Border Gateway Protocol (BGP) is the de facto routing protocol responsible for interdomain routing. There is support for public internet routing and private IGP routing domains. It is an exterior gateway protocol (EGP) path vector protocol. BGP is essentially a distance vector protocol that advertises AS path information to neighbors with routing updates.
Figure 1 BGP Private Range Autonomous Systems (ASN)
Load balancing is not supported, however load sharing is an option via path attribute policies. As a classless routing protocol, routes advertised are not automatically summarized by default. The architecture is non-hierarchical with autonomous system numbers (ASN) assigned to internal BGP (iBGP) or external (eBGP) connections. eBGP neighbor peers are assigned to different autonomous systems while iBGP peers are assigned to the same ASN.
BGP autonomous system numbers (ASN) are either private or public. Internet connectivity require assignment of public range ASN from a service provider. The designated public range is from 1 to 64511 for BGP internet routable connections. Private AS number range from 64512 to 65535 is for iBGP and/or eBGP network usage. There are some larger enterprise network domains that implement iBGP and/or eBGP autonomous systems.
Path Attributes
There is a path selection algorithm that selects the best route based on path attribute policies. Each path attribute also has a default setting that is used when no policy is configured. BGP has a neighbor table that is created when adjacency is formed with BGP neighbors. There is a topology table as well that stores all BGP routes advertised from neighbors. Finally, the router builds the BGP routing table with only best path routes installed from the topology table. For example, when multiple paths exist to a destination, the path with highest weight is selected first. The route with highest local preference is selected if weight attributes are equal.
BGP Characteristics
- Path vector routing protocol
- Exterior Gateway Protocol (EGP)
- Administrative distance: eBGP 20, iBGP 200
- IP only routing
- IPv6 support (MP-BGP)
- Open standard
- Application layer (TCP port 179)
- Metric = path attributes
- Best path selection algorithm
- Classless routing
- Flat topology with autonomous systems
- Neighbor topology view only
- Full routing updates every 30 seconds (eBGP), 5 seconds (iBGP)
- Keepalive timer = 60 seconds, Hold timer = 180 seconds
- Loop prevention: AS-path attribute (eBGP), split horizon (iBGP)
- Standard subnet masks
- Message authentication: none, text, password, MD5
- Route auto-summarization: disabled (default)
- Scalable to global enterprise and internet domains
- Load balancing: none