Connectivity Guide
Supported releases 10.4.1.0 or later
Border Gateway Protocol
Border Gateway Protocol (BGP) is an interautonomous system routing protocol that transmits interdomain routing information within and
between autonomous systems (AS). BGP exchanges 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) identies each network with a unique AS number (ASN). AS numbers 64512 through
65534 are reserved for private purposes. 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 trac 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.
Classless interdomain routing
BGPv4 supports classless interdomain routing (CIDR) with aggregate routes and AS paths. CIDR denes a network using a prex consisting
of an IP address and mask, resulting in ecient use of the IPv4 address space. Using aggregate routes reduces the size of routing tables.
Path-vector routing
BGP uses a path-vector protocol that maintains dynamically updated path information. Path information updates which returns 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
Layer 3
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