BGP Route
A BGP route consists of two parts, a prefix and a set of path attributes. It is not uncommon
to use the term path to refer to a BGP route, although that term technically refers to one
of the path attributes of that route.
Routing Information Base
BGP routes are stored in a BGP speaker’s routing information base (RIB), also known as
its routing table, which conceptually consists of the following three parts:
•
Adj-RIBs-In store unprocessed routes learned from update messages received by the
BGP speaker.
•
Loc-RIB contains local routes resulting from the BGP speaker applying its local policies
to the routes contained in its Adj-RIBs-In.
•
Adj-RIBs-Out store routes that the BGP speaker will advertise to its peers in the update
messages it sends.
Prefixes and CIDR
A prefix describes a set of IP addresses that can be reached using the route. For example,
the prefix 10.1.1.0/24 indicates all IP addresses whose first 24 bits contain the value 10.1.1.
The term network is sometimes used instead of prefix to describe a set of addresses. To
reduce confusion, this chapter restricts network to its more common usage, to refer to a
physical structure of routers and links.
Prefixes are made possible by classless interdomain routing (CIDR). CIDR addresses
have largely replaced the concept of classful addresses (such as Class A, Class B, and
Class C) in the Internet. Classful addresses have an implicit, fixed-length mask
corresponding to the predefined class boundaries. For example, 192.56.0.0 is a Class B
address with an implicit (or natural) mask of 255.255.0.0.
CIDR uses network prefixes and explicit masks, represented by a prefix length, enabling
network prefixes of arbitrary lengths. CIDR represents the sample address above as
192.56.0.0/16. The /16 indicates that the high-order 16 bits (the first 16 bits counting from
left to right) in the address mask are all 1s.
CIDR enables you to aggregate multiple classful addresses into a single classless
advertisement, reducing the number of advertisements that must be made to provide
full access to all the addresses. Suppose an ISP has customers with the following
addresses:
192.168.128.0
192.168.129.0
192.168.130.0
192.168.131.0
9Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring BGP Routing
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