Answer / jagadiesh

The Border Gateway Protocol (BGP) is the core routing
protocol of the Internet. It maintains a table of IP
networks or 'prefixes' which designate ...

he 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.

Answer / akash

Just want to add BGP is application oriented protocol and
In IGP there must be a fullmesh topology used to establish the connection.

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