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Why We Need to Explicitly Activate the address-family ipv4

Separation of Address Families in BGP:

    • In modern versions of Cisco IOS, BGP is designed to support multiple address families beyond just IPv4. BGP can handle:
      • IPv4 unicast (standard routing for IPv4 addresses)
      • IPv6 unicast (for IPv6 routing)
      • VPNv4 and VPNv6 (for MPLS Layer 3 VPNs)
      • Multicast for IPv4 or IPv6
      • And other extensions like EVPN or MPLS VPN.
    • The address-family command is used to tell BGP which specific type of routes you want to activate. By default, no address-family is active, so you need to manually specify which ones BGP should work with.

  2. Default BGP Behavior:

    • In older Cisco IOS versions, BGP only supported IPv4 unicast by default, so this wasn't an issue. However, newer versions of IOS require explicit activation of the IPv4 unicast address family to avoid ambiguity and to support flexibility for other address families.
    • Without the address-family ipv4 activation, even though you configure neighbors in BGP, no routes would be exchanged because BGP doesn’t know which address family (e.g., IPv4, IPv6, VPNv4) it should advertise.

  3. BGP Flexibility:

    • Activating the address-family ipv4 ensures that BGP is processing and exchanging IPv4 unicast routes between neighbors. This structure allows you to later extend your configuration for:
      • IPv6 routing (by activating the address-family ipv6).
      • MPLS VPN routing (by activating address-family vpnv4).

What Happens If You Don’t Activate the Address Family?

  • Without explicitly activating address-family ipv4 unicast, BGP sessions may come up (you’ll see the BGP neighbor state as Established), but no IPv4 routes will be exchanged between the routers.
  • This is because the BGP process hasn’t been told to handle and exchange IPv4 routes. By activating the IPv4 address family, you are instructing BGP to advertise and accept IPv4 routes from the neighbors.

Conclusion:

Even though you're using IPv4 in the network, activating the address-family ipv4 command is essential in modern Cisco IOS versions to ensure BGP knows to process and advertise IPv4 unicast routes between peers. This flexibility also allows for easy extension of your BGP configuration to support other address families like IPv6 or VPNv4 in the future.

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