Troubleshooting Cross-VLAN Connectivity: Ping Timeouts with SSH Not Working

Introduction

Establishing connectivity across two Virtual Local Area Networks (VLANs) can be a common challenge for network administrators. Devices in different VLANs may not communicate with each other using protocols like SSH or traceroute, unless a ping request is initiated first. In this article, we will discuss the causes of ping timeouts and SSH connectivity issues when attempting to communicate across VLANs.

Ping Timeouts

Ping timeouts occur when the ping command does not receive a response from the target device within a specified time frame. This issue can be caused by several factors:

• Subnet Mismatch: Ensure that the source and destination devices are in the same subnet or have a route to reach each other.
• Firewall Rules: Check if there are any firewall rules blocking ICMP (Internet Control Message Protocol) packets, which are used by the ping command.
• Routing Issues: Verify that routing tables on both devices are correctly configured.

SSH Not Working

SSH (Secure Shell) connectivity issues can be caused by several factors:

• Public Key Authentication: Ensure that the public key of the client is added to the authorized_keys file on the server.
• Private Key: Make sure the private key file is not damaged and is accessible to the user attempting to connect.
• Firewall Rules: Check if there are any firewall rules blocking SSH traffic on the target device.
• Network Connectivity: Verify that the devices can communicate using ping before attempting to establish an SSH connection.

Troubleshooting

To troubleshoot cross-VLAN connectivity issues, follow these steps:

1. Check Subnet Mismatch: Use the ip addr show command to verify the IP addresses and subnet masks of both devices.
2. Check Routing Tables: Use the ip route show command to check the routing tables on both devices.
3. Check Firewall Rules: Use the iptables or netfilter command to check and modify firewall rules.
4. Check Public Key Authentication: Use the ssh-keygen command to generate new keys or add existing keys to the authorized_keys file on the server.