BGP Path Vector

• BGP routers exchange network reachability information, called path vectors, made up of path attributes.
• The path vector information includes:
  • A list of the full path of BGP AS numbers (hop-by-hop) necessary to reach a destination network.
• Other attributes include:
  • IP address to get to the next AS (next hop attribute)
  • Information about how the networks at the end of the path were introduced into BGP (origin code attributes).
  • There are other attributes that will be discussed later.
• The BGP AS path is guaranteed to be loop free.

A router running BGP does not accept a routing update that already includes its AS number in the path list, because the update has already passed through the AS, and accepting it again we result in a routing loop.

• By applying routing-routing policies to the path of BGP AS numbers, routing behavior can be enforced at the AS level to determine how data will flow through the AS.
  • These policies can be implemented for:
    • All networks owned by an AS.
    • Certain CIDR block of network numbers (prefixes).
    • Individual networks or subnetworks.
  • These policies are based on the attributes carried in the routing information and configured on the routers.
• BGP can advertise only the routes it uses.

BGP specifies that a BGP router can advertise to its peers in neighboring autonomous systems only those routes that it uses. This rule reflects the hop-by-hop routing paradigm generally used throughout the current Internet.

• There are some policies that cannot be supported by hop-by-hop routing and thus require other technique in order to implement.
  • One example is that BGP does not allow one AS to send traffic to a neighboring AS with the goal of manipulating the traffic to take a different route from that taken by traffic originating in that neighboring AS.
  • In other words, you cannot influence how a neighboring AS will route your traffic, but you can influence how your traffic gets to a neighboring AS.
• To illustrate the idea enumerated on the two bullet points above, consider the following example:
  • In the above diagram, AS 64520 advertises to AS 64512 only its best path: 64520 64600 64700
  • This path is the only path through 64520 that AS 64512 sees.
  • All packets that are destined for 64700 via 64520 take this path, because it is the AS-by-AS (hop-by-hop) path that AS 64520 uses to reach the networks in AS 64700.
    • AS 64520 doe not announce any other paths because it does not choose any of the other possible paths as the best paths, based on the BGP routing policy in AS 64520.
  • Even if AS 64512 knows of any other paths through AS 64520 and wants to use it, AS 64520 will not allow the packets to route to any other paths, because AS 64520 selected 64520 64600 64700 as its best path, and all AS 64520 routers will use that path based on BGP policy.
    • BGP does not enable one AS to send traffic to a neighboring AS intending that the traffic take a different route from that taken by traffic originating in the neighboring AS.
  • AS 64512 has an option to use AS 64520 or AS 64530 to reach AS 64700 based on its own BGP routing policies.

When to Use BGP

• BGP is more appropriate to use when at least on of the following conditions exists:
  • The AS allows packets to transit through it to reach other autonomous systems (for example, it is a service provider).
  • The AS has multiple connections to other autonomous systems.
  • Routing policy and route selection for traffic entering and leaving the AS must be manipulated.
• BGP allows an enterprise to differentiate between its traffic and traffic from its ISP. Therefore BGP is an option if this differentiation is required.
  • A static route to an ISP will not distinguish whether a certain traffic is from the enterprise or from the ISP.
• BGP is the protocol that is used to implement an agreement between to or more autonomous systems to exchange updates.

When Not to Use BGP

• BGP should not be used if the following conditions are true:
  • A single connection to the Internet or another AS
  • Lack of memory or processor power on routers to handle constant BGP updates.
  • Limited understanding of route filtering and the BGP path-selection process.

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet, as well as following the links on the resources section of this entry.