Dell PowerConnect 6248 Configuration Guide - Page 172
DHCP Snooping Scenario, NSF and VoIP
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VoIP Figure 9-2 shows how nonstop forwarding maintains existing voice calls during a management unit failure. Assume the top unit is the management unit. When the management unit fails, the call from phone A is immediately disconnected. The call from phone B continues. On the uplink, the forwarding plane removes the failed LAG member and continues using the remaining LAG member. If phone B has learned VLAN or priority parameters through LLDP-MED, it continues to use those parameters. The stack resumes sending LLDPDUs with MED TLVs once the control plane restarts. Phone B may miss an LLDPDU from the stack, but should not miss enough PDUs to revert its VLAN or priority, assuming the administrator has not reduced the LLDPDU interval or hold count. If phone B is receiving quality of service from policies installed in the hardware, those policies are retained across the management unit restart. Figure 9-2. NSF and VoIP Phone A LAG Phone B DHCP Snooping Scenario Figure 9-3 illustrates an L2 access switch running DHCP snooping. DHCP snooping only accepts DHCP server messages on ports configured as trusted ports. DHCP snooping listens to DHCP messages to build a bindings database that lists the IP address the DHCP server has assigned to each host. IP Source Guard (IPSG) uses the bindings database to filter data traffic in hardware based on source IP address and source MAC address. Dynamic ARP Inspection (DAI) uses the bindings database to verify that ARP messages contain a valid sender IP address and sender MAC address. DHCP snooping checkpoints its bindings database. 172 Utility