External Gateway Routing Protocol

Border Gateway Protocol (BGP) is an exterior routing protocol that connects two separate networks that are owned by two completely different authorities. This is the protocol that is used in the Internet.


BGP always work with an IGP to route the traffic. BGP is not intelligent it does not calculates bandwidth and any other mechanism for traffic transmission, all it does is choose the route that has shortest path. We can refer path as its metric, it counts the number of autonomous systems to get to the destination and picks the route that has less autonomous systems. For example the packet is destined for XYZ network and there are two routes to the destination one has 5 routers and other has 3 routers, BGP will pick the path that has 3 routers and will send the traffic.



When to use BGP?

BGP is not run when you rent public IPs from you ISP, it is not run when you have only one way to the Internet.
It is only run when you own the block of public IPs and you want to send your traffic for more than one ISP that’s where you need to run BGP.

ISPs daily use and work with this protocol.

BGP Configuration


Interface Gig0/1 is the port where I have pool of public IPs.

First assign autonomous number to your IPs and assign the network to autonomous number. Most autonomous numbers are between 1 to 65535.


Now assign the network to autonomous system that you will use to connect the ISP.


Now show the routing table to make sure we are not still using BGP route.


Now we need to tell the BGP about our neighbor and its autonomous number, this is a static process.


As soon as you connect with your ISP autonomous number your ISP will provide you three options for routing tables.

1 – Provide you a list of entire Internet routing table this contains hundreds of thousands of routing tables which your router will not be able to support.

2 – Provide you the routing table list of your ISP routing table this gives you the list of autonomous number and routing tables that your ISP holds.

3 – Provide you nothing this will give you nothing and you will just share your autonomous number with Internet.

Now we can see routing table again and there must be some BGP routes in our table.



Following is the routing information of BGP.
* symbol means that the route is valid.
> means it is the best route.



BGP by default does not support redundancy the best practice to make redundancy is to define the IP and neighbor on the Loopback interface.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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