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Unavailability of Some Services due to Port Mask Configuration Error

Publication Date:  2012-07-27 Views:  484 Downloads:  0

Issue Description

To open services between the provincial center and its subordinate cities and towns, the OSPF is adopted on the core layer and the static route is adopted between MANs and the core layer. After services are opened, some customers comment that the service between the provincial center and some subordinate cities are unavailable. Upon checking the device information, the route information is correct. PING cannot be executed between different service hosts. The unavailable services in all the subordinate cities and towns are in the same category. Terminals of the subordinate cities and towns can be interconnected but they fail to be connected with the provincial capital. The network segments of this service are extracted unifiedly from a category-C address space.

Alarm Information


Handling Process

Modify the mask of the IP address of the device ports. As the forwarding mechanism of the Ethernet port is related with ARP, the Ethernet port interconnection is required. In addition, IP addresses of all devices in a same network segment should be configured with the same mask.

Root Cause

In the network topology, the convergent router of the provincial center and devices on the core layer are interconnected via an Ethernet port. When the service is opened, the IP addresses of the interconnected devices are configured to contain 30-bit masks as planned. These IP addresses are extracted from the same Category-C address space and the devices should perform the L3 forwarding to IP addresses of other network segments in the Category-C address space. The maintenance engineer has modified the 30-bit mask of the IP address of the uplink interface to be 24 bits by mistake. Due to this mistake, to process all the other addresses in this Category-C address space on this convergent router, the interconnected devices must make an ARP request first to get the MAC address of the corresponding address, and then perform the L2 forwarding rather than L3 forwarding. There is only one device at the peer end, which will not give response to the ART request. Thus, the service is disrupted. For example, if the IP addresses of the interconnected devices are and, these two devices need to perform the L3 forwarding to the addresses in the network segment of Once is modified to be, the devices will regard the addresses in the network segment of to be the associated address and will make an ARP request. There is only one host in the same domain, so the service will be unavailable.