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.

The exhibit shows a BGP peering scenario between three routers: router1 and router2 are part of the same AS (AS65000), while the SP router is in a different AS (AS65101). This indicates an EBGP (External BGP) peering between the SP router and router1, and IBGP between router1 and router2.

Step-by-Step Breakdown:

Next-Hop Behavior in BGP:

IBGP: In IBGP, the next-hop address is not modified when advertising routes within the same AS. Thus, when router1 advertises routes learned from router2 to the SP router, it will keep the next-hop address of router1, not router2.

EBGP: In EBGP, the next-hop address is modified. When router1 receives routes from the SP router, it will advertise them to router2 with the next-hop address of router1.

Route Propagation:

Routes received by router1 from router2 will be advertised to the SP router with router1 as the next hop.

Similarly, routes advertised by the SP router will be passed on to router2, with router1 remaining as the next hop.

Juniper Reference:

BGP Next-Hop: Juniper's BGP implementations follow standard BGP next-hop behavior, where the next-hop is modified in EBGP but not in IBGP, ensuring proper route advertisement across autonomous systems.

In OSPF (Open Shortest Path First), areas are used to segment a network into smaller, more manageable pieces to improve scalability. By dividing a network into areas, OSPF can reduce the size of the link-state database (LSDB), which helps routers process updates more efficiently.

Step-by-Step Breakdown:

Purpose of OSPF Areas:

OSPF areas allow for hierarchical routing within the OSPF domain. Routers in the same area have identical LSDBs, but routers in different areas do not exchange full link-state information. Instead, they exchange summarized routes, which reduces the LSDB size and CPU/memory usage.

Benefits:

Reducing the LSDB size improves scalability and ensures faster convergence in larger networks. Area 0 is the backbone area, and all other areas must connect to it, forming a hierarchical structure.

Juniper Reference:

OSPF Configuration: Areas in OSPF are configured to optimize network performance by limiting the scope of link-state advertisements (LSAs) to within an area.

The source MAC address in the Ethernet header is used to populate the bridging table (also called the MAC address table) on a switch. When a frame arrives at a switch, the switch examines the source MAC address and records it along with the ingress port in its MAC address table.

Step-by-Step Breakdown:

Learning Process:

When an Ethernet frame arrives on a switch port, the switch looks at the source MAC address and adds this MAC address to the MAC table along with the port it was received on. This process is called MAC learning.

Purpose:

The switch uses this information to determine the correct port to send frames destined for that MAC address in future transmissions, thus ensuring efficient Layer 2 forwarding.

Juniper Reference:

Ethernet Switching: Juniper switches use source MAC addresses to build and maintain the MAC address table, which is essential for Layer 2 switching.

An IP fabric is a network architecture commonly used in data centers to provide scalable, high-throughput connectivity using a spine-leaf topology.

Step-by-Step Breakdown:

Layer 3 Routing Protocol:

An IP fabric relies on a Layer 3 routing protocol, typically BGP or OSPF, to provide routing between the leaf and spine switches. This ensures efficient traffic forwarding across the network.

Single Connection Between Spine and Leaf:

In an IP fabric, each leaf switch connects to every spine switch with a single connection. This ensures that traffic between any two leaf switches can travel through the spine layer in just two hops.

Juniper Reference:

Spine-Leaf Design: Juniper's IP fabric implementations are designed for scalability and low-latency routing, often using protocols like BGP for Layer 3 control.