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The GE Interfaces Could Not Perceive the Status Changes of the Peer Interfaces Because of a Transmission Device in Between

Publication Date:  2012-07-27 Views:  77 Downloads:  0
Issue Description
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Transmission device |
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On the bearer network constituted by four NE40Es, taking the shape of a square, the Option A solution is adopted for the cross-domain VPNs between AR-A and ASBR-A and between AR-B and ASBR-B. MP-BGP runs in an AS for AR-A and AR-B. MP-BGP runs in another AS for ASBR-A and ASBR-B. Generally, the traffic passes through one plane instead of from plane A to plane B or from plane B to plane A. Therefore, GE interfaces are configured for all devices.
Symptom: After the interface where AR-A was connected to the transmission device went Down, the status change of the BGP peer was not immediately displayed on ASBR-A. Thus, the attached service was interrupted for three minutes. 
Alarm Information
Handling Process
1. Configure BFD for BGP between ARs and ASBRs for the local device to rapidly perceive the status of the peer. The test proved that this configuration could realize fast switchover.
2. Change the interface type to POS and configure alarm sensitive on the POS interfaces. The test proved that this configuration could realize fast switchover. 
Root Cause
Checking the BGP status on the ASBRs, the engineer found that the BGP status was Established. This indicated that the status of the BGP peer did not change and the fast convergence of BGP was not implemented. That is to say, the ASBRs did not receive the condition for triggering the status change of the BGP peer.
In the networking, a transmission device existed between AR-A and ASBR-A. When the GE interfaces were connected to the transmission device, the local device could not perceive the status change of the interface of the peer device. Thus, ASBR-A still forwarded the traffic to the next-hop AR-A, hence the lost traffic and interrupted service. ASBR-A triggered the status change of the BGP peer for routing convergence only after the timeout of the local BGP (180s). 
For the GE interfaces interconnected through a transmission device, note that the local device cannot perceive the status change when the interface of the peer device goes Down. Therefore, corresponding protective measures must be taken to ensure the security of the traffic.