Incorrect MAC Address Learning from BPDUs Cause MAC Address Flapping

 Network Topology

Physical Network Topology

Figure 1-1 MAC Address Flapping

Fault Description

As shown in the preceding figure, packet loss occurs when S9300-1 pings S5300-2.

Configuration Files

NA

 Troubleshooting Location

Troubleshooting Procedure

1.       Rapid Ring Protection Protocol (RRPP) is configured on S5300-1 and S5300-2 to prevent loops. GE0/0/048 of S5300-2 is blocked.

[53-2] display rrpp verbose domain 1
Domain Index   : 1
Control VLAN   : major 20    sub 21
Protected VLAN : Reference Instance 1
Hello Timer    : 1 sec(default is 1 sec)  Fail Timer : 6 sec(default is 6 sec) 
 
RRPP Ring      : 1
Ring Level     : 0
Node Mode      : Master
Ring State     : Complete
Is Enabled     : Enable                       Is Active: Yes
Primary port   : GigabitEthernet0/0/47         Port status: UP
Secondary port : GigabitEthernet0/0/48         Port status: BLOCKED

2.       MAC address flapping occurs on S5300-1. Normally, G0/0/48 of S5300-1 learns the MAC address of S5300-2. However, G0/0/47 of S5300-1 also learns the MAC address of S5300-2.

[53-1]display mac-address flapping  record        
 S  : start time                         
 E  : end time                   
(Q) : quit VLAN                   
(D) : error down               
-------------------------------------------------------------------------------                                                     
Move-Time                 VLAN MAC-Address  Original-Port  Move-Ports   MoveNum                                                     
-------------------------------------------------------------------------------                                                     
S:2018-08-22 17:15     2000    0010-5100-0011 GE0/0/47      GE0/0/48    65535                                                      
E:2018-08-22 17:15                
-------------------------------------------------------------------------------                                                      
Total items on slot 0: 1
 

3.       Packets on G0/0/47 of S5300-1 are captured and analyzed. It is found that the flapping MAC address is the source MAC address of bridge protocol data units (BPDUs).

The destination MAC address range of the BPDUs is as follows:

[53-1]display bpdu mac-address              
 Remaining configurable number: 122       
 ----------------------------------------------------------------------------  
 (001) 0180-c200-0000/ffff-ffff-fff0    (002) 0180-c200-0010   
 (003) 0180-c200-0020/ffff-ffff-fff0    (004) 0180-c200-008a  
 (005) 0180-c200-8585                   (006) 010f-e200-0001

4.       Most of the BPDUs are Layer 2 protocol packets. To resolve the live network problem, disable the corresponding protocols.

After LLDP, HGMP, NDP, and NTDP are disabled on S5300-2, the problem is solved.

Root Cause

If a device incorrectly learns the source MAC address of BPDUs, MAC address flapping will occur.

To resolve the problem, disable the protocols that send BPDUs on the peer end.

A blocked interface does not send or receive data packets, but can send some BPDUs for certain protocols.

Normally, the peer device of the blocked interface does not learn the source MAC address of BPDUs, so that the BPDUs are correctly sent.