In this topic we will focus on just one command and the default behavior of BGP on Cisco devices. For the Cisco IOS we just configure a neighbor under the global BGP process to enable the IPv4 unicast peering, like following:

R1(config)#router bgp 1 R1(config-router)#neighbor 10.10.10.2 remote-as 2 R1(config-router)#neighbor 10.10.10.2 ebgp-multihop

So that when we like to do a VPNv4 Peering with the same neighbor we have to “Activate” it under a certain address-family:

R1(config-router)#address-family vpnv4 unicast R1(config-router-af)#neighbor 10.10.10.2 activate

Now you’re maintaining two types of BGP peering with the neighbor “IPv4 & VPNv4”, So the question which might come up in your mind “What if I don’t want to maintain IPv4 peering but only VPNv4?”

Yeah, good question and so important for networks which allocate a Route-Reflector for VPNv4 routes separate from the IPv4 Route-Reflector if exist, We want to reduce the headache of maintaining an unused IPv4 session hitting the router’s processor.

We can change the default behavior of the Cisco IOS from maintaining any type of BGP peering till mentioning it, this magic command is “no bgp default ipv4-unicast”, lets take a look of how it works:

We have two routers connected through a direct link “10.10.10.0/24”, with the following configurations:

R1(config)#router bgp 1 R1(config-router)#no bgp default ipv4-unicast R1(config-router)#neighbor 10.10.10.2 remote-as 2 R1(config-router)#neighbor 10.10.10.2 ebgp

R2(config)#router bgp 2 R2(config-router)#no bgp default ipv4-unicast R2(config-router)#neighbor 10.10.10.1 remote-as 1 R2(config-router)#neighbor 10.10.10.1 ebgp

If we checked the BGP sessions configured we will find nothing, that is because we didn’t mention the address-family of the neighbor as we disabled the default behavior of negotiating IPv4 connection.

R1#show ip bgp summary R1#show bgp vpnv4 unicast all summary % Uninitialized context

Now we can Activate the neighbor under any address-family available by the mention of the address-family and then type the command “neighbor X.X.X.X activate”

Here are the available address-families for Cisco IOS:

R1(config-router)#address-family ? ipv4   Address family ipv6   Address family l2vpn  Address family nsap   Address family vpnv4  Address family vpnv6  Address family

It is recommended to try it yourself in lab and Enjoy 😉