Information Technology Reference
In-Depth Information
Full iBGP Mesh with No Confederation to AS 2
To A S 2
To A S 1
AS 100
B
A
G
C
F
D
E
To A S 3
To A S 6
To A S 4
To A S 5
iBGP Connections Reduced with Confederation
To A S 2
To A S 1
B
AS 10
AS 20
Confederation 100
A
G
C
F
D
E
To A S 3
To A S 6
To A S 4
To A S 5
Figure 11-12
BGP Confederations
BGP Path Attributes
BGP uses several attributes for the path-selection process. BGP uses path attributes to
communicate routing policies. BGP path attributes include next hop, local preference, au-
tonomous system path, origin, multi-exit discriminator (MED), atomic aggregate, and ag-
gregator. Of these, the autonomous system path is one of the most important attributes: It
lists the number of autonomous system paths to reach a destination network.
BGP attributes can be categorized as
well known
or
optional
. Well-known attributes are
recognized by all BGP implementations. Optional attributes do not have to be supported
by the BGP process.
We l l - k n o w n a t t r i b u t e s c a n b e f u r t h e r s u b c a t e g o r i z e d a s
mandatory
or
discretionary
.
Mandatory attributes are always included in BGP update messages. Discretionary attrib-
utes might or might not be included in the BGP update message.
Optional attributes can be further subcategorized as
transitive
or
nontransitive
must advertise the route with transitive attributes to its peers even if it does not support
