BGP
Like Interior Gateway Protocols (IGPs), BGP exchanges cartography advice in adjustment for routers
to eventually apprentice the best routes to a set of IP prefixes. Unlike IGPs, BGP does not use a metric
to baddest the best avenue amid alternating routes to the aforementioned destination. Instead, BGP uses several
BGP aisle attributes (PAs) and an complex accommodation action back allotment amid multiple
possible routes to the aforementioned subnet.
BGP uses the BGP free arrangement aisle (AS_PATH) PA as its absence metric apparatus when
none of the added PAs has been ever set and configured. Generally speaking, BGP uses PAs to
describe the characteristics of a route; this introduces and explains a advanced array of BGP PAs. The
AS_PATH aspect lists the path, as authentic by a arrangement of free arrangement numbers (ASNs)
through which a packet charge canyon to ability a prefix. Amount 11-1 shows an example.
Figure 11-1 BGP AS_PATHs and Aisle Agent Logic
Figure 11-1 shows a archetypal case of how BGP uses aisle agent argumentation to accept routes. In the figure, R1
learns of two AS_PATHs by which to ability 9.0.0.0/8—through ASNs 2-3 and through ASNs 5-4-3.
If none of the routers has acclimated acquisition behavior to access added PAs that access BGP’s
choice of which avenue is best, R1 will accept the beeline AS_PATH—in this case, AS_PATH 2-3.
In effect, BGP treats the AS_PATH as a vector, and the breadth of the agent (the cardinal of ASNs
in the path) determines the best route. With BGP, the appellation avenue still refers to acceptable hopby-
hop IP routes, but the appellation aisle refers to the arrangement of free systems acclimated to reach
a accurate destination.
ASN 2
R4
ASN 5
R5
ASN 1
R1
ASN 3
R7
Network
9.0.0.0/8
9.0.0.0/8
AS_PATH 2, 3
AS_PATH 5, 4, 3
ASN 4
R6
This affiliate follows a agnate arrangement as several of the IGP chapters. First, the argument focuses on
neighbor relationships, followed by how BGP exchanges acquisition advice with its neighbors.
The affiliate ends with a area accoutrement how BGP adds IP routes to a router’s IP acquisition table
based on the BGP cartography table.