Connectivity Guide

The Internet Assigned Numbers Authority (IANA) identifies 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 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.
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 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
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.
Sessions and peers
A BGP session starts with two routers communicating using the BGP. The two end-points of the session are called peers. A peer
is also called a neighbor. Events and timers determine the information exchange between peers. BGP focuses on traffic routing
policies.
Sessions
In operations with other BGP peers, a BGP process uses a simple finite state machine consisting of six states—Idle,
Connect, Active, OpenSent, OpenConfirm, and Established. For each peer-to-peer session, a BGP implementation
tracks the state of the session. The BGP defines the messages that each peer exchanges to change the session from one state
to another.
Idle
BGP initializes all resources, refuses all inbound BGP connection attempts, and starts a TCP connection
to the peer.
410 Layer 3