LAN switches use forwarding tables, such as Layer 2 and CAM tables, to direct traffic to specific ports based on the VLAN number and the destination MAC address of the frame. When there is no entry that corresponds to the destination MAC address of the frame in the incoming VLAN, the (unicast) frame is sent to all forwarding ports within the respective VLAN. This causes flooding. The very cause of flooding is that the destination MAC address of the packet is not in the Layer 2 forwarding table of the switch. In this case, the packet is flooded out of all forwarding ports in its VLAN, except the port it is received on.
The default ARP table aging time is 4 hours while the CAM holds the entries for only 5 minutes. The switch sends out a frame to all forwarding ports within the respective VLAN when the destination MAC address is aged out from the CAM table. You need a CAM aging timer greater or equal to the ARP timeout in order to prevent unicast flooding. As a workaround, you can issue one of these commands in order to increase the CAM aging timer for the VLAN you are having trouble with to match the ARP aging time:
For CatOS, issue the set cam agingtime command.
For Cisco IOS software, issue the mac-address-table aging-time command.
Note: In any Catalyst environment that runs a Hot Standby Router Protocol (HSRP), it is recommended that you ensure the CAM and ARP timers are synchronized.
Refer to Unicast Flooding in Switched Campus Networks for information on possible causes and implications of unicast packet flooding in switched networks
CAM 테이블은 5분, ARP Table은 4시간이 default aging Time.
결국 이 두 테이블의 시간 차이로 인해서 CAM Table에서는 사라지고 ARP Table에서는 남아있는 경우에
Unicast Flooding이 발생한다는..
HSRP를 사용할 경우에는 반드시 CAM Table과 ARP Table의 시간을 동기화 시켜줘야 한다고 한다.
특히나, Cat OS와 IOS간의 연결된 포트의 VLAN의 경우에는 양단간의 ARP Table과 CAM Table과의 시간을 동기화 해주어야 하는데, Table Size가 너무 커질 수 있으므로, 5분에 맞춘다. 뭐 크게 성능상에 문제가 없다면.. 4시간으로 맞춰도 상관 없음.