Self-loop Problems due to Inappropriate Route Summarization Configuration

Publication Date:  2012-07-27 Views:  135 Downloads:  0
Issue Description
A customer prepares for the route information remedy for the provincial backbone network and each Metropolitan Area Network (MAN) egress should apply the route summarization to summarize the local route information into the least routes and distribute them to the provincial backbone network. After the remedy, the traffic on the uplink interface of the egress of the MAN is extremely large occasionally; however, it is larger than the traffic of all downlink interfaces and this problem will influence the speed of the downlink service. The problem has never occurred before the remedy.
Alarm Information
none
Handling Process
No matter in which routing protocol the route summarization is applied, pay attention to the black hole route relevant to the summarized route of the local so as to avoid route loop due to fault in partial network.
Root Cause
Through the mirror packet capture on the uplink interface of the MAN core device, a large number of TTL=0 packets are found. For general services, the ttl cannot be 0 unless the route loop exists. According to the location of the TTL=0 packet, it can be judged that the loop is between the MAN core device and the uplink provincial backbone device. The destination address of the TTL=0 packet belongs to a downlink service network segment and the network segment incurs faults when the problem occurs. According to the analysis for the route information on each device, because of the route summarization on the MAN core device, the uplink device only knows the summarized route in this region/city without knowing the detailed route, that is, the uplink service cannot sense the fault on this network segment. However, when the fault occurs, the longest matched route with the network on the MAN core device is the default route (the default route points to the uplink provincial backbone device), so the packets accessing this fault network segment circulate between the MAN core device and provincial backbone device. The problem does not occur before the route summarization, because the provincial backbone device can sense any change of the detailed route in the region/city, and when the route information of the relevant network segment, the packets will be discarded.

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