Default Gateway Unreachable — Router Failure or Routing Table Loss
All users on one or more LAN segments lose internet and inter-site connectivity when the router or Layer 3 switch providing the default gateway crashes, reboots, or loses its routing table. HSRP/VRRP failover may also fail silently, leaving no active gateway.
Indicators
- Clients have valid IP addresses but cannot reach anything outside their subnet
- Ping to default gateway IP times out from workstations
- Traceroute stops at first hop — no response from gateway IP
- All sites sharing this router lose WAN connectivity simultaneously
- HSRP/VRRP active router down — virtual IP no longer responding
Likely causes
- Router or L3 switch crashed or rebooted (power, memory exhaustion, software bug)
- HSRP/VRRP failover failed — standby router not taking active role
- Static default route lost after reboot because running config was not saved
- BGP peer dropped — upstream ISP routes withdrawn
- STP topology change caused root bridge election, blocking gateway uplink port
Diagnostic steps
-
Test gateway reachability from workstation: Test-NetConnection -ComputerName <gateway-IP>; if no response, the physical device or virtual IP is down
-
Access router via out-of-band (serial console, IPMI/iLO/iDRAC, 4G OOB); check show version for crash reason and uptime; review syslog for exception/traceback
-
For HSRP: show standby brief — confirm active/standby priority and virtual IP state; for VRRP: show vrrp brief — confirm master and virtual MAC responding
-
Check routing table: show ip route — verify default route 0.0.0.0/0 is present; if missing, add static route or investigate BGP/OSPF convergence
-
For BGP: show bgp summary — peer state should be Established; if Idle/Active, check TCP 179 connectivity to ISP peer and BGP hold-timer expiry
-
Check STP: show spanning-tree on L3 switch — verify gateway uplink is in Forwarding state and root bridge is as expected
Resolution path
- Reach device via out-of-band console to confirm state
- Trigger HSRP/VRRP failover to standby if primary is unrecoverable
- Restore default route from saved config or re-add manually
- Re-establish BGP peers with upstream ISP after connectivity restored
- Write memory after recovery to prevent recurrence on next reboot
Prevention
- Always save running config after changes: write memory / copy run start
- Deploy HSRP or VRRP for gateway redundancy on all critical segments
- Provision out-of-band access (4G OOB router or serial console server) for every edge device
- Monitor gateway IP availability with ICMP polling at 60-second intervals
Tools
- Router CLI (Cisco IOS/XE, Juniper JunOS, MikroTik RouterOS, VyOS)
- show ip route / show bgp summary / show standby brief
- Out-of-band console (iDRAC, iLO, serial terminal, 4G OOB unit)
- SNMP / ICMP monitoring (PRTG, Zabbix, LibreNMS)
- Syslog server (for crash/reboot event history)