Network design.

A standard model is a hierarchy of core, distribution and access. Click here to see some notes from Cisco's Internetwork Design Guide, which does a good job of talking about some of the basic design concepts.

An "Autonomous System" or AS is a collection of networks under one administrative domain. Usually you will use a IGP (Interior Gateway Protocol) between routers within an AS. You would use an EGP (Exterior Gateway Protocol) between AS's.

Routers that connect one Autonomous Systems to another are said to be AS border routers. But within an AS you might group networks together (perhaps based on IP address hierarchy) into Areas. Routers that link together Areas are known as area border routers (ABR). Remember this terminology, it will be used later when we learn OSPF.

Redundancy and Layer 2 design vs Layer 3 design

We try to design networks to be redundant, so if any equipment fails the network still functions. Redundancy can be built at layer 2 or at layer 3. Layer 2 redundancy on Ethernet is provided by spanning tree protocol (STP). When a switch fails, it can take a few minutes for spanning tree to converge (re-organize itself and bring up redundant paths). Timers can be adjusted to let this happen faster. Spanning tree has one issue in that all switches must participate. A switch that doesn't participate can easily cause a loop and melt down the network. For higher reliability we try to keep layer two domains (broadcast domains) small. Layer 3 redundancy is provided by network routing protocols. These can converge faster, and also have timers that can be adjusted.

At layer 2 on Ethernet there is a concept of an "access" switch/hub. This is the switch or hub that a endstation connects to. If this fails, the station loses connectivity. At layer 3 there is a concept of a "access" router. The access router is the default route for an endstation. If the access router fails the end station can lose connectivity. There are techniques to allow redundancy if this fails.

Our class networks

Our lab network consists of 10.XX.0.0/16, which is subnetted into a small number of networks. The "core" network (net 0, 10.XX.0.0/24) connects the AS border router (BR) and the two distribution routers (R2 and R3). The distribution routers connect to one access router (R1). Each distribution router connects via a point to point /30 network to the access routers. Having the access router (R1) connected to two distribution routers provides some redundancy. R1 provides access to two networks net1 (10.XX.1.0/25) and net2 (10.XX.1.128/26)

Router R3 also acts as an access router for network 3. (10.XX.2.0/24). A router can function as a an access router and a core or distribution router at the same time.