BGP Path Attributes: Multi-Exit Discriminator (MED)

Following on from checking the origin type, the next path attribute that is checked is the MED, or Multi-Exit Discriminator.

Multi-Exit Discriminator utilises a 32-bit value (0 – 4,294,967,295) known as a metric. The lower the metric the more preferred a route is.

The MED is set automatically to the IGP path metric during network advertisement or redistribution. If this MED is received via a eBGP session, it can be advertised to other iBGP peers, though not sent outside the AS from where it was received.

A MED without a value should be considered to have a MED metric of 0 – the highest possible priority.