Building peer sessions

:A neighbor connection (also referred to as a peer connection) between two routers can be established within the same AS, in which case BGP is called internal BGP (IBGP). Likewise, a peer connection between routers in different ASs is referred to as external BGP (EBGP).

:The neighbor negotiation process is mainly the same for internal and external neighbors as far as building the TCP connection at the transport level. 

:Neighbors can reach one another via some Interior Gateway Protocol (IGP), the BGP session is established, and BGP messages are exchanged.

:Generally, for external BGP sessions, a route through a directly connected interface establishes IP reachability. Indirectly connected external neighbors require extra configuration.

:A BGP session formed between external BGP peers that are not physically connected is  referred  to as multihop EBGP. 

:The current authentication features available in BGP-4 use the message-digest version 5 (MD5) algorithm.

:it is important to maintain a full IBGP mesh within the AS.

:By definition, the default behavior of BGP requires that it must be synchronized with the IGP before BGP may advertise transit routes to external ASs. The consequence of injecting BGP routes inside an IGP is costly. Redistributing routes from BGP into the IGP will result in major overhead on the internal routers, primarily from an IGP scalability perspective, because (as discussed earlier) IGPs are not designed to handle that many routes. That said, by far the most common configuration in Internet-connected networks is to disable BGP synchronization and rely on a full mesh of IBGP routers.

reference :: Internet Routing Architecture

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.