> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zayo.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Looking Glass Tools

> Master the essential Looking Glass and ARP table tools for active, real-time network troubleshooting.

Before tackling complex scenarios, it is essential to master the foundational tools for active troubleshooting. The Looking Glass utility and the ARP table provide definitive, real-time answers to fundamental questions about connectivity and reachability.

## Looking Glass

The **Looking Glass** tool is a suite of active diagnostic utilities that allow you to test the network path from the perspective of the platform itself.

| Tool                   | Function                                        | Primary Use Case                                                               |
| ---------------------- | ----------------------------------------------- | ------------------------------------------------------------------------------ |
| **Route Lookup**       | Verifies route visibility from a location.      | "Is my new VPC route correctly advertised to my branch office?"                |
| **Ping**               | Tests end-to-end reachability and latency.      | "Can my server reach the public internet?"                                     |
| **Traceroute**         | Traces the hop-by-hop path to a destination.    | "Where is the latency or packet loss occurring on the path to my application?" |
| **Path MTU Discovery** | Determines the maximum packet size on a path.   | "Is a mismatched MTU causing performance degradation?"                         |
| **Packet Capture**     | Captures raw network traffic for deep analysis. | "I need to analyze the full packet flow to debug a complex protocol issue."    |

## ARP Table

The **Address Resolution Protocol (ARP) Table** is another fundamental tool, used to validate Layer 2 connectivity and provide crucial clues for Layer 3 troubleshooting. By navigating to **Network -> Cloud Router -> ARPs**, you can see the mapping of IP addresses to MAC addresses on a given interface.

### Use Case: Troubleshooting BGP

The presence of an ARP entry for a BGP peer's IP address is a definitive confirmation that Layer 2 connectivity (e.g., the correct VLAN, physical link) is established.

If BGP peering is failing, but a valid ARP entry exists for the peer, the problem is almost certainly a Layer 3 BGP configuration error (such as an incorrect AS number or a mismatched authentication key) and not a physical or VLAN issue. This allows you to immediately focus your efforts on the correct configuration domain.