You manage the Cisco FabricPath network in the following exhibit:
A host that is directly connected to L2 sends a unicast packet to an unknown host elsewhere in the Cisco FabricPath network.
Which of the following will L2 do first?
- perform a lookup for the destination host in its MAC address table
- update the classic Ethernet MAC table with the sending host’s source MAC address
- encapsulate the packet into the FabricPath header
- receive an unknown unicast packet from the sending host
L2 will first receive an unknown unicast packet from the sending host if that host sends a unicast packet to an unknown host elsewhere in the Cisco FabricPath network. In this scenario, L2 is a leaf switch to which the sending host is directly connected. However, the sending host does not have information about how to send a unicast packet to the destination host. Therefore, the sending host sends an unknown unicast packet to the directly connected leaf switch. When L2 receives the unknown unicast, packet, the following occurs:
– L2 updates the classic Ethernet Media Access Control (MAC) table with the sending host’s source MAC address.
– L2 performs a lookup for the destination MAC address of the destination host.
– L2 encapsulates the classic Ethernet frame in a FabricPath header and floods it to the multidestination forwarding tree with L2’s switch ID as the outer source address (OSA).
Because of the spine-and-leaf configuration of the topology, L2 floods the packet to its directly connected spine switches, S1 and S2, which then flood the packet to the other leaf switches. Each leaf switch then performs a MAC address lookup. The switch that has the destination host in its MAC address table adds the sending host to its own MAC address table. At this point, the packet is sent to the destination host.
By contrast, when L2 receives a known unicast packet, which is a unicast packet with a destination address that the switch knows how to reach, that is intended for a host on a different switch the following occurs:
– L2 performs a MAC address lookup and locates the destination switch.
– L2 looks up the switch ID of the destination switch and chooses a FabricPath core port on which to send the packet.
– L2 encapsulates the packet in a FabricPath header, including its own switch ID in the OSA field and the destination switch’s ID in the outer destination address (ODA) field.
– L2 sends the packet.
If L2 receives a unicast packet that is destined for a host that is also directly connected to L2, the switch will locally deliver the packet and will not send or flood it to spine switches.