Cisco Nexus 2000 Series Fabric Extenders are designed to provide connectivity for rack and blade servers, as well as converged fabric deployments. Cisco Nexus 5000, Nexus 6000, Nexus 7000, and Nexus 9000 Series Switches, as well as Cisco UCS Fabric Interconnect, act as parent switches for the fabric extenders.
Enabling PIM on an interface also enables Internet Group Management Protocol (IGMP) operation on that interface. An interface can be configured to be in dense mode, sparse mode, or sparse-dense mode. The mode describes how the Cisco IOS software populates its multicast routing table and how the software forwards multicast packets it receives from its directly connected LANs.
A sparse mode interface is used for multicast forwarding only if a join message is received from a downstream router or if group members are directly connected to the interface. Sparse mode assumes that no other multicast group members are present. When sparse mode routers want to join the shared path, they periodically send join messages toward the RP. When sparse mode routers want to join the source path, they periodically send join messages toward the source; they also send periodic prune messages toward the RP to prune the shared path.
Initially, a dense mode interface forwards multicast packets until the router determines that there are group members or downstream routers, or until a prune message is received from a downstream router. Then, the dense mode interface periodically forwards multicast packets out the interface until the same conditions occur. Dense mode assumes that multicast group members are present. Dense mode routers never send a join message. They do send prune messages as soon as they determine they have no members or downstream PIM routers. A dense mode interface is subject to multicast flooding by default.
Two servers in a cluster setup - CAS01 and CAS02. Each has a pair of interfaces to Nexus2k (same switch). vPC is setup for each server pair and then setup a NLB address. Problem is when bringing up the CAS02 on the PC, it is unable to ping the CAS01 server or get Internet access but can be seen by other devices in the network. If one of the CAS02 interfaces is down it is then able to ping CAS01 and regain Internet access. But this leaves the server unable to join the NLB.
Following is the configuration of Nexus 5500 managing the Nexus 2000 connecting to NLB config.