculations.

Filter-Based Forwarding (FBF) in Junos OS allows traffic to be routed based on specific criteria such as source address, rather than just the destination address. This is useful in scenarios like policy routing or providing multiple paths for different types of traffic.

Step-by-Step Breakdown:

Instance-Type Forwarding:

You must create an instance-type forwarding routing instance. This routing instance allows for different routing tables based on the incoming packet filter.

Command:

set routing-instances FBF-instance instance-type forwarding

Match Filter:

You need to create a filter to match the traffic that will be forwarded according to your custom routing policy. This filter is applied to an interface to determine which traffic will use the custom forwarding instance.

Command Example:

set firewall family inet filter FBF-filter term 1 from source-address

In a three-stage IP fabric (also known as a Clos fabric), traffic between any two points (ingress to egress) in the fabric is only two hops away.

Step-by-Step Breakdown:

Three-Stage IP Fabric:

Leaf Layer: Leaf switches connect directly to servers and edge devices.

Spine Layer: Spine switches provide connectivity between leaf switches but do not connect to each other directly.

Two-Hop Communication:

In this architecture, every leaf switch is connected to every spine switch. Therefore, when a packet enters the fabric via an ingress leaf switch, it is forwarded to a spine switch, which then directs the packet to the correct egress leaf switch. This path always involves exactly two hops:

Ingress leaf Spine Egress leaf.

Benefits:

This consistent two-hop path ensures predictable latency and makes the network highly scalable while maintaining low complexity.

Juniper Reference:

IP Fabric Architecture: This two-hop property of Clos fabrics is a hallmark of spine-leaf designs, as supported by Juniper's QFX and EX switches in data centers.

An aggregate route is a summarized route that is created by combining multiple specific routes into a single, broader route. In Junos OS, when an aggregate route is configured, its default next hop is set to reject.

Step-by-Step Explanation:

Aggregate Route:

Aggregate routes are used to reduce the size of routing tables by representing a collection of more specific routes with a single summary route. They help improve routing efficiency and scalability, especially in large networks.

Default Next Hop Behavior:

When you configure an aggregate route in Junos OS, it has a reject next hop by default.

The reject next hop means that if a packet matches the aggregate route but there is no more specific route in the routing table for that destination, the packet will be discarded, and an ICMP 'destination unreachable' message is sent to the source.

This behavior helps to prevent routing loops and ensures that traffic isn't forwarded to destinations for which there is no valid route.

Modifying Next Hop:

If needed, the next hop behavior of an aggregate route can be changed to discard (which silently drops the packet) or to another specific next hop. However, by default, the next hop is set to reject.

Juniper Reference:

Junos Command: set routing-options aggregate route <route> reject to configure an aggregate route with a reject next hop.

Verification: Use show route to verify the presence and behavior of aggregate routes.