Dell PowerSwitch S4128F-ON OS10 Enterprise Edition User Guide Release 10.4.1.0 - Page 294

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 Stub AS Transit 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. Connected to only one 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 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. 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 294 Layer 3