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Cable modem problems with Cisco 871

The undesired intermittent bridging behavior of Cisco 871 using old ROMMON software can lead to hard-to-diagnose problems if you're connected to an Internet access network through a cable modem that accepts only a single MAC address. The right sequence of events can leave the router/modem combination in a state with no external connectivity requiring a modem power-cycle:

  1. The router and the cable modem are power-cycled.
  2. The router starts to bridge between all LAN interfaces, effectively connecting inside workstations directly to the cable modem.
  3. One of the workstations could detect a LAN failure (due to router reload) and restart the DHCP process (a Windows XP host would definitely do that).
  4. The DHCP requests from the workstation are bridged straight to the cable modem which caches the workstation's MAC address and forwards the DHCP request.
  5. The workstation is assigned a public IP address (at this time, the workstation is connected directly to Internet and thus vulnerable).
  6. The router loads Cisco IOS and reinitializes the Ethernet interfaces. Bridging between internal and external interfaces is stopped.
  7. The router sends DHCP request on the outside interface, but the modem ignores it, as the MAC address of the DHCP request differs from the previously cached one.

In most cases, the cable modem has to be power-cycled to lose the cached MAC address.

This behavior can be observed only if the router and the cable modem are reset at the same time and the cable provider doesn't care much about MAC security and allows the modem to learn the MAC address. If you reset only the cable modem, the router is not bridging (no problem); if you reset just the router, the cable modem still caches the router's MAC address and ignores the DHCP request from the inside workstation(s).

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