Configure a Routing Link for BGP Peers

Determine Requirements

In order to configure your your router to peer with other BGP routers, you will need to identify several parameters associated with the routing configuration. First, identify your ASN and select a unique IP address that you will assign as the router ID. Next determine which interface to use for the routing link, and identify the address ranges to be advertised to neighboring routers.

We'll use the following values in this example: - Interface for routing link: vlan10 - Autonomous System Number (ASN): 65000 - Router ID: 172.23.0.129 - Advertise Network(s): 172.23.0.0/24

Prepare Interfaces

To begin, we need to prepare the interface that we'll be using to peer with neighboring routers. In the past, this would have meant configuring IPv4 addresses for each side of the connection; however, we will be taking advantage of unnumbered BGP to identify and connect to our peers based on auto-configured IPv6 addresses. This simplification is welcome since the addresses we assign on point-to-point routing links generally serve no purpose other than facilitating communication between the adjacent routers.

Our main task is to enable auto-configuration for IPv6 and to disable auto-configuration privacy extensions so that our MAC address will be encoded into the link-local IPv6 address for the interface. We also need to ensure that the routing interface will listen for and accept ICMPv6 router advertisements from its peer.

Configure interface

Create or edit a networkd .network configuration for the routing link, e.g., /etc/systemd/network/24-vlan10.network and ensure that the following IPv6 settings are in place. No IPv4 configuration will be performed on this link.

[Match]
Name=vlan10

[Network]
# Enable link local addressing for IPv6 only
LinkLocalAddressing=ipv6
# Watch for router advertisements so that we can learn about neighboring routers
IPv6AcceptRA=yes
# Required due to a bug in FRR (https://github.com/FRRouting/frr/issues/2205)
IPv6PrivacyExtensions=no

Configure FRRouting

Set up address for BGP router ID

The router can use any locally defined address as a router ID attached to BGP advertisements. Though we could use any one of our directly attached addresses, it is common to configure an ID on the loopback interface of the network device to ensure that it is available regardless of link status -- static addresses attached to physical interfaces are offline when the link is down.

We'll perform this configuration within the FRR VTY shell (launch using vtysh):

configure terminal
interface lo
ip address 172.23.0.129/32
quit

Additional unnumbered routing configuration

Routers in an unnumbered peering relationship rely on ICMPv6 router advertisements to discover peers on a routing link. We previously configured our interface to accept these messages, but we still need to set up the routing daemon to generate the messages on the specified interface.

The following commands will turn on ICMPv6 router advertisements and set an interval of one advertisement every 5 seconds (as recommended by FRR documentation).

Enter the following commands in the VTY shell:

configure terminal
interface vlan10
ipv6 nd ra-interval 5
no ipv6 nd suppress-ra
quit

Configure BGP Routing

Once the preliminary configuration is complete, we can setup the BGP daemon to peer on the specified interface. The following example will enable a new router in ASN 65000 that peers over the vlan10 interface and advertises routes for 172.23.0.0/24.

Enter the following commands in VTY shell: 

configure terminal
router bgp 65000 
bgp router-id 172.23.0.129 
neighbor vlan10 interface remote-as external
network 172.23.0.0/24
quit

Supporting multiple routing links on a single router requires a simple modification to this script to add additional neighbor statements. If you've already configured other details, you don't need to repeat them to add an aditional neighbor. Enter the following commands to resume editing your existing router:

configure terminal
router bgp 65000

Save Your Changes

Don't forget that live updates to FRR are not persistent. Rebooting the router or restarting frr will dispose of any settings that are not written to the startup config.

To save your changes, you need to be in enable mode. Exit out of configure mode, if necessary, and call copy running-config startup-config or write memory.

From the Linux commandline, you can view the current startup configuration within /etc/frr/frr.conf.