The following is an extract from a recently submitted update of mine to the Quagga documentation. Republished here under the licence of the Quagga documentation1:
The BGP MED (Multi_Exit_Discriminator) attribute has properties which can cause subtle convergence problems in BGP. These properties and problems have proven to be hard to understand, at least historically, and may still not be widely understood. The following attempts to collect together and present what is known about MED, to help operators and Quagga users in designing and configuring their networks.
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