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BGP Overview<br />
Multi-Exit Discriminator<br />
The multi-exit discriminator (MULTI_EXIT_DISC or MED) attribute is used to influence eBGP neighbors<br />
to select a certain path for inbound traffic into the AS that is advertising the metric.<br />
The lower the MED metric, the more desirable a route becomes.<br />
The MED metric is merely a suggestion to the external neighbor as to which path should be<br />
used inbound to the local AS. This is because the external AS that is receiving the MED<br />
metric might be using other BGP attributes for route selection.<br />
Origin<br />
The origin attribute identifies the source of a learned route. There are three possible values for this<br />
attribute:<br />
• IGP - The route has been learned by an iBGP neighbor that is internal to the local AS. This value is set<br />
when the network command is issued from BGP configuration mode to inject a route into BGP.<br />
• EGP - The route has been learned from an eBGP neighbor.<br />
• Incomplete - The origin of the route is not known or learned in some other way. A route that has been<br />
redistributed into BGP or specified using the network command is set to this value.<br />
AS Path<br />
The AS path (AS_PATH) attribute consists of an ordered list of AS numbers that the route advertisement<br />
has crossed. Each time a router advertisement passes through an AS, the AS number is added to the list.<br />
The AS_PATH attribute is used by BGP as a loop avoidance mechanism. A route is rejected anytime a<br />
router detects its own AS number in a route advertisement.<br />
The AS path can also be used to determine the best route to a given destination. If two identical routes are<br />
learned and all other attributes are equal, the one with the fewest number of traversed autonomous systems<br />
is preferred.<br />
Next Hop<br />
The next hop (NEXT_HOP) attribute is the IPv4 address that is used to reach the advertising router. Since<br />
BGP routes traffic from AS to AS, the default next hop that is advertised is the next AS.<br />
<strong>Community</strong><br />
The community attribute provides a way to group routes together in communities and apply a consistent<br />
policy to the group. The policies can be set to control routing decisions, such as acceptance, preference,<br />
and redistribution.<br />
For more detailed information on many of these BGP attributes and the related commands<br />
used to influence path selection, refer to Additional BGP Configuration on page 12.<br />
61200860L1-29.4E Copyright © 2012 <strong>ADTRAN</strong>, Inc. 5