Cisco developed the idea for defining a multilayer campus model in order to improve campus network performance, availability as well as scalability. The campus multilayer model is made up of three layers along with associated network services for each layer. The layers are the access, distribution and core layer.
The Access Layer is the network edge, which is the place where desktops and other peripherals are connected. Access switches are either a Layer 2 or 3 switches for routed access layer designs. These days, access switch connectivity is usually with the standard 1Gb or 1000Mbps links. When forwarding traffic, the access switch uplinks used are Gigabit also and they sometimes use EtherChannel for better performance. This layer defines VLANs and assigns service quality to various types of traffic. Also, this is where security is applied and packet filtering is deployed, in order to enhance performance and reduce traffic sent to distribution switches.
ACCESS LAYER SERVICES
Access Control Lists
Power over Ethernet
The Distribution Layer is where traffic from the access layer switches is forwarded to the core layer. Distribution multilayer switches route and balances the load of traffic. Policy based routing is also applied as well as route filtering of traffic toward the access layer. These routes are summarised to the core layer for reduced device use and enhance performance. This layer is usually the default gateway for all employees who may be connected to the access switches.
A common default gateway protocol is the HSRP protocol and it provides gateway redundancy as well as load sharing. VLANs are terminated at distribution switches and are pruned there in order to reduce broadcast traffic going between switches. The distribution switch receives the server farm traffic from where it is forwarded to a connected access layer switch with attached servers at the data center. Client-server applications utilize multiple web application as well as database servers which are connected to several access switches.
The Core Layer engages in high-speed packet forwarding of traffic coming from the distribution layer, the WAN core as well as the Internet DMZ. Popular core layer switches are the Nexus 7000 and 6500 switches in order to get an enhanced performance. At the core switches, dynamic routing is deployed in order to forward packets. The multilayer distribution switches and WAN core routers, make most of the routing decisions. The 10 GE interface uplinks are becoming more popular for use in core layer as well as distribution layer switch connectivity.
Aside from high-speed packet forwarding, distribution and core layers are usually designed with equal cost links in an effort to improve network convergence with ECMP. This improves network performance and also optimises load balancing.
High-Speed Packet Forwarding Point to Point Load Balancing Decreased Peering of Distribution Switches for Scalability Forward Internet DMZ traffic
Service Block as part of newer campus switching design is an architectural component. It's the main purpose of the migration and also, centralization of data center services and IPv6 and wireless LAN controllers. A key feature to note is tunneling, which is used to integrate traffic all over the switching infrastructure for different purposes.
Data Center Centralized Services
Modularizing and Adding of Services Easier
IPv4 and IPv6 Dual Stack Migration
Centralized Wireless LAN Controller
Tunneling of Traffic for Compatibility
MULTILAYER WAN DESIGN
Cisco developed the multilayer WAN design model to improve network performance, availability as well as scalability across the company WAN. The model has three layers with network services for each. These layers include the branch, distribution, and core layer.
The Branch Layer is the network WAN edge made up of smaller branch offices usually made up of about 25 to 100 employees. WAN infrastructure doesn't have as much bandwidth as campus switching infrastructure. The branch layer has control of various services that reduce traffic that is sent to the distribution layer. At branch routers, route filtering, as well as summarization, is deployed in order to reduce advertisements and also, bandwidth usage. QoS is also deployed at the branch routers and switches in order to classify and mark voice, video as well as data traffic.
Summarization to the Distribution Layer
QoS Edge Classification and Marking
Voice and Video Services
The Distribution Layer is the aggregation layer for the traffic gotten from the connected branches going to and coming from the data center. This layer is in control of various services which enhance routing and reduce broadcasts. Due to this, dynamic routing, as well as policy based routing and also summarization, are deployed at the routers of this layer accompanied by route filtering to the branch layer. At this layer, load balancing services are deployed also, so as to increase the rate of data transfer between the branch and core layers. Worthy of note is the fact that it is not completely for branch and core offices to have a large number of employees. They could be strategic locations for the purpose of traffic forwarding.
DISTRIBUTION WAN SERVICES
Branch Layer Aggregation
Summarization to the Core Layer
Policy Based Routing
Route Filtering to the Branch Layer
Load Balancing to the Core Layer
The WAN Core Layer is the aggregation layer for traffic coming from and going to the data center as well as for connected offices of the distribution layer. This layer is in charge of a high-speed routing of packets. Owing to this, the routers of this layer are deployed with dynamic routing and policy based routing. Considering the amount of traffic at this layer, packet classification, marking and also, queuing, is needed for bandwidth optimization.
Load balancing to the distribution layer and the data center ensures an enhanced rate of data transfer.
CORE WAN SERVICES
Distribution and WAN Core Aggregation
QoS to the Distribution and WAN Core Layer
High-Speed Packet Forwarding
Policy Based Routing
Load Balancing to the Distribution Layer
Internet DMZ services are edge connectivity for the traffic going to and coming from the Internet. The services forward packets from employees, telecommuters, business partners as well as customers.
INTERNET DMZ SERVICES
ISP Internet Connectivity
Security Services (Firewall, IDS, IPS)
Public Servers (DNS, FTP, Email, The Web