PCs Cannot Obtain IP Addresses Through DHCP After Broadcast Suppression Is Configured on Interfaces

Publication Date:  2015-06-09 Views:  441 Downloads:  0
Issue Description
An enterprise needs to improve reliability of its campus network, to ensure non-stop services.

The network reconstruction includes:

1. Replace stand-alone MA5200Gs with ME60_1 and ME60_2 that work in active/standby mode.

2. Replace the original aggregation switches with a stack consisting of S5700_1 and S5700_2. In addition, VRRP and BFD are configured on S5700_1 and S5700_2.

3. Eth-Trunk is configured between access switches and S5700_1/S5700_2.

However, after the network reconstruction is complete, some users cannot obtain IP addresses in peak hours.

Figure 2-1 Networking diagram



Handling Process
1. Collect the configurations on the problematic PCs and determine the access switches to which these PCs connect.

2. Check the interfaces on S5700_1 and S5700_2 connected to access switches. A large number of DHCP DISCOVER and OFFER packets are discarded on the interfaces.

3. Check interface configurations, and delete the broadcast-suppression packets 100 configuration on the interfaces. The fault is rectified.
Root Cause
Clients broadcast DHCP DISCOVER packets to discover DHCP servers. When many clients broadcast DHCP DISCOVER packets simultaneously, broadcast suppression configured on interfaces may cause a failure to allocate IP addresses to clients.
Solution
Delete broadcast suppression on interfaces.
Suggestions
1. The fault may be caused by the broadcast suppression configuration on interfaces.

2. Set and adjust the broadcast suppression threshold on interfaces according to actual situation. The configuration should not affect service.

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