Troubleshooting guide

BGP Overview

Multi-Exit Discriminator

The multi-exit discriminator (MULTI_EXIT_DISC or MED) attribute is used to influence eBGP neighbors

to select a certain path for inbound traffic into the AS that is advertising the metric.

Origin

The origin attribute identifies the source of a learned route. There are three possible values for this

attribute:

• IGP - The route has been learned by an iBGP neighbor that is internal to the local AS. This value is set

when the network command is issued from BGP configuration mode to inject a route into BGP.

• EGP - The route has been learned from an eBGP neighbor.

• Incomplete - The origin of the route is not known or learned in some other way. A route that has been

redistributed into BGP or specified using the network command is set to this value.

AS Path

The AS path (AS_PATH) attribute consists of an ordered list of AS numbers that the route advertisement

has crossed. Each time a router advertisement passes through an AS, the AS number is added to the list.

The AS_PATH attribute is used by BGP as a loop avoidance mechanism. A route is rejected anytime a

router detects its own AS number in a route advertisement.

The AS path can also be used to determine the best route to a given destination. If two identical routes are

learned and all other attributes are equal, the one with the fewest number of traversed autonomous systems

is preferred.

Next Hop

The next hop (NEXT_HOP) attribute is the IPv4 address that is used to reach the advertising router. Since

BGP routes traffic from AS to AS, the default next hop that is advertised is the next AS.

Community

The community attribute provides a way to group routes together in communities and apply a consistent

policy to the group. The policies can be set to control routing decisions, such as acceptance, preference,

and redistribution.

The lower the MED metric, the more desirable a route becomes.

The MED metric is merely a suggestion to the external neighbor as to which path should be

used inbound to the local AS. This is because the external AS that is receiving the MED

metric might be using other BGP attributes for route selection.

For more detailed information on many of these BGP attributes and the related commands

used to influence path selection, refer to Additional BGP Configuration on page 12.