Private ASNs (64,512–65,535) should not be used in AS_PATHs advertised into the Internet beyond a single ISP. One purpose of this private range is to conserve the ASN space by assigning private ASNs to customers that only connect to that single ISP. Then, the ISP can simply remove the private ASN before advertising any routes for that customer outside its network. Figure 13-9 shows the typical case for using a private AS. While the concept is relatively simple, the configu..
Checks Before Becoming BGP Neighbors
Similar to IGPs, BGP checks certain requirements before another router may become a neighbor, reaching the BGP established state. Most of the settings are straightforward; the only tricky part relates to the use of IP addresses. The following list describes the checks that BGP performs when forming neighbor relationships: 1. The router must receive a TCP connection request with a source address that the router finds in a BGP neighbor command. 2. A router’s ASN (on the ro..
Having a feasible successor provides the best convergence. A feasible successor is a backup path, and it can be substituted for a lost path at any point. When a path is lost and no feasible successor exists, the router will send queries to its remaining neighbors. If a neighbor does not know of an alternative path, it will recursively ask its neighbors. Recursive queries can loop without being resolved, forcing the router to time-out the query. This situation is known as stuc..
Integrated Services Digital Network (ISDN) is used by telephone companies to carry digitized voice and data over the existing telephone system. ISDN has emerged as one of the leading technologies for telecommuting and remote office connection into the corporate headquarters. On the seven layer model, ISDN operates at the physical, data link, and network layers. ISDN basic rate interface (BRI) provides two barrier channels (commonly referred as B channels). Each of these B ch..
A campus network’s core layer provides connectivity of all distribution-layer devices. The core, sometimes referred to as the backbone, must be capable of switching traffic as efficiently as possible. Core devices, sometimes called campus backbone switches, should have the following attributes: ■ Very high throughput at Layer 3 ■ No costly or unnecessary packet manipulations (access lists, packet filtering) ■ Redundancy and resilience for high availability ■ Advanc..
