8.2. Routing targets

A route can specify various targets for the packet :-

Table 8.1. Example route targets

TargetNotes
an Ethernet interface (locally-atached subnet)requires ARP or ND to find the device on the LAN to which the traffic is to be sent.
a specific IP address (a "gateway")the packet is forwarded to another router (gateway) ; routing is then determined based on the gateway's IP address instead
tunnel interface such as L2TP, PPPoE or FB105 tunnels.such routes are created as part of the config for the interface and relate to the specific tunnel.
special targetse.g. the FB2900 itself, or to a black hole (causes all traffic to be dropped)

These are covered in more detail in the following sections.

8.2.1. Subnet routes

Whenever you define a subnet or one is created dynamically (e.g. by DHCP), an associated route is automatically created for the associated prefix. Packets being routed to a subnet are sent to the Ethernet interface that the subnet is associated with. Traffic routed to the subnet will use ARP or ND to find the final MAC address to send the packet to.

In addition, a subnet definition creates a very specific single IP (a "/32" for IPv4, or a "/128" for IPv6) route for the IP address of the FB2900 itself on that subnet. This is a separate loop-back route which effectively internally routes traffic back into the FB2900 itself - i.e. it never appears externally.

A subnet can also have a gateway specified, either in the config or by DHCP or RA. This gateway is just like creating a route to 0.0.0.0/0 or ::/0 as a specific route configuration. It is mainly associated with the subnet for convenience. If defined by DHCP or RA then, like the rest of the routes created by DHCP or RA, it is removed when the DHCP or RA times out.

Example: <subnet ip="192.168.0.1/24"/> creates a route for destination 192.168.0.0/24 to the interface associated with that subnet. A loop-back route to 192.168.0.1 (the FB2900's own IP address on that subnet) is also created.

8.2.2. Routing to an IP address (gateway route)

Routes can be defined to forward traffic to another IP address, which will typically be another router (often also called a gateway) For such a routing target, the gateway's IP address is then used to determine how to route the traffic, and another routing decision is made. This subsequent routing decision usually identifies an interface or other data link to send the packet via - in more unusual cases, the subsequent routing decision identifies another gateway, so it is possible for the process to be 'recursive' until a 'real' destination is found.

Example: <route ip="0.0.0.0/0" gateway="192.168.0.100"/> creates a default IPv4 route that forwards traffic to 192.168.0.100. The routing for 192.168.0.100 then has to be looked up to find the final target, e.g. it may be to an Ethernet interface, in which case an ARP is done for 192.168.0.100 to find the MAC to send the traffic.

There is logic to ensure that the next-hop is valid - the gateway specified must be routable somewhere and if that is via an Ethernet interface then the endpoint must be answering ARP or ND packets. If not, then the route using the gateway is supressed and other less specific routes may apply.

8.2.3. Special targets

It is possible to define two special targets :-

  • 'black-hole' : packets routed to a black-hole are silently dropped. 'Silent' refers to the lack of any ICMP response back to the sender.
  • 'nowhere' (also called Dead End) : packets routed to 'nowhere' are also dropped but the FB2900 generates ICMP error responses back to the sender.

The blackhole and nowhere top-level objects are used to specify prefixes which are routed to these special targets. In the User Interface, these objects can be found under the Routes category icon.

When using BGP you can also define a network which is announced by default, along with any dead-end-community, and treated otherwise the same as nowhere.