concise BGP notes CCNP/CCIE

BGP :

BGP is a path vector routing protocol

EBGP  route  has the distance of 20

IBGP route  has the distance of 200

BGP private autonomous system range 64512-65535

TTL value is 1 by default for EBGP session

use EBGP multi hop command to increase TTL value  it will   increase  to 255 .

in EBGP loop prevention happens based on AS-path information

if IBGP neighbor learns a network from another IBGP neighbor , it can not

advertise the same   route to another IBGP  neighbor if it does not have neighbor configured , that is where route reflector  comes into play.

BGP routers only advertise best path route to their  neighbor

BGP  route based on TCP  port 179 and it does not show in routing table

that the route  learn through an interface , unlike IGP protocols

, BGP  always shows that route learned from an ip address.

in BGP  ( ? )  mark in origin code shows that route either redistributed to BGP

or route injected through BGP  inject map.

weight and local preference typically used to effect outbound traffic coming in

As-path and MED  used to manipulate to inbound traffic going out to neighbor

in BGP neighbor ship who ever having highest router id , will initiate a TCP session to port 179 to its neighbor from a random port , meaning it will act as BGP  client,

to its neighbor , and neighbor will reply from TCP port 179 to a random port to its destination, that will act as TCP server.

BGP  update are incremental , meaning if there is change it triggers an update

BGP packet type :

open :  start the session

keealive :

update :  network reach ability exchange

notification: something bad has happened close session

BGP Tables :

neighbor table : connected peers

BGP table :  all routes router know about

routing table : list of  best route

EBGP multi hop command required when peering with EBGP neighbor if neighbor not connect directly . if neighbor connected directly no need EBGP multi hop command .

Rule BGP synchronization :

A BGP speaking router do not use or advertise a EBGP route via IBGP , until there is a valid route learned for that   through IGP. 

BGP next-hop processing :

for EBGP peers :  change next hop address on advertise routes.

for IBGP peers : do not change next hop address on advertise routes

BGP split horizon :

BGP split horizon rule , if a route  receive via IBGP update from a IBGP peer

don’t send it back to other IBGP peer.

BGP neighbor relationship status:

1. idle : verifying route to neighbor

2. active : attempting connectivity to neighbor

3.open sent : open message (hello) sent to neighbor

4. open confirm : neighbor replied to open message

4.active : neighbor failed to reply or mismatched parameters

5.Establised  

BGP well know attribute :

weight

as=path

next hop address

origin

local preference

MED  :   to influence other autonomous system routing path , meaning updates are going out can be modified through MED

the difference between weight and local preference is weight locally significant

with in the router , local preference gets advertise with in Autonomous system

by a router .