Routing

Following a MED math, the next decision is where did the path come from, in this order of preference: eBGP peers are most desired Confederation member AS peers are next iBGP peers are least desired.

Following on from checking the origin type, the next path attribute that is checked is the MED, or Multi-Exit Discriminator. Multi-Exit Discriminator utilises a 32-bit value (0 – 4,294,967,295) known as a metric. The lower the metric the more preferred a route is. The MED is set automatically to the IGP path metric during network…

Following AS Path, origin type is evaluated next in the order of preference. The BGP attribute is well-known and is known simply as ‘origin’. Origin defines where networks originated from, how they were advertised into BGP. Networks that were advertised into BGP via a network statement in the BGP configuration are given an ‘i’ indicator,…

After AIGP Path Attributes, the next deciding factor is evaluating the AS Path of a route. The AS Path attribute typically corresponds to the number of AS hops are needed to reach a destination. The shorter the AS path, the more preferred it is. There can be times when AS paths are not relative to…

After Weight and Local Preference, the third decision is to determine if the route originated locally. Preference is given in this order: Was the route advertised locally Was this part of a network that has been aggregated locally Was this route learned by a BGP peer

Local Preference is a well-known discretional path attribute. It is included with advertisements throughout an AS. It is the second preferred path attribute when deciding a best path, after weight. It’s default value is 100, and a higher value is a preferred value. Local preference is a 32-bit value, between 0 to 4,294,967,295 Local preference…

BGP Weight is a Cisco defined path attribute and the first step on selecting the best path in BGP. The path with the higher weight is preferred. Weight is a 16-bit value, between 0 to 65,535. A path that originates on the router has a default weight of 32,768. A path that originates from elsewhere…

BGP consists of two parts, Network Layer Reachability Information and Path Attributes. The path attributes are a huge factor in deciding the preferred path for traffic to take. On top of this only the best path is advertised to neighbours in BGP. BGP automatically installs the first route it receives for a new destination as…

Typically routers always select the path by examining the prefix length of as network entry, with the chosen path being based on prefix length. The longest prefix length always being preferred. The logic can be used to influence path selection in BG. Path attributes can be used to influence traffic too, but there may be…