Figure 39-3 BGP Local Preference Attribute
Multi-Exit Discriminator Attribute
The multi-exit discriminator (MED) or metric attribute is used as a suggestion to an external AS regarding the preferred route into the AS that is advertising the metric.
The term suggestion is used because the external AS that is receiving the MEDs may be using other BGP attributes for route selection. We will cover the rules regarding route selection in the next section. In Figure 39-4, Router C is advertising the route 172.16.1.0 with a metric of 10, while Route D is advertising 172.16.1.0 with a metric of 5. The lower value of the metric is preferred, so AS 100 will select the route to router D for network 172.16.1.0 in AS 200. MEDs are advertised throughout the local AS.
Origin Attribute
The origin attribute indicates how BGP learned about a particular route. The origin attribute can have one of three possible values:
- IGP—The route is interior to the originating AS. This value is set when the network router configuration command is used to inject the route into BGP.
- EGP—The route is learned via the Exterior Border Gateway Protocol (EBGP).
- Incomplete—The origin of the route is unknown or learned in some other way. An origin of incomplete occurs when a route is redistributed into BGP.
The origin attribute is used for route selection and will be covered in the next section.
Figure 39-4 BGP Multi-Exit Discriminator Attribute
AS_path Attribute
When a route advertisement passes through an autonomous system, the AS number is added to an ordered list of AS numbers that the route advertisement has traversed. Figure 39-5 shows the situation in which a route is passing through three autonomous systems.
AS1 originates the route to 172.16.1.0 and advertises this route to AS 2 and AS 3, with the AS_path attribute equal to {1}. AS 3 will advertise back to AS 1 with AS-path attribute {3,1}, and AS 2 will advertise back to AS 1 with AS-path attribute {2,1}. AS 1 will reject these routes when its own AS number is detected in the route advertisement. This is the mechanism that BGP uses to detect routing loops. AS 2 and AS 3 propagate the route to each other with their AS numbers added to the AS_path attribute. These routes will not be installed in the IP routing table because AS 2 and AS 3 are learning a route to 172.16.1.0 from AS 1 with a shorter AS_path list.
Next-Hop Attribute
The EBGP next-hop attribute is the IP address that is used to reach the advertising router. For EBGP peers, the next-hop address is the IP address of the connection between the peers. For IBGP, the EBGP next-hop address is carried into the local AS, as illustrated in
Figure 39-6.
Figure 39-5 BGP AS-path Attribute
Figure 39-6 BGP Next-Hop Attribute
Router C advertises network 172.16.1.0 with a next hop of 10.1.1.1. When Router A propagates this route within its own AS, the EBGP next-hop information is preserved. If Router B does not have routing information regarding the next hop, the route will be discarded. Therefore, it is important to have an IGP running in the AS to propagate next-hop routing information.
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