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FAQ - which are the reasons for discarded frames on the uplink port

Publication Date:  2012-07-25 Views:  55 Downloads:  0
Issue Description

Q:
When querying the port statistics of the uplink port of MA5600T, sometimes you can find that there are some discarded frames. This happens if the uplink is on SCU or uplink board also.

Alarm Information


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Handling Process

A:
A packet capture by uplink port mirror that also includes VLAN tag can be the base to prove why there are discarded frames on the uplink port.
This next section describes how to check each case:
1) Run the display traffic-suppress { frameid/slotid | all } command.
You can make an impression about the traffic on the uplink port by running the display port traffic command. If the total number of packets per second is less than the suppress level, that means there is no way for this to be the cause of the discarded frames. If the total number of packets is above the suppress level, it is theoretically possible that there are discarded frames due to traffic suppression.
2) Display the ACLs running on that port.
display packet-filter port frame/slot/port
If a packet filter is applied on that ACL, that means that packets that meet the deny rule will be counted as discarded frames.
3) The internal ACL rules issued by the system are generally used to capture protocol packets, using the method of copying and sending packets to the CPU and discarding the original packets. If functions like PITP and DHCP option 82 are enabled, an increase in discarded frames on the uplink port is possible. You can check them in the configuration:
[abs-config]
<abs-config>
dhcp option82 enable
pitp enable pmode
4) The same VLANs need to be pemitted on both MA5600T uplink port and uplink equipment port. If MA5600T receives a packet that is tagged with a VLAN that is not configured on that port, the discarded frames counter will increase.
The VLANs configured on that port can be queried by issuing the following command:
display port vlan frameid/slotid/portid
5) In some cases a VLAN is configured only on the uplink port. If the MA5600T receives a packet on that VLAN and that packet needs to be broadcasted, the MA5600T will fail to find the egress node and the packet will be discarded. Such packets also include STP packets.
MA5600T_1(config)#display vlan 2704
{ <cr>|to<K> }:
  Command:
          display vlan 2704
  VLAN ID: 2704
  VLAN type: smart
  VLAN attribute: common
  VLAN description:
  VLAN forwarding mode: vlan-mac
  VLAN broadcast packet forwarding policy: forward
  VLAN multicast packet forwarding policy: forward
  VLAN unknown unicast packet forwarding policy: forward
  ------------------------------
   F/S /P     Native VLAN  State
  ------------------------------
   0/9 /0               1  up                                  (only this port is bound to VLAN 2704)
  ------------------------------
  Standard port number: 1
  Service virtual port number: 0
Also in some cases, the VLAN is bound to multiple ports but only one of them is in up state. So, for example protocol packets such as STP in that VLAN can’t be sent to other ports and they are discarded.
MA5600T_1(config)#display vlan 2600
{ <cr>|to<K> }:
  Command:
          display vlan 2600
  VLAN ID: 2600
  VLAN type: smart
  VLAN attribute: common
  VLAN description: IPTV_mcast
  VLAN forwarding mode: vlan-mac
  VLAN broadcast packet forwarding policy: forward
  VLAN multicast packet forwarding policy: forward
  VLAN unknown unicast packet forwarding policy: forward
  ------------------------------
   F/S /P     Native VLAN  State
  ------------------------------
   0/9 /0               1  up 
   0/20/0               1  down           (the other port bound to VLAN 2600 is in down state)
  ------------------------------
  Standard port number: 2
  Service virtual port number: 0

Root Cause


The possible causes of packet discard are as follows:
1) Packets are discarded due to upstream broadcast/unicast/multicast traffic suppression.
2) Packets are discarded according to the matched user-defined ACL rules defined by user.
3) Packets are discarded according to the matched default ACL rules in the system, and function-triggered ACL rules.
4) VLAN mismatching occurs.
5) There is no forwarding port corresponding to the VLAN or the forwarding port is abnormal.

Suggestions

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