BGP performs interdomain routing in Transmission-Control Protocol/Internet Protocol (TCP/IP) networks. BGP is an exterior gateway protocol (EGP), which means that it performs routing between multiple autonomous systems or domains and exchanges routing and reachability information with other BGP systems.
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| Aggregation | The BGP term for summarization. |
| Attribute | Similar to metric, these are variables that describe characteristics of the path to the destination. Once difined, the characteristics can be used to make decision as to which path should be taken, based on the characteristic or attribute of the paths available. |
| Autonomous System | Definition for the organizational boundary. Routers share the same routing protocol within the organization. The autonomous system defines all the router within an administrative domain, where each router has full knowledge of the subnets within the domain. If you are connecting directly to the Internet using BGP, the autonomous system number must be unique from the Internet addressing committees. |
| Exterior Gateway Protocol (EGP) | The generic term for a protocol that runs between autonomous systems. There is also a specific protocol with this name, which was the precursor to BGP. |
| External BGP (eBGP) | eBGP sends routing information between different autonomous system. |
| Interior Gateway Protocol (IGP) | This is a routing protocol that runs within an autonomous system. In the past gateway was used to define a router. |
| Internal BGP (iBGP) | This is when BGP is used within an autonomous system. The routers are not required to be physical neighbors and often sit on edges of the autonomous system. iBGP is used between eBGP speakers in the same autonomous system. |
| Originator-ID | This is a BGP attribute. It is an optional nontransitive attribute that is created by the route reflector. The attribute contains the router ID of the router that originated the route in the update. The purpose of theis attribute is to prevent a routing loop. If the originating router recieves its own update, it ignores it. |
| policy-based routing | This allows the administrator to program the routing protocol by defining how the traffic is routed. This is a form of static routing enforced by access-lists called route-maps. Policy-based routing (PBR) is protocol independent and uses route-maps by creating a separate process to force routing decisions in a sophisticated way by matching and changing attributes and other criteria. |
| prefix-list | The prefix-list is used as an alternative to distribute-lists to control how BGP learns or advertises updates. Prefix-lists are faster, more fexible, and less processor intensive than distribute-lists. |
| Route reflector | This is the router that is configured to forward routes from other identified iBGP clients. This removes the necessity for a fully meshed iBGP network, preserving network resources. A fully meshed network has a great deal of overhead and does not scale. |
| Route reflector client | A client is a router that has a TCP session with its iBGP peer that is acting as a route reflector. It forwards routes to the route reflector, which propagates these on to other routers. The client does not have peer connections with other clients. |
| Route reflector cluster | A cluster is a group consisting of a route reflector and clients. There can be more than one route reflector in a cluster. |
| Synchronization rule | This rule states that a router cannot forward a route to an eBGP peer unless the route is in its local IP routing table. This requires that the IGP and BGP routing tables are synchronized. This is to prevent BGP from advertising routes that the autonomous system cannot cirect to the destination. If BGP is fully meshed, it does not have to rely on the IGP routing tables and you can disable synchronization. Use the Router(config-router)#no synchronization command to turn off synchronization. |
| Transit autonomous system | An autonomous system that is used to carry BGP traffic across to another autonomous system. None of the traffic is destined for any router within the transit autonomous system; it is simply being routed through it. |
| Category | Description |
| Well-known: | |
| Mandatory | These attributes are required and are therefore recognized by all BGP implementations. |
| Discretionary | It is not required that these attributes be present in the update messages, but if they are present, all routers running BGP will recognize and act on the information contained. |
| Optional: | |
| Transitive | The router might not recognize these attributes, but if this is the case, it marks the update as partial and sends the update, complete with attributes, to the next router. The attributes traverse the router unchanged, if they are not recognised. |
| Nontransitive | Nontransitive attributes are dropped if they fall onto a router that does not understand or recognize the attribute. |
| Attribute Name | Category | Code | Preference |
| Origin | Well-known, mandatory | 1 | Lowest origin code. IGP < EGP < Incomplete |
| AS_Path | Well-known, mandatory | 2 | Shortest path |
| Next Hop | Well-know, mandatory | 3 | Shortest part or IGP metric |
| Multiple Exit Discriminator | Optional, nontransitive | 4 | Lowest value |
| Local preference | Well-known, discretionary | 5 | Highest value |
| Atomic aggregate | Well-known, discretionary | 6 | Information not used in path selection |
| Aggregator | Optional, transitive | 7 | Information not used in path selection |
| Community | Optional, transitive | 8 | Information not used in path slection |
| Originator ID | Optional, nontransitive | 9 | Information not used in path selection |
| Cluster ID | Optional, nontransitive | 10 | Information not used in path selection |
| Weight | Cisco-defined | Highest value |
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