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Campus and WAN Multilayer Network Design

Multilayer Campus Design

The idea for defining a multilayer campus model was developed by Cisco to improve campus network performance, availability and scalability. There are three layers that comprise the campus multilayer model with associated network services for each layer. The layers include the access layer, distribution layer and core layer.

Access Layer is the network edge where desktops, network servers and various peripherals including video equipment and IP phones are connected. The access switch is either a Layer 2 switch or a Layer 3 switch for routed access layer designs. Connectivity to the access switch is typically with standard Gigabit (1000 Mbps) links today.

The access switch uplinks used for forwarding traffic to the distribution layer are Gigabit as well, sometimes with EtherChannel for increased performance. The access layer is responsible for defining VLANs and assigning quality of service to various traffic types. In addition, security is applied at the access layer and packet filtering is deployed to optimize performance and decrease traffic sent to the distribution switches.

Access Layer Services

VLAN Assignment
Access Control Lists
Static Routing
QoS Policies
Switch Stacking
Port Security
DHCP Snooping
Broadcast Filtering
NIC Teaming
Power over Ethernet

Distribution Layer is the aggregation layer where traffic from the access layer switches is forwarded to the core layer. The distribution multilayer switches are responsible for routing and load balancing of traffic. In addition policy based routing is applied and route filtering of traffic toward the access layer. Routes are summarized to the core layer for improved performance and decreased device utilization. The distribution switch is typically the default gateway for all employees connected to the access switches.

The HSRP protocol is a common default gateway protocol that provides gateway redundancy and load sharing. The default gateway will ARP for server MAC addresses and maintain the ARP and CAM table for switch connectivity. The VLANs are terminated at the distribution switches and any VLANs are pruned there to decrease broadcast traffic between switches. The server farm traffic is forwarded to the distribution switch where it is forwarded to a connected access layer switch with attached servers at the data center. Client-server applications use multiple web, application and database servers that are connected to multiple access switches.

Distribution Layer Services

Dynamic Routing
Load Balancing
Terminate VLANs
Policy Based Routing
Route Filtering To Access Layer
Route Summarization To Core Layer
First Hop Redundancy Protocol (HSRP, GLBP, VRRP)
VLAN Pruning
ARP Services
Server Farm Connectivity
Firewall, IPS, SSL, Load Balancer Service Modules

Core Layer is responsible for high speed packet forwarding of traffic from the distribution layer, WAN core and the Internet DMZ. The Nexus 7000 switches and 6500 switches are popular as core layer switches for optimized switching performance. Dynamic routing is deployed at the core switches however the purpose is to forward packets. Most of the routing decisions are made by the multilayer distribution switches and WAN core routers. The 10 GE interface uplinks are becoming the standard for core layer and distribution layer switch connectivity.

In addition to high speed packet forwarding, the distribution and core layers are often designed with equal cost links to improve network convergence with ECMP. That allows for optimized load balancing to increase network performance.

Core Layer Services

High Speed Packet Forwarding
Point to Point Load Balancing
Decreased Peering of Distribution Switches for Scalability
Forward Internet DMZ traffic

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