Distribution Layer

The distribution layer of the network is the demarcation point between the access and core layers of the network. The distribution layer can have many roles, including implementing the following functions:

• Policy (for example, to ensure that traffic sent from a particular network should be forwarded out one interface, while all other traffic should be forwarded out another interface)

• Address or area aggregation or summarization

• Departmental or workgroup access

• Broadcast/multicast domain definition

• Routing between virtual LANs (VLANs)

• Media translations (for example, between Ethernet and Token Ring)

• Redistribution between routing domains (for example, between two different routing protocols)

• Demarcation between static and dynamic routing protocols

Several Cisco IOS software features can be used to implement policy at the distribution layer, including the following:

• Filtering by source or destination address

• Filtering on input or output ports

• Hiding internal network numbers by route filtering

• Static routing

• Quality of service mechanisms (for example, to ensure that all devices along a path can accommodate the requested parameters)

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