3.7.d Implement, optimize and troubleshoot routing policies

3.7.d [i] Attribute manipulation

BGP is a protocol that uses route attributes to select the best path to a destination.

BGP uses several attributes for the path-selection process. BGP uses path attributes to communicate routing policies. BGP path attributes include next hop, local preference, AS path, origin, multi-exit discriminator (MED), atomic aggregate, and aggregator. Of these, the AS path is one of the most important attributes: It lists the number of AS paths to reach a destination network.

BGP attributes can be categorized as well-known or optional. Well-known attributes are recognized by all BGP implementations. Optional attributes do not have to be supported by the BGP process; they are used on a test or experimental basis. Well-known attributes can be further subcategorized as mandatory or discretionary. Mandatory attributes are always included in BGP update messages. Discretionary attributes might or might not be included in the BGP update message.

Next-Hop Attribute

The next-hop attribute is the IP address of the next IP hop that will be used to reach the destination. The next-hop attribute is a well-known mandatory attribute. With eBGP, the eBGP peer sets the next hop when it announces the route. Multi-access networks use the next-hop attribute where there is more than one BGP router.

Local Preference Attribute

The local preference attribute indicates which path to use to exit the AS. It is a well-known discretionary attribute used between iBGP peers and is not passed on to external BGP peers. In Cisco IOS Software, the default local preference is 100. The higher local preference is preferred.

Origin Attribute

Origin is a well-known mandatory attribute that defines the source of the path information. Do not confuse the origin with comparing whether the route is external (eBGP) or internal (iBGP). The origin attribute is received from the source BGP router.

There are three types of origin attributes:

● IGP— Indicated by an i in the BGP table. Present when the route is learned by way of the network statement.

● EGP— Indicated by an e in the BGP table. Learned from EGP.

● Incomplete— Indicated by a ? in the BGP table. Learned from redistribution of the route.

In terms of choosing a route based on origin, BGP prefers routes that have been verified by an IGP over routes that have been learned from EGP peers, and BGP prefers routes learned from eBGP peers over incomplete paths.

AS_Path Attribute

The AS path is a well-known mandatory attribute that contains a list of AS numbers in the path to the destination. Each AS prepends its own AS number to the AS path. The AS path describes all the autonomous systems a packet would have to travel to reach the destination IP network. It is used to ensure that the path is loop-free. When the AS path attribute is used to select a path, the route with the fewest AS hops is preferred.

MED Attribute

The MED attribute, also known as a metric, tells external BGP peers the preferred path into the AS when multiple paths into the AS exist. exist. In other words, MED influences which one of many paths a neighboring AS uses to reach destinations within the AS. It is an optional non-transitive attribute carried in eBGP updates. The MED attribute is not used with iBGP peers. The lowest MED value is preferred, and the default value is 0. Paths received with no MED are assigned a MED of 0. The MED is carried into an AS but does not leave the AS.

Community Attribute

Although it is not an attribute used in the routing-decision process, the community attribute groups routes and applies policies or decisions (accept, prefer) to those routes. It is a group of destinations that share some common property. The community attribute is an optional transitive attribute of variable length.

Atomic Aggregate and Aggregator Attributes

The atomic aggregate attribute informs BGP peers that the local router used a less specific (aggregated) route to a destination without using a more specific route. If a BGP router selects a less specific route when a more specific route is available, it must attach the atomic aggregate attribute when propagating the route. The atomic aggregate attribute lets the BGP peers know that the BGP router used an aggregated route. A more specific route must be in the advertising router’s BGP table before it propagates an aggregate route.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3969-3973).  . Kindle Edition.

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