BGP

BGP is the protocol which used in ISPs.
Port no: 179
It will be classified into two:

1.EBGP:

BGP Session established between different AS
AD : 20

2.IBGP;

Establishing BGP connection between routers which is located same AS.
AD : 200

BGP  is distance vector protocols.

Best Path Selection:

We can configure BGP to use a path based on our requirement using Weights and Priority.

AS_PATH:

It is a BGP attributes which has all the BGP AS number it has crossed. It is mainly used in loop prevention mechanism.

BGP don't try to discover neighbours. it only forms neighbour with directly connected router. with multihop it required existing route like static or dynamic route.Most of the time default route is won't work out.

BGP is control plan protocol.

BGP uses TCP for it's communications

BGP messages :

1.Open.
2.Update
3.Notification
4. Keep alive


OPEN:

It is used to form BGP adjacency.
It contains BGP version details, AS detail, Identifier(Mostly IPv4 as a router-id), Hold time and so on

Update:

This will carry the information about route changes.
This massage also act as a keep alive message to avoid unnecessary message.

Keep Alive :

This message only interact with neighbors to confirm that they are alive or not.

Notification:

This message will be send if there is a problem arise on running BGP setup.

BGP Neighbor State:


BGP is maintaining his neighbor in  6 states.

1. Idle
2. Connect
3. Active
4. OpenSent
5. OpenConfirm
6. Established

Idle is the starting stage of TCP three way handshake . This state it listens all the new connections.
Connect state is after successful connection establishment.
OpenSend- Sending  Open message to other neighbor and once receive the reply it matches the BGP details like Version , IP and so on.
OpenConfirm is once it confirmed that the message is verified and confirmed.
 Established state all information has been shared and wait for hold timer expire.

 Now we can see the Basic configuration of BGP:

 BGP configuration is same as other routing protocols
EX:

router bgp 62300
neighbor 10.8.65.2 remote-as 65423
address-family ipv4 unicast

Note:

Address family activation differ based on the IOS version.

SHOW commands:

show bgp ipv4 summary.

Prefix Advertisement:

BGP maintains its prefix and path attributes details in three Tables

1. Adj-RIB-in(information before inbound route information receives)

2. Adj-local(prefix details about route received from nearest routers)

3.Adj-RIB-out(prefix details about output bound

Policy have received)

BGP Best-Path calculation:

Best path calculation will happen based on below events:

1. Next hop reach-ability change 

2.Interface failure connected to an EBGP

3.Redistribution change

4.New paths for a route

BGP path attributes can be modified upon receipt or advertisement to influence routing in the local AS or neighboring AS.

IBGP:

IBGP is about BGP communication between the same AS.

If there is IGP connection it can form a relation over it.

EBGP:

This exchange network prefixes between AS. It differ compare from IBGP from TTL and advertising prepend and modifies route .

Route reflector:

IBGP peering can be configured so it reflects routes to another IBGP peer.

BGP multihoming and Multi pathing.

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It is helpful when we have to use ISP redundancy  

If both ISPs(all redundancy ISPs) from a same network then It uses same AS number and multipathing option

BGP route reflectors.

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It helps to reduce the routers load by forming neighbor with each routers on all ASs.

Which gives a loads, hence, route reflector gives alternated that it sends all neighboring routes to all in the AS routers . which helps to reduce the load of the routing table.

BGP confederations:

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BGP confederation is the method of AS hierarchy.

It is way to merge a multiple AS into a single AS. like Master and slaves.

BGP community:

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It is a tag which is placed on  the BGP routes. 

It helps to play with upstream and downstream routers traffic using routing policies.

BGP Atomic aggregate :

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When we do the summarization, it may sent routes to other routes with not having full details such as MED, community.

BGP groups:

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We can create a BGP groups and implement the prefix list from a router instead of having a prefix list on each router.