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The third-party AP connected to the S125X-AF fails to obtain the configuration template from their AC

2022-09-26 11:00:23 Published
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Network Topology

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Problem Description

After replacing the S12508 with the S12508X-AF switch (FX board), a large number of connected third-party APs fail to obtain the configuration template from the AC side, but the connectivity is normal.

Process Analysis

There is a configuration problem. The PAPI fragmented packets obtained by the AP in the configuration file match the policy of rule 26 (reject UDP packets with destination port number 0) in the packer-filter packet filtering delivered by the interface. As a result, the connected AP fails to obtain the configuration. Related configuration as follows: 

Interface related configuration

packet-filter 3010 inbound

packet-filter 3010 outbound

#

acl number 3010

rule 0 deny tcp destination-port eq 445

rule 10 deny tcp destination-port eq 135

rule 15 deny tcp destination-port eq 137

rule 20 deny tcp destination-port eq 138

rule 25 deny tcp destination-port eq 139

rule 26 deny udp destination-port eq 0  // root cause

rule 30 permit ip

#

1. Based on the packet analysis of the interaction between the AP and the AC when the customer fails, there are PAPI fragmented packets in the environment, that is, UDP fragmented packets 

2. From the feedback message, the first packet of the corresponding UDP fragmented packet carries the UDP port number; but from the subsequent fragmented packets, the second and subsequent UDP fragmented packets are actually data packets, not port number 

3. The protocol type UDP(17) field, according to the fixed offset of 12 bytes, parses the destination port number. When fragmenting the packet, the actual parsed content is the DATE content. If the DATE content is all 0, it will be Matching the following ACL policy causes packet-filter to take effect, resulting in packet loss. rule 26 deny udp destination-port eq 0 

4. According to the above analysis, the laboratory performs packet streaming in a 1:1 environment, which can reproduce the packet loss scenario. Because the old 125 is a Marvell chip, the chip implementation mechanism is different from that of the S125X-AF. When the above ACL policy is configured, only the packets whose udp destination interface is 0 will be matched, and other scenarios will not match.

Solution

1. Delete the rule 26 deny udp destination-port eq 0 policy in acl num 3010 


 2. Add the fragment tag fragment match, and delete the port outbound packet filtering policy, that is, undo packet-filter 3010 outbound 

 An example of adding configuration is as follows:

acl number 3010

rule 0 deny tcp destination-port eq 445

rule 10 deny tcp destination-port eq 135

rule 15 deny tcp destination-port eq 137

rule 20 deny tcp destination-port eq 138

rule 25 deny tcp destination-port eq 139

rule 26 permit ip fragment

rule 27 deny udp destination-port eq 0 


Note: The order of the rule should be consistent with the example, that is, the rule of the fragment needs to be in the front. In addition, since the outbound direction of the FX board does not support fragment tag matching, you need to delete the packet-filter policy configured in the outbound direction of the port.

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