Tomasz Wendlandt

W architekturze 6500 przy zastosowaniu DFC następuje rozproszone przetwarzanie ramek (distributed switching). W takiej kofiguracji każda karta liniowa ma własną tablice CAM. Co w praktyce oznacza, że każda karta z DFC uczy się MACów niezależnie i przechowuje je w tablicy przez określony czas (aging time). Naturalne jest wówczas, że Supervisor nie widzi ruchu dla danego MAC adresu, ponieważ ruch ten zostanie wysłany z pominięciem Supervisora (bezpośrednio przez kartę liniową), a ponieważ przez dłuższy czas Supervisor nie będzie widział ruchu do danego MACa, zostanie on usunięty z jego tablicy CAM.

Co się dzieje dalej? Ponownie przychodzi ruch dla danego MACa, ale ponieważ inne karty w chassis nie mają go w CAMie ruch ten jest wysyłany do wszystkich kart. W efekcie widzimy unicast flooding w ramach jednego vlanu. Jak można rozwiązać taki problem? Należy włączyć synchronizację tablic MAC adresów DFC z PFC Supervisora oraz opcjonalnie wydłużyć czas aging-time:

(config)#mac-address-table synchronize
(config)#mac-address-table aging-time 900

#sh mac address-table address 0014.0000.aabb all
Legend: * - primary entry
        age - seconds since last seen
        n/a - not available

  vlan   mac address     type    learn     age              ports
------+----------------+--------+-----+----------+--------------------------
Module 2[FE 1]:
   10  0014.0000.aabb   dynamic  Yes        180   Gi9/21
Module 2[FE 2]:
   10  0014.0000.aabb   dynamic  Yes        180   Gi9/21
Module 3[FE 1]:
   10  0014.0000.aabb   dynamic  Yes        145   Gi9/21
Module 3[FE 2]:
   10  0014.0000.aabb   dynamic  Yes        145   Gi9/21
Module 4[FE 1]:
   10  0014.0000.aabb   dynamic  Yes        180   Gi9/21
Module 4[FE 2]:
   10  0014.0000.aabb   dynamic  Yes        180   Gi9/21
Active Supervisor:
   10  0014.0000.aabb   dynamic  Yes         15   Gi9/21
Module 7[FE 1]:
   10  0014.0000.aabb   dynamic  Yes         20   Gi9/21
Module 7[FE 2]:
   10  0014.0000.aabb   dynamic  Yes         20   Gi9/21
Module 8[FE 1]:
   10  0014.0000.aabb   dynamic  Yes        170   Gi9/21
Module 8[FE 2]:
   10  0014.0000.aabb   dynamic  Yes        170   Gi9/21
Module 9[FE 1]:
*  10  0014.0000.aabb   dynamic  Yes         15   Gi9/21
Module 9[FE 2]:
*  10  0014.0000.aabb   dynamic  Yes        180   Gi9/21

#sh mac address-table synchronize statistics

MAC Entry Out-of-band Synchronization Feature Statistics:
---------------------------------------------------------

    Module [2]
    -----------

    Module Status:
Statistics collected from module                    :  2
Number of L2 asics in this module                   :  2

    Global Status:
Status of feature enabled on the switch             :  on
Default activity time                               :  160
Configured current activity time                    :  160

    Statistics from ASIC 0 when last activity timer expired:
Age value in seconds from age byte register         :  0x3D
Current activity interval start time for seconds    :  0x20
Current activity interval end time for seconds      :  0x40
Current inactive interval start time for seconds    :  0x0
Current inactive interval end time for seconds      :  0x20
Age value in minutes from age byte register         :  0x2F
Current activity interval start time for minutes    :  0x2C
Current activity interval end time for minutes      :  0x2F
Current inactive interval start time for minutes    :  0x29
Current inactive interval end time for minutes      :  0x2C
Age value in hours from age byte register           :  0x27
Current activity interval start time for hours      :  0x26
Current activity interval end time for hours        :  0x27
Current inactive interval start time for hours      :  0x26
Current inactive interval end time for hours        :  0x26
Age value in days from age byte register            :  0x16
Current activity interval start time for days       :  0x15
Current activity interval end time for days         :  0x16
Current inactive interval start time for days       :  0x15
Current inactive interval end time for days         :  0x15
Number of active entries read                       :  1372435
Number of entries ignored with update to age byte   :  3812323
Number of entries updated with age byte             :  1781719
Number of entries created new                       :  4194

    Statistics from ASIC 1 when last activity timer expired:
Age value in seconds from age byte register         :  0x3D
Current activity interval start time for seconds    :  0x20
Current activity interval end time for seconds      :  0x40
Current inactive interval start time for seconds    :  0x0
Current inactive interval end time for seconds      :  0x20
Age value in minutes from age byte register         :  0x2F
Current activity interval start time for minutes    :  0x2C
Current activity interval end time for minutes      :  0x2F
Current inactive interval start time for minutes    :  0x29
Current inactive interval end time for minutes      :  0x2C
Age value in hours from age byte register           :  0x27
Current activity interval start time for hours      :  0x26
Current activity interval end time for hours        :  0x27
Current inactive interval start time for hours      :  0x26
Current inactive interval end time for hours        :  0x26
Age value in days from age byte register            :  0x16
Current activity interval start time for days       :  0x15
Current activity interval end time for days         :  0x16
Current inactive interval start time for days       :  0x15
Current inactive interval end time for days         :  0x15
Number of active entries read                       :  2043307
Number of entries ignored with update to age byte   :  3928935
Number of entries updated with age byte             :  993682
Number of entries created new                       :  0

Źródło:
http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00807347ab.shtml