Routing Protocol: BGP (CCNP Notes)

BGP

It is the protocol of the internet.  It basically connects different AS networks.  Creates a TCP session on port  179. Routers between different AS exchange information on how to reach and are also called as edge routers or border routers. All routers within a single AS and participating in BGP must be configured in full mesh and each router must be configured as a peer to one another.(This creates a problem as number of routers increases quadtratically as router increases. A solution to this is

Any two routers that have opened a TCP connection to each other for the purpose of exchanging routing information are known as peers or neighbors.

EBGP peers are directly connected while IBGP peers are not directly connected.

Usually, the two EBGP speakers are directly connected (for example, over a wide-area network [WAN] connection). Sometimes, however, they cannot be directly connected. In this special case, the neighbor ebgp-multihop router configuration command is used.

Note: Multihop is used only for EBGP, but not for IBGP.

 

The BGP synchronization rule states that if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP.(Reason for this rule Is otherwise there is packet loss.  Turned off because if you are running BGP it will be on all the routers in the network)

·         Inter AS routing

·         Uses port 179 and TCP

·         Two types , eBGP and iBGp

·         Path Attributes and BGP routes.

 

BGP propagates the best path to its neighbors. The decision is based on the value of attributes (such as next hop, administrative weights, local preference, the origin of the route, and path length) that the update contains and other BGP-configurable factors. This section describes the following attributes and factors that BGP uses in the decision-making process:

  • AS_path Attribute
  • Origin Attribute
  • Next Hop Attribute
  • Weight Attribute
  • Local Preference Attribute
  • Multi-Exit Discriminator Attribute
  • Community Attribute

Slowest routing protocol. BGP is technically a distance routing protocol.

Wihout tuning BGP behaves just like RIP.

Packets:

Open: Start the session

KeepAlive: Are u der???

Update Message: Network rechability exchanges

Notification: Something bad happened; close connection

Tables:

Neighbour Tables: The connected BGP friends

BGP Table:  List of all BGP routes(can be very big)

Routing table: list of all best routes

IBGP vs EGBP

Ibgp is used within the same AS and EBGP is used between diff AS

IBGP  relationship can form through the routers.

multihop. EBGP relationships between routers by default must be directly connected.

Two ways to get networks into BGP:

-Network command(Tells what networks to advertise)

-Redistribution

BGP next Hop processing:

For EBGP peers: change next hop address on advertised routes

For IBGP peers : Do not change next hop address on advertised routes

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