Because there is no classical DHCP lease state maintained for local PPPoX pools, the IP addresses are not synchronized by the DHCP server. Instead they are synchronized through PPPoX clients. After the PPPoX subscriber is synchronized, the respective IP address lease is updated in the respective local pool.
For example:
A PPPoE client is created on 7750 SR and 7450 ESS ‛A’.
The IP address is assigned from the local poll on 7750 SR and 7450 ESS ‛A’.
The PPPoE client is synchronized to the peering node 7750 SR and 7450 ESS ‛B’.
After the client is synchronized in the 7750 SR and 7450 ESS ‛B’, the IP address assignment is synchronized by the internal PPPoE process on the 7750 SR and 7450 ESS ‛B’ with the local pool.
One artifact of this behavior (IP address assignment in local DHCP pools is synchronized through PPPoX clients and not by DHCP server synchronization mechanism) is that during the node boot, the DHCP server must wait for the completion of PPPoX subscriber synchronization by MCS so that it learns which addresses and prefixes are already allocated on the peering node. Because the DHCP server can theoretically start assigning IP addresses before the PPPoX sync is completed, a duplicate address assignment may occur. For example, an IP address lease can be granted by DHCP local pools while PPPoX sync is still in progress. After the PPPoX sync is completed, the DHCP server may discover that the granted IP lease has already been allocated by the peering node. The most recent lease is kept and the other is removed from both systems. To prevent this scenario, a configurable timer can be set to an arbitrary value that renders sub-if non-operational until the timer expires. The purpose of this timer is to allow the PPPoX sync to complete before subscribers under the sub-intf can be served.