Configuring Multi-VLAN DHCP
March 25, 2025 - Reading time: 4 minutes
Dynamic Host Configuration Protocol (DHCP) allows a router to automatically assign IP addresses and other network parameters (such as default gateway and DNS server) to hosts. In a router-on-a-stick setup, one physical interface on the router is divided into multiple subinterfaces, each subinterface corresponding to a VLAN. The router will act as a DHCP server for each VLAN.
Password Recovery
March 11, 2025 - Reading time: 2 minutes
The Password Recovery procedure is used to regain administrative access to a device when passwords are lost or forgotten. This method involves interrupting the boot process to enter ROMMON mode, bypassing the startup configuration, and then resetting the necessary credentials. The recovery process requires precise use of configuration register changes to ensure the device boots properly without the previous settings. By following these steps, administrators can restore secure access while maintaining the integrity of the existing configuration.
BGP Troubleshooting
March 4, 2025 - Reading time: 2 minutes
Common Mistakes and How to Avoid Them
When configuring BGP, several common mistakes can cause session failures, missing routes, or improper route propagation. These mistakes can cause sessions to remain in Idle/Active states or prevent advertised routes from being installed in the routing table.
Below are the most frequent errors, their causes, and how to troubleshoot them effectively. To view a detailed explanation for each issue, simply click the provided links.
BGP Issue: Reachable Next-Hop
February 23, 2025 - Reading time: 3 minutes
When troubleshooting BGP, it’s not enough for the eBGP session to be up and for routes to be learned—they must also be installed in the routing table to be actively used. One common issue occurs when BGP learns routes, but they never appear in the routing table. This typically happens when the next-hop IP is unreachable, causing BGP to mark the routes as valid but not best.
BGP Issue: Auto-Summary Fix
February 23, 2025 - Reading time: ~1 minute
When you notice that only classful networks are being advertised in your BGP configuration, the culprit is often the auto-summary feature. Auto-summary, enabled by default, aggregates subnet routes to their classful boundaries, which may lead to unintended summarization of your specific routes.
BGP Issue: Sync Fix
February 23, 2025 - Reading time: 2 minutes
When BGP routes are not being advertised to eBGP peers, the likely culprit is the synchronization rule. By default, BGP will only advertise routes to eBGP peers if those routes are also present in the IGP. This behavior is meant to ensure that the routes are truly reachable. However, in many modern networks, this strict requirement can be unnecessary.