Border Gateway Protocl (BGP) provides loop-free routing between different autonomous systems or organizations, or domains. It uses a path-vector routing algorithm to exchange routing information between BGP speakers. Based on this information, each BGP speaker determines a path to reach a particular destination while detecting and avoiding paths with routing loops. The routing information includes the actual route prefix for a destination, the path of autonomous systems to the destination, and additional path attributes.
BGP selects a single path, by default, as the best path to a destination host or network. Each path carries well-known mandatory, well-known discretionary, and optional transitive attributes that are used in BGP best-path analysis. You can influence BGP path selection by altering some of these attributes by configuring BGP policies.
BGP also supports load balancing or equal-cost multipath (ECMP), where next-hop packet forwarding to a single destination can occur over multiple “best paths” that tie for top place in routing metric calculations. It potentially offers substantial increases in bandwidth by load-balancing traffic over multiple paths.
Cisco NX-OS supports BGP version 4, which includes multiprotocol extensions that allow BGP to carry routing information for IP multicast routes and multiple Layer 3 protocol address families. BGP uses TCP (Port 179) as a reliable transport protocol to create TCP sessions with other BGP-enabled devices.
The BGP autonomous system (AS) is a network controlled by a single administration entity. An autonomous system forms a routing domain with one or more Interior Gateway Protocols (IGPs) and a consistent set of routing policies. BGP supports 16-bit and 32-bit autonomous system numbers.
External BGP autonomous systems dynamically exchange routing information through external BGP (eBGP) peering sessions. BGP speakers within the same autonomous system can exchange routing information through internal BGP (iBGP) peering sessions.
BGP supports 2-byte or 4-byte AS numbers. Cisco NX-OS displays 4-byte AS numbers in plain-text notation (that is, as 32-bit integers). You can configure 4-byte AS numbers as either plain-text notation (for example, 1 to 42,94,967,295) or AS.dot notation (for example, 1.0).
A BGP speaker does not discover and peer with another BGP speaker automatically. You must configure the relationships between BGP speakers. A BGP peer is a BGP speaker that has an active TCP connection to another BGP speaker.
BGP uses TCP port 179 to create a TCP session with a peer. When a TCP connection is established between peers, each BGP peer initially exchanges all of its routes—the complete BGP routing table—with the other peer. After this initial exchange, the BGP peers send only incremental updates when a topology change occurs in the network or when a routing policy change occurs. In the periods of inactivity between these updates, peers exchange special messages called keepalives. The hold time is the maximum time limit that can elapse between receiving consecutive BGP update or keepalive messages. Cisco NX-OS supports the following peer configuration options:
Individual IPv4 or IPv4 address: BGP establishes a session with the BGP speaker that matches the remote address and AS number.
IPv4 or IPv6 prefix peers for a single AS number: BGP establishes sessions with BGP speakers that match the prefix and the AS number.
Dynamic AS number prefix peers: BGP establishes sessions with BGP speakers that match the prefix and an AS number from a list of configured AS numbers.
Cisco NX-OS accepts a range or list of AS numbers to establish BGP sessions and does not associate prefix peers with dynamic AS numbers as either interior BGP (iBGP) or external BGP (eBGP) sessions until after the session is established.
For example, if you configure BGP to use IPv4 prefix 172.16.2.0/8 and AS numbers 10, 30, and 100, BGP establishes a session with 172.16.2.1 with AS number 30 but rejects a session from 172.16.2.2 with AS number 20.
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