Definition of backboning in The Network Encyclopedia.

What is Backboning (in networking)?

The process that enables the transmission of messages between similar messaging systems by making use of a different type of intermediate messaging system. For example, Microsoft Exchange Server can use an existing public or private backbone messaging system to implement an Exchange Server–based messaging system. Exchange Server can also be used for linking other mail systems such as legacy Microsoft Mail networks.

Simple Backboning example using exchange

How it works

A simple backboning example is the connecting of two or more Lotus cc:Mail postoffices using an Exchange Server organization as the messaging backbone. By installing the cc:Mail Connector on Exchange Server, messaging connectivity can be established with connected cc:Mail postoffices. Messages can then be routed from one postoffice through the Exchange organization to other postoffices on the network.

Another example of backboning is the connecting of different sites in an Exchange Server organization using a public or private messaging network. Here are two possible scenarios:

  • Using Simple Mail Transfer Protocol (SMTP) hosts on the Internet to connect Exchange sites. To do this, the Internet Mail Service can be installed on appropriate Exchange servers.
  • Using a public or private X.400 messaging system for connecting Exchange sites using the X.400 Connector.

The term “backboning” is sometimes used to describe the core messaging paths set up for a large Exchange organization, regardless of whether messaging systems other than Exchange are involved.


When using a public messaging backbone (or a private one owned by a different company) for connecting your Exchange sites, you should consider the following:

  • Installing and configuring appropriate messaging connectors on suitable messaging bridgehead servers
  • Establishing and maintaining directory replication between Exchange sites
  • Handling background traffic from other users of the backbone
  • Tuning messaging performance to optimize use of the backbone
  • Implementing a suitable topology for the messaging backbone

For very large Exchange organizations, use a hub and spoke topology instead of a mesh topology. Hub and spoke topologies have less redundancy and fault tolerance, but mesh topologies have routing tables that grow exponentially with the number of sites involved. Mesh topologies for large organizations can lead to routing tables that are so large they degrade the performance of the message transfer agents, even on high-performance servers.