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Chapter 7

BGP's Basic Components

By turning on the first bit, this flag indicates that all well-known attributes

must be passed along to downstream peers after the peers receive and process
the message. BGP does not require every implementation to support every
option. The second bit specifies how implementations handle options they
do not recognize. If the second bit, which is known as the transitive flag, is
on, then if the option is recognized it will pass the information downstream
to its BGP neighbors. If the bit is turned off, then the option is ignored and
not passed downstream to other neighbors.

All well-known attributes are considered transitive.

Some attributes appear only in iBGP or in eBGP. For this book, we con-

sider that iBGP and eBGP are the same protocol but with differences in the
peering points and the types of attributes of each. Remember where each is
used. In iBGP, each peer communicates between speakers in the same AS,
and in eBGP, peers communicate between speakers in different ASes.

Path attributes can be considered as the metrics used by BGP routers that

are passed in UPDATE messages to other BGP peers. These messages can
contain notifications of local routes, foreign routes, or route topology
changes. An attribute can be placed in one of four categories, as listed below:

Well-known mandatory

Well-known discretionary

Optional transitive

Optional non-transitive

Let's take a look at the characteristics of each attribute in the next sections

and the attributes that can be associated with each type.

WELL-KNOWN MANDATORY

A well-known mandatory attribute is used by a totally compliant BGP imple-
mentation to propagate all the network's BGP neighbors. Well-known man-
datory attributes must appear in all BGP update messages. This means that
a well-known mandatory attribute must appear in an advertised route and

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