Whether you are planning on taking the Building Scalable Cisco
Networks (BSCN) or the Building Scalable Cisco Internetworks
(BSCI) exam on the path to the CCNP certification, you are going
to have to know BGP. For some reason, I have always had a hard
time remembering the BGP attributes. This article is
dedicated to the better understanding of BGP attributes.
A BGP attribute, or path attribute, is a metric used to
describe the characteristics of a BGP path. Attributes are
contained in update messages passed between BGP peers to
advertise routes. There are four categories of BGP attributes.
These are:
Optional transitive attributes may also be marked as partial.
Well-known attributes are attributes that all BGP
implementations must recognize and are propagated to all BGP
neighbors. Mandatory well-known attributes must appear in the
route description. Discretionary attributes do not have to
appear in the route description (they are not mandatory).
Optional attributes may not be supported by all BGP
implementations. The transitive bit determines if an optional
attribute is passed to BGP neighbors. If an optional attribute
is transitive and not supported by a neighbor, it is passed and
marked as partial. If it is non-transitive, a neighbor that
does not implement the attribute deletes it.
The attributes that are commonly used in the Cisco
Implementation of BGP are as follows:
WELL-KNOWN, MANDATORY
AS-path: A list of the Autonomous Systems (AS) numbers that a
route passes through to reach the destination. As the update
passes through an AS the AS number is inserted at the beginning
of the list. The AS-path attribute has a reverse-order list of
AS passed through to get to the destination.
Next-hop: The next-hop address that is used to reach the destination.
Origin: Indicates how BGP learned a particular route. There are
three possible types -- IGP (route is internal to the AS), EGP
(learned via EBGP), or Incomplete (origin unknown or learned in
a different way).
WELL-KNOWN, DISCRETIONARY
Local Preference: Defines the preferred exit point from the local
AS for a specific route.
Atomic Aggregate: Set if a router advertises an aggregate causes
path attribute information to be lost.
OPTIONAL, TRANSITIVE
Aggregator: Specifies the router ID and AS of the router that
originated an aggregate prefix. Used in conjunction with the
atomic aggregate attribute.
Community: Used to group routes that share common properties so
that policies can be applied at the group level.
OPTIONAL, NON-TRANSITIVE
Multi-exit-discriminator (MED): Indicates the preferred path
into an AS to external neighbors when multiple paths exist.
A list of path attributes is contained in BGP update messages.
The attribute is variable length and consists of three fields:
Attribute type consisting of a 1-byte attribute flags field and
a 1-byte attribute code field, Attribute length field that is 1
or 2 bytes, and a variable length attribute value field. The
attribute type codes used by Cisco are: 1-origin, 2-AS-path,
3-Next-hop, 4-MED, 5-Local preference, 6-Atomic aggregate,
7-aggregator, 8-community, 9-originator-ID, and 10-cluster list.
This article discussed the BGP attributes with the
intent of preparing you for those confusing questions on the
BSCN or BSCI exams. Hopefully, there is no more confusion on
the various attributes that are used in the Cisco
implementation of BGP.
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