OS10 Enterprise Edition User Guide Release 10.4.0E(R3)


Border gateway protocol

Border gateway protocol (BGP) is an interautonomous system routing protocol that transmits interdomain routing information within and between autonomous systems (AS). The primary function of BGP is to exchange network reachability information with other BGP systems. BGP adds reliability to network connections by using multiple paths from one router to another. Unlike most routing protocols, BGP uses TCP as its transport protocol.

Autonomous systems

BGP autonomous systems are a collection of nodes under a single administration with shared network routing policies. Each AS has a number, which an Internet authority assigns—you do not assign the BGP number.

The Internet Assigned Numbers Authority (IANA) identifies each network with a unique AS number (ASN). The AS numbers 64512 through 65534 are reserved for private purposes. The AS numbers 0 and 65535 cannot be used in a live environment. IANA assigns valid AS numbers in the range of 1 to 64511.

Multihomed AS
Maintains connections to more than one other AS. This group allows the AS to remain connected to the Internet if a complete failure occurs to one of their connections. This type of AS does not allow traffic from one AS to pass through on its way to another AS.
Stub AS
Connected to only one AS.
Transit AS
Provides connections through itself to separate networks. For example, Router 1 uses Router 2—the transit AS, to connect to Router 4. Internet service providers (ISPs) are always a transit AS because they provide connections from one network to another. An ISP uses a transit AS to sell transit service to a customer network.

When BGP operates inside an AS - AS1 or AS2, it functions as an internal border gateway protocol (IBGP). When BGP operates between AS endpoints - AS1 and AS2, it functions as an external border gateway protocol (EBGP). IBGP provides routers inside the AS with the path to reach a router external to the AS. EBGP routers exchange information with other EBGP routers and IBGP routers to maintain connectivity and accessibility.

External BGP

Classless interdomain routing

BGPv4 supports classless interdomain routing (CIDR) with aggregate routes and AS paths. CIDR defines a network using a prefix consisting of an IP address and mask, resulting in efficient use of the IPv4 address space. Using aggregate routes reduces the size of routing tables.

Path-vector routing

BGP uses a path-vector protocol which maintains dynamically updated path information. Path information updates which return to the originating node are detected and discarded. BGP does not use a traditional internal gateway protocol (IGP) matrix but makes routing decisions based on path, network policies, and/or rule sets.

Full-mesh topology

In an AS, a BGP network must be in “full mesh” for routes received from an internal BGP peer to send to another IBGP peer. Each BGP router talks to all other BGP routers in a session. For example, in an AS with four BGP routers, each router has three peers; in an AS with six routers, each router has five peers.

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