In scenarios where subscriber host IPv4 address lies within the configured subscriber-interface subnet, the default-gw IPv4 address for the host is one of the subscriber-interface IPv4 addresses. In this case, the service provider is aware of the IPv4 addressing scheme in the BNG and supplies the DHCP client with the appropriate default-gw IPv4 address by LUDB, RADIUS or DHCP server (in that order of priority).
In scenarios where the retail service provider wants to maintain independence from the IPv4 addressing scheme deployed in the BNG (that is controlled by wholesaler), the retailer can always supply its own IPv4 address, the subnet mask and the default-gw IPv4 address. But if the default-gw IPv4 address and subnet mask is not supplied by the retailer, then they are auto-generated by the BNG. After the default-gw IPv4 address is auto-generated, it is sent to the requesting DHCP client by DHCP offer in option 3 (RFC 2132, Router Option, section 3.5). There is no additional configuration needed for this action. The BNG automatically detects whether the default-gw IPv4 address is supplied by LUDB, RADIUS or DHCP server and acts correspondingly.
The default-gw IPv4 address is auto-generated based on the assigned IPv4 address/mask by setting the last bit of the assigned host IPv4 address to binary 01 or binary 10. For example if the subscriber host’s assigned IPv4 address is 10.10.10.10 255.255.255.0, then the default-gw IPv4 address is set to 10.10.10.1. If the assigned IPv4 address is 10.10.10.1 255.255.255.0, then the auto-generated default gateway IPv4 is set to 10.10.10.2.
The default gateway IPv4 address always must be within the subscriber’s subnet. If it is not, the behavior may be inconsistent. For example:
RADIUS (or DHCP) returns IP@, mask and def-gw:
IP 10.10.10.1
Def-gw 10.10.0.254
Subnet mask 255.255.255.0
The subscriber is successfully instantiated in the BNG, but the client may not ARP for a default-gw outside of its configured subnet. Whether the client does or does not ARP for a default-gw outside of its configured subnet depends on the implementation in the RG and CPE.
RADIUS returns IP@ and subnet mask.
In this case the auto-generated default-gw IPv4 address is always within subscriber’s subnet.
Flexible IPv4 addressing with auto-generated default-gw is supported only in Routed Central Office (RCO) model with routed residential gateways (RGs) or CPEs. In RCO model with bridged residential gateways or CPEs, the default-gw IPv4 addresses and the assigned IPv4 addresses may overlap. After the IPv4 address of the default-gw is auto-generated, it is possible that the second host behind the bridged residential gateway or CPE is assigned the same IPv4 address as the IPv4 address of the default gateway of the first host. Such hosts would not be able to communicate with outside world.
For example:
RADIUS or DHCP server assigns IPv4 address and subnet mask to the first host in a bridged environment:
IP1: 10.10.10.1
Auto-generated default-gw IPv4 address: 10.10.10.2
Because the RADIUS and DHCP server are not aware of the auto-generated default-gw, they may assign the following IPv4 address to the second host that comes on-line:
IP 2: 10.10.10.2 (same IPv4 address as the default-gw IPv4 address of the first host)
Auto-generated default-gw IPv4 address: 10.10.10.1
Now the first host forwards all traffic outside of the configured subnet to the second hosts which discards this traffic, effectively rendering this operation model non-deployable. And the other way around.