Directly Connected External BGP Neighbors Not Coming Up Cause Layer 2 Is Down Preventing Communication with Directly Connected BGP Neighbor

IP connectivity cannot occur until Layer 2 in the OSI reference model is up. Whether this Layer 2 information is learned dynamically or is configured statically, each router must have a correct Layer 2 rewrite information of adjacent routers. Ethernet, Frame Relay, ATM, and so on are most commonly used Layer 2 technologies. Most network administrators configure Layer 2 parameters in router configurations correctly; sometimes, basic cabling issues also can cause Layer 2 issues. Among cabling issues, misconfiguration in router configuration can cause ARP, DLCI mapping, and VPI/VIC encapsulation failures, which are the most common Layer 2 failures. It is beyond the scope of this book to address how this Layer 2 information is obtained. Case(s) in this section try to address how to troubleshoot BGP problems when the cause of the EBGP neighbor relationship failure is Layer 2.

Figure 15-2 shows the flowchart to follow to fix this problem.

Figure 15-2. Problem-Resolution Flowchart

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