Discussion

way, a policy applied to a group overrides a BGP-wide policy, and a policy applied to
a peer overrides both a group and BGP-wide policy.
Multiprotocol BGP (MBGP), defined in RFC 2858, is an extension to BGP that supports
other protocols, including IPv6, MPLS, and VPNs.
In choosing routes toward a destination, if there is more than one route to the same
destination, BGP uses an algorithm to select a single route to use (see the Introduction
to Chapter 8). Note that other router vendors may follow a slightly different set
of rules to determine the active route.
For more information about BGP, see BGP4: Inter-Domain Routing in the Internet
(Addison-Wesley).
BGP Attributes
BGP routers exchange routes, or NLRI, with their neighbors. An NLRI consists of a
route prefix and the BGP attributes associated with the route. Attributes contain
information about a route, such as where it came from and how to reach it, that BGP
uses to choose the best path to a destination. A number of attributes were defined in
the original BGP specification, and, over time, attributes have been added to extend
the functionality of BGP. Compared to IGP routes, which generally just carry the
route, a next hop, metric, and an optional tag, BGP routes typically have about a
dozen attributes associated with them.
There are several types of attributes. Well-known attributes are supported by all BGP
implementations. Mandatory attributes are included with every prefix. If they are
missing, the receiving BGP router will generate an error message. Discretionary
attributes are those that BGP routers must recognize and support but don’t have to
be included with every prefix. When a BGP router passes a prefix to its peers, it
includes all well-known, mandatory, and discretionary attributes associated with the
prefix, either in the state they were received or in the state after they were modified
when they passed through the local AS.
BGP routers can also include optional attributes with prefixes, or those that are not
necessarily supported by all BGP routers. Optional attributes can be transitive, which
means that BGP must include the information when sending the prefix to another
router even if the sending router doesn’t understand the option, or nontransitive,
which allows a router that doesn’t understand the option to silently drop it when
advertising the prefix.
The following are some of the common BGP attributes. Most BGP implementations
understand these attributes.
ORIGIN (well-known, mandatory)
Designates how BGP learned about the route. It can be one of the following:
I
Route was originally learned from an IGP in the originating AS.
420 | Chapter 13: BGP
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