Access-driven DHCP server redundancy model ensures uninterrupted IP address assignment service from a single IP address-range and prefix if an access link forwards BNG fails. To avoid duplicate address allocation, there must be a single active path available from the clients to only one of the SR OS-based DHCP servers in redundant configuration. This single active path is ensured by a protection mechanism in dual-homed environment in the access side of the network. The supported access redundancy mechanisms are SRRP or MC-LAG.
In the access-driven DHCP redundancy model, the DHCP relay in each router of the redundant pair must point only to the IP address of its local DHCP server. In other words, the DHCP messages received on one DHCP server should never be relayed to the other. Because the IP address-ranges and prefixes are shared between the DHCP servers, accessing both DHCP servers with the same DHCP request can cause DHCP lease duplication. Moreover, the IP addresses of both DHCP servers must be the same in both nodes. Otherwise the DHCP renew process would fail.
Granting new leases out of the shared IP address-ranges or prefixes that are configured as access-driven is not dependent on the state of the inter-chassis communication link (MCS). Instead, the new leases can be granted from both nodes simultaneously and it is the role of the protection mechanism in the access to ensure that a single path to either server is always active.
This model allows the newly active node, after a SRRP/MC-LAG switchover, to be able to serve new clients immediately from the same (shared) IP range or prefix. At the same time, upon the switchover, the corresponding subscriber-interface route is re-evaluated for advertisement with a higher routing metric to the network side by SRRP awareness. The result is that by aligning the subscriber-interface routes or prefixes with access-driven DHCP address ranges or prefixes in DHCP server, the IP address ranges or prefixes are advertised to the network side only from the actively serving node (SRRP master state or active MC-LAG node). This is performed indirectly with the corresponding subscriber-interface route that is aligned (by configuration) with the DHCP address range or prefix in access-driven mode.
If SRRP or MC-LAG is not deployed in conjunction with the access-driven configuration option, the IP address duplication could occur.
Multi-chassis redundancy relies on MCS to synchronize various client applications. (subscriber states, DHCP states, IGMP states, and so on) between the two chassis. Therefore, the links over which MCS peering session is established must be highly redundant. Failed MCS peering session renders dual-chassis redundancy non-operational. Considering this fact, the DHCP failover scenario with SRRP/MC-LAG and shared IP address range should be evaluated considering the following cases described in Table: DHCP failover scenarios .
| Access related failure | Inter-chassis link state | Number of failures | Action |
|---|---|---|---|
None |
None |
None |
Only the multi-chassis active BNG (SRRP master state) grants IP leases. The subnet tied to the SRRP instance is advertised to the network side on the multi-chassis active BNG. |
None |
COMM-INT |
Possibly multiple |
Only the (SRRP master state) BNG (SRRP master state) grants IP leases. Communication link between the two chassis has failed and the DHCP states cannot be synchronized. Operator is required to restore the links between the chassis. |
None |
PARTNER-DOWN |
Possibly multiple |
Same as above. The premise of this deployment model in general (SRRP/MC-LAG and access-driven) is that there is only one path leading to the DHCP server active. This path is governed by SRRP. Therefore, there is no change in behavior between the PARTNER-DOWN and COMM-INT states in this scenario. |
Access Link Towards the multi-chassis active BNG (SRRP master state) |
NORMAL |
Single |
SRRP switches over. The new IP lease grants continues from new multi-chassis active BNG (SRRP master state) using the same IP address range. IP leases are synched OK. Subnets are advertised from new multi-chassis active BNG by SRRP state awareness. |
Access Link Towards the multi-chassis active BNG (SRRP master state) |
COMM-INT |
Multiple |
SRRP switch over. The new leases can be handed from the DHCP server on the newly multi-chassis active BNG (SRRP master state). However, this DHCP server may not have its lease state table up to date because the inter-chassis communication link is non-operational. Consequently, the new multi-chassis active BNG may start handing out leases that are already allocated on the node with the failed link. In this case, IP address duplication would ensue. Therefore it of the utmost importance that the intercommunication chassis link is well protected and the only event that causes it to go down is when the entire node goes down. Otherwise the nodes becomes isolated from each other and synchronization becomes non-operational. |
Access Link Towards the multi-chassis active BNG (SRRP master state) |
PARTNER-DOWN |
Multiple |
Same as above. |
Access Link Towards the multi-chassis standby BNG (SRRP standby state) |
NORMAL |
Single |
No effect on the operation because everything is active on the multi-chassis active BNG (SRRP master state) anyway. |
Access Link Towards the multi-chassis standby BNG (SRRP standby state) |
COMM-INT |
Multiple |
Intercommunication link is broken. The multi-chassis active BNG (SRRP master state) continues handing out the new leases and renewing the old ones. However, they are not synchronized to the peering node. |
Access Link Towards the multi-chassis standby BNG (SRRP standby state) |
PARTNER-DOWN |
Multiple |
Same as above. |
Entire multi-chassis active BNG (SRRP master state) |
COMM-INT |
Single |
SRRP switches over. However, lease duplication may occur on the new multi-chassis active BNG because the intercommunication link is broken and this new multi-chassis active BNG is not aware of DHCP leases that the peer (failed node) may have allocated to the clients while the intercommunication-link was broken. |
Entire multi-chassis active BNG (SRRP master state) |
PARTNER-DOWN |
Single |
Same as above. |
Entire multi-chassis standby BNG (SRRP standby state) |
COMM-INT |
Single |
Operation continues, but multi-chassis redundancy is lost. |
Entire multi-chassis standby BNG (SRRP standby state) |
PARTNER-DOWN |
Single |
Same as above. |
A possible deployment scenario is shown in Figure: Failover scenario with SRRP and DHCP in access-driven mode.
