How to advertise networks in BGP

CCNA 200-301

CCNP Enterprise

CCNP Security

CCIE Enterprise Lab

CCIE Security Lab

CCNP Service Provider

CCNP Data Center

CCNP Collaboration

CCIE DC Lab

2019-09-27

In this lesson, we’ll take a look at how you can advertise networks in BGP. There are two methods of how we can do this:

1. Network command

2. Redistribution

Like our IGP, we can use the network command to advertise something, or we can reassign the network to BGP. However, there is a big difference in the behavior of BGP’s network commands.

when you use any igp (rip, OSPF or eigrp), the network command is used to activate igp on all interfaces within the scope of the network command.

BGP doesn't care about interfaces, it doesn’t even look at them. When we use the network command in BGP then BGP will only look at the routing table. When it finds the network that matches the network command, it will install it in the BGP table.

Let me show you some examples to explain what I’m talking about. We will use the following two routers:

R1 and R2 are in different autonomous systems, so we use eBGP. This is the BGP configuration:

there is nothing special here, just the normal eBGP. between R1 and R2

Let’s broadcast some networks in BGP

Network Command

Let's create a loopback interface with a network and advertise it in BGP:

above we created a loopback interface with network 1.1.1.0/24, which is the interface we will release in BGP. Since we created the loopback interface, the network will be connected directly to R 1：

Everything here is it's own. Simply use the network command to put the required network into the BGP table. One thing you must know is that you must use the exact network and subnet mask for the network command. Let me give you an example:

I created a loopback interface using the network 11.11. 11.11/32. BGP uses the network command to issue 11.11. 11.0/24. Because the subnet mask does not match, the network is never placed in the BGP table:

Be aware of this. Make sure you type the exact network address and subnet mask when advertising something in BGP. Let’s fix this:

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