Subnet sharing between the hosts in flexible IPv4 addressing model is supported. In other words, in flexible IPv4 addressing model the operator can assign all IPv4 addresses (minus one, the default-gw IPv4 address) from a specific subnet. In this fashion, all subscribers (routed RGs or CPEs) within a single subnet can share the same default gateway.
For example, if the operator owns the IPv4 subnet 10.10.10.0/24, then one IPv4 address can be set aside for the default-gw (for example 10.10.10.254) and the remaining addresses can be assigned to the subscriber (routed RGs or CPEs). An example would be:
RG1: IP=10.10.10.1/24 def-gw 10.10.10.254
RG2: IP=10.10.10.2/24 def-gw 10.10.10.254
RG3: IP=10.10.10.3/24 def-gw 10.10.10.254
:
RG100: IP=10.10.10.100/24 def-gw 10.10.10.254
The subnet sharing is also supported in conjunction with auto-generated default-gw IPv4 address. The implication of this is that the IPv4 address of the default-gw can collide with the same IPv4 address already assigned to an existing subscriber. This is not an issue for routed RGs or CPEs because the BNG always answers ARPs for the IPv4 address of the default-gw with its own MAC address. However, local-proxy ARP functionality in the 7450 ESS and 7750 SR BNG must be enabled to support this.
This behavior can be further clarified with the following example.
Let’s assume that we have scenario with two routed RGs:
RG-1, IP=10.10.10.0/24, default-gw IP=10.10.10.1
RG-2, IP=10.10.10.1/24, default-gw IP=10.10.10.0
After RG-1 ARPs for its default gateway of 10.10.10.1, the BNG replies with its own MAC address.
Now that host RG-1 has resolved ARP for it default-gw (MAC address pointing to the router), it can send traffic to the outside world by the BNG. When such traffic arrives to the router, the destination IPv4 address of the received packet determines the forwarding decision within the router. If the destination IPv4 address matches the IPv4 address of any subscriber (RG) instantiated within the system, the traffic is forwarded to the that RG. This also includes the case where the destination IPv4 address is the default-gw IPv4 address (10.10.10.1), which represents just another RG within the router. The traffic is consequently passed from RG-1 by 7450 ESS and 7750 SR to RG-2.