Dophiのブログ

    "The CPU loading of ERS8600 is too high!!" "What causes the CPU loading exceed 80%?" "Why is the traffic so slow and I can't get any data from our ERP server?" Well, these are the scenarios we can get in our daily life. A switch can get high CPU loading with many different reasons. In most cases, high CPU utilization indicates problems in network. In here, I would like to arrange some common reasons for high CPU loading.

1. Broadcast and multicast storms:

    Broadcast and multicast storms are the most common cause of CPU loading. Broadcast and multicast packets are flooded onto all ports of the VLAN except IGMP control is enabled to control multicast packets. In addition, there is no limited packet lifetime in layer 2 environments, they may loop forever and spread quickly depending on the capability of a switch. The major reason making high CPU loading is usually wrong cabling or invalid device configuration especially in SMLT environment.

    To prevent high CPU utilization, there are many features on ERS8600 such as CP-lmit and extended CP-limit. In addition, loop breaking technique including SLPP (Simple Loop Prevention Protocol) and Spanning Tree can be used to associate with CP-limit or extended CP-limit.


2. Spanning Tree:


    Another common cause for high CPU load on ERS8600 is instable Spanning Tree. For example, an edge switch changes port status and send out a message to notify a topology change in network. Upon the core switch reverie this message, core switch sends out BPDUs and cause Spanning Tree database in all switches is changed. This cloud make flooding of traffic and let CPU loading be high because of the update of forwarding database.


3. ARP Processing:


    ERS8600 is a layer 3 device which maintains an ARP table and layer 2 forwarding table. In order to maintain these tables, ERS8600 sends out ARP packets to validate the activity of a device when a entry age out in layer 2 forwarding table. If there are many devices in a network, this might cause a great amount of ARP packets and make CPU load be high.


    To avoid this situation, we can increase the timer of forwarding table on ERS8600.


4. MAC address learning:


    A packet with an unknown source MAC address or a known source MAC on a new port will be sent to the CPU. The CPU process it and try to build a entry of forwarding table. In some cases, the same source MAC address might come from different ports in a mis-configuration network. This make CPU continuously process forwarding table building and finally be high loading. This situation happens usually in a mis-configuration SMLT, clustering, or loops of port mirroring.


    ERS8600 provides a useful command “trace” to determine the reason of high CPU utilization. The usage of this command is listed as below



    trace level 9 3



    This command records the packet sent to CPU and gives us a detail information of CPU process with a special format.


[05/23/08 01:10:48:49] tMainTask CPP: cpp.c : 2889: cppProcRxFrame: dst=ff-ff-ff-ff-ff-ff src=00-0b-ab-05-ed-ba typ=0806 len=60 port=4/6 vid=0x001 pid=2 code=000a qos=0 pkthdr 0x801d0044 0x20010280, PktProcCode 0


    This log shows source and destination MAC addresses, Ethernet length or protocol type, and which port that frames were received or transmitted. Details about the frames can be analyzed by encoding the diagnostic codes