Hierarchical Network Design
- predictable behavior
- low maintenance
- high availability
- recover from failure and topology changes in a pre-determined manner
- scale to support future expansion and upgrades
- design around traffic flows rather than a particular type of traffic
- keep end users at consistent distances from resources
- cisco approach enables designers to organize the network into distinct layers of devices
- access
- distribution
- core
- the resulting network is:
- efficient
- intelligent
- scalable
- easily managed
- access layer switches are aggregated at the distribution switch layer
- distribution layer switches are aggregated as the core switch layer
Access Layer:
- end users connected
- low cost per switch port
- high port density
- scalable uplinks to higher layers
- user access functions:
- VLAN membership
- traffic and protocol filtering
- QoS
- resiliency through multiple uplinks
Distribution Layer:
- interconnection between the campus network’s access and core layers
- aggregation of multiple access-layer devices
- high layer 3 throughput for packet handling
- security and policy-based connectivity functions through access lists or packet filters
- QoS
- scalable and resilient high-speed links to the core and access layer
- switches capable of handling the total volume of throughput from all connected devices
- high port density of high speed links to to support the collection of access layer switches
- VLAN’s and broadcast domains converge at this layer and require routing, filtering and security
- usually a layer 3 boundary where routing meets the VLAN’s of the access layer
Core Layer:
- connectivity of all distribution layer devices
- AKA backbone
- needs to switch traffic as efficiently as possible
- very high throughput at layer 3
- no costly or unnecessary packet manipulations (ACL’s, filtering)
- Redundancy and resilience
- advanced QoS
- designed with simplicity and efficiency in mind
Switch Block:
- a group of access layer switches together with their distribution layer switches
- all switch blocks connect into the core block
- balanced mix of layer 2 and layer 3
- distribution layer shields the switch block from certain failures or conditions in other parts of the network. eg. broadcasts are not propagated from the switch block into the core and other switch blocks
- STP is confined to each switch block where a VLAN is bounded
- VLAN’s should not be extended beyond distribution switches
- the distribution layer should always be the boundary of VLAN’s, subnets and broadcasts
- VLAN traffic should not traverse the network core
- when sizing a switch block, consider:
- port density for access layer switches
- traffic types and patterns
- amount of layer 3 switching capacity at the distribution layer
- number of users connected to access layer switches
- geographic boundaries of of subnets or VLAN’s
- size of spanning tree domains
- usually no more than 2000 users should be placed within a single switch block, although sizing should be based primarily on:
- traffic types and behaviour
- size and number of common workgroups
- a switch block is too large if:
- the routers (MLS’s) at the distribution layer become traffic bottlenecks, possibly due to high volumes of inter VLAN traffic, intensive CPU processing or switching times required by policy or security functions
- broadcast or multicast traffic slows the switches in the block
- best practice is for all layer 2 connectivity to be contained within the access layer
Core Block:
- the campus network’s backbone
- required to connect 2 or more switch blocks in a campus network
- must be as efficient and resilient as possible as all traffic passing to and from all switch blocks must cross it
- carries more traffic than any other block
- links to/from the distribution layer can be L3 or L2 (using a small vlan bounded by the switches, and an SVI to provide routing)
- for sizing core switches, each one must be able to handle each of it incoming distribution links at 100% capacity
Collapsed Core:
- core is collapsed into distribution layer
- dist and core functions provide by the same switch devices
- used in smaller campus networks where a separate core is not warranted
- not an independent building block but integrated into the distribution layer of individual switch blocks
- each access layer switch has a redundant link to each dist/core layer switch
- all L3 subnets in the access layer terminate at the dist switches L3 ports
- dist/core switches connect to each other by one or more L3 links for redundancy/failover
Dual Core:
- connects 2 or more switch blocks in a redundant fashion
- independent from any other switch block
- 2 identical, redundant switches
- redundant links connect the distribution layer of a switch block to each of the dual core switches
- routing protocols plus provide equal cost load balancing between dist and core switches