Hierarchical Network

Networks could divide Router or Switch into several logical grouping#. In turn, this simplifies the routing updates as they will be confined within the grouping, and drastically reduces the traffic overhead. Notice its similarity to VLAN.

Although Hierarchical Network is a good idea for network design, we should be aware of the network diameter (number of devices that a packet need to cross before reaching its destination), bandwidth/link aggregation, and redundancy. If the network diameter is too large, it will introduce high latency for the communication. The advice is to have a predictable number of hops between the source and destination devices.

A well-designed Hierarchical Network is scalable, redundant, highly performant, secure, manageable and easier to maintain.

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Variable-Length Subnet Masking (VLSM)

We could conserve even more IP addresses by subnetting the subnetted subnet if it is still too large and there are plenty of spaces left to include more hosts after one round of VLSM. It could build up a Hierarchical Network.
TRS2251 Chapter 8: Switching Technology

Hierarchical Network#
Switched LAN Architecture

In a typical Local-Area Network (LAN)# network, it is designed as a Hierarchical Network# involving multiple Switches# with three layers: core layer, distribution layer, access layer. Access layer consists of end devices, and defines a basic configuration and constraint on them on network connection. Distribution layer is where we will define routing policies, Virtual LAN (VLAN)#, access control, broadcast domains, and access layer traffic aggregation or funnelling. Core layer should be capable of handling large amounts of traffic with high performance, this means that it needs to be highly available and redundant. For cost saving, we can collapse or combine distribution layer and core layer into one single layer.

Note: If user applications need higher bandwidth than a single switch link could allow, it is useful to aggregates multiple switch port links (up to 8 GB) together in order to provide higher throughput between switches especially if they have bandwidth discrepancies. See Hierarchical Network#.
Link-State Routing Protocol

It is best used in a Hierarchical Network and fast convergence# of the network is crucial.
Classless Inter-Domain Routing (CIDR)

By using CIDR, we could reduce the size of the Routing Table as the supernet (network that is at a higher hierarchy) is able to summarise multiple network addresses with a subnet mask that is less than the classful subnet mask. Classless Routing Protocols# will be used for routing update.
Autonomous System (AS)

AS is a #logical network segment that contains several Network Devices# especially Routers# under a common administration sharing a common routing strategy. An AS is typically identified using 16-bit autonomous system number (ASN) given by the American Registry of Internet Numbers (ARIN), an Internet Service Provider (ISP) or a network administrator. ~~It is a basic unit in~~ Routing inside AS involves #IGP whereas routing outside AS involves #EGP.

#networking