Path Attributes

A path attribute provides some additional information about a route. If a BGP speaker has more than one route to the same destination prefix, it selects one of those routes to use (the “ best” route) based on the path attributes. BGP as implemented on the E-series router specifies detailed and complex criteria for picking the best route; this helps ensure that all routers will converge to the same routing table, a necessary behavior to avoid routing loops. See Selecting the Best Path for more information.

The following are some of the most important path attributes:

• AS-path specifies the sequence of autonomous systems that must be crossed to reach a certain destination. This path attribute is used to avoid routing loops and to prefer shorter routes over longer routes.
• Next-hop specifies the IP address of the ingress router in the next autonomous system on the path to the destination.
• Local-pref and multiexit discriminator (MED) are metrics that administrators can tune to ensure that certain routes are more attractive over other routes. The local-pref attribute specifies a degree of preference that enables a router to select among multiple routes to the same prefix. The MED is used for ASs that have more than one connection to each other. The administrator of one AS sets the MED to express a degree of preference for one link versus another; the BGP peer in the other AS uses this MED to optimize traffic.
• Originator-ID specifies the IP address of the router that originates the route. The router ignores updates that have this attribute set to its own IP address.
• Atomic-aggregate and aggregator inform peers about actions taken by a BGP speaker regarding aggregation of routes. If a BGP speaker aggregates routes that have differing path attributes, it includes the atomic-aggregate attribute with the aggregated prefix to inform update recipients that they must not deaggregate the prefix. A BGP speaker aggregating routes can include the aggregator attribute to indicate the router and AS where the aggregation was performed.
• Community and extended community identify prefixes as sharing some common attribute, providing a means of grouping prefixes and enacting routing policies on the group of prefixes. A prefix can belong to more than one community. You can specify a community name as a 32-bit string, a standards-defined well-known community, or an AS number combined with a 32-bit number to create a unique identifier. An extended community name consists of either an IP address or an AS number, combined with a 32-bit or 16-bit number to create a unique identifier.

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