Design Review Template - NETS

Hierarchical (MultiLayer) Network Design
This layer interfaces the Access layer with the Core Layer; it is a major aggregation point and plays a very
important role. The distribution layer terminates all access vlans from the layer 2 perspective. High performing
Catalysts 6k are typically used to route the vlans efficiently at wire speed towards the backbone. The switches
have routing intelligence to participate in any routing protocol with core routers as well as providing HSRP
functionality to load share traffic coming from the access layer. The Distribution switches do not inject routing
updates into the access layer in normal circumstances.
Although it consists only of two L3 switches, the way we implement and interconnect them to other layers is not
trivial.
Helpful hints and features to deploy in the access layer:
• Aggregates the Access Layer (wiring closet) switches
• Provides uplink connectivity to the Core LayerReduces the need of high density peering with a Core-only
Layer
• Redundancy (Availability), Load Balancing and Capacity Planning (Provisioning) are the most important
aspects.
o Use Layer 3 Switching in this Layer
o Use HSRP and HSRP-Tracking for first-hop redundancy
The Core Layer
The backbone of the network. Aggregates the Distribution Layer switches and plays the primary role of
connecting other network building blocks. It should provide redundancy, rapid convergence using high-speed
connections and high-speed Layer 3 switching.
The Server Farm:
A server farm is implemented as a high-capacity building block attached to the campus backbone, and is treated
as its own distribution block. Server farm is an aggregation point for much of the traffic from the whole campus.
As such, it makes sense to design the server farm with less over subscription of switch and trunk resources than
the normal building block. For example, if the other campus building blocks connect to the backbone with Fast
Ethernet, then the server farm should probably attach to the backbone with Gigabit Ethernet.
How do we handle Campus or Enterprise-Wide VLANs
These are definitely incompatible with the Multilayer model, which is a strictly routed environment. There are
generally two possible ways to force these protocols on the model, either we build Campus-Wide vlans with
complex Spanning Tree topologies spanning the whole network or we need to bridge these protocols on the
routers. Both alternatives undermine the robustness of the network.
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