BGP Concepts and attribute

BGP Path Vector Characteristics 
BGP routers exchange network reachability information, called path vectors, made up of path attributes.

Why should BGP use
- AS has multiple connections to other AS.
- AS allows packets to transit through it to reach other AS.
- To manipulate the traffic entering and leaving the AS.

Why should not BGP use
- a single connection to internet or another AS.
- if border router can't handle bgp updates. eg,.lack of memory and cpu power
- don't much familier about route filtering.
In these case, use static or default routes.

BGP Synchronization 
- is disabled by default in Cisco IOS Software Release  12.2(8)T and later.

BGP Table
- keeps it own table for storing BGP information.
- give best path of BGP table to routing table.

BGP Message Types
open, update, notification, keepalive

BGP Message Header
All messages begin with the same 3 field headers.

Path Attribute
- are a set of BGP metrics.
- BGP uses the path attributes to determine the best path to the networks.
Some attributes are mandatory and auto included in update messages while others are manually configurable.
- use to enforce routing policy.

A BGP update message includes a variable-length sequence of path attributes describing the route. Each path attribute is a triple -
1. attribute type
2. attribute length
3. attribute value

Attribute Types
Four different attribute types -
1. well-known Mandatory (AS_PATH, NEXT_HOP, ORIGIN)
2. well-known Discretionary (LOCAL_PREF, ATOMIC_AGGREGATE)
3. Optional Transitive (community)
4. Optional Nontransitive (MED)

Cisco Weight Attribute
- cisco proprietary attribute.
- similar to the local preference.
- locally configured on a router and is not propagated to any other routers.
- use 16 bit (0-65535)
- default is 32768.

BGP Route Selection Process

reference :: cisco networking academy

BGP Terminology

Autonomous System
An AS is a group of routers that share similar routing policies and operate within a single AD. Unique AS number is managed by IANA.
- use 16 bit(1-65535)
- public AS number is 1-64511.
- private AS number is 64512-65535.

Internet Assigned Numbers Authority
- Divided into five RIRs(Regional Internet Registry). AFRINIC, APNIC, ARIN, LACNIC, RIPE NCC.
- assigned unique AS number and IP Address.

BGP
Internet is a collection of AS that are interconnected to allow communication among them. BGP provides the routing between these AS.
- is a path vector protocol.
- use TCP.
- latest version is BGPv4.

Later, I will discuss Comparison of BGP and IGPS.

EBGP - run between routers in different AS.
IBGP - run between routers in the same AS.

EBGP neighbors need to be directly connected.
- need TCP session (three way handshake)
- different AS numbers , AD - 20.
IBGP neighbors must be reachable usually by using an IGP. Loopback IP are used to identify IBGP neighbors.
- must be established TCP session.
- same AS number, AD - 200.

Transit AS
A transit AS is an AS that routes traffic from one external AS to another external AS.
IBGP in a transit AS - fully meshed BGP internetwork and BGP runs on all internal routers                                    and all routers establish IBGP sessions.
IBGP in a Non-transit AS - To avoid routing loop within an AS, BGP specifies that routes                                             learned through IBGP are never propagated to other IBGP                                                 peers.