How to configure Cisco 881 Router with cable modem

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The Cisco 881 Series Integrated Services Router (ISR) is a popular small‑office/branch router that combines enterprise routing features with a built‑in 4‑port Ethernet switch, making it ideal for simple deployments that still need strong control over WAN routing, NAT, and security. Unlike many consumer routers, the Cisco 881 separates its ports by function: FastEthernet4 (FE4) is the dedicated WAN interface, while FastEthernet0–FastEthernet3 (FE0–FE3) behave as LAN switch ports (Layer‑2). Because the LAN ports operate like a switch, your LAN gateway IP is typically configured on interface Vlan1 (SVI) rather than directly on FE0–FE3—an important detail that trips up many first‑time configurations.

 

1) Topology & what you’re configuring

Cable Modem (bridge / passthrough) → Cisco 881 WAN (FastEthernet4) → Cisco 881 LAN switchports (FE0–FE3 via VLAN1) → PCs/Firewall/Switch

On the Cisco 881, the WAN port is FastEthernet4 (FE4) and the LAN ports are FastEthernet0–FastEthernet3 (FE0–FE3).
Those LAN ports behave as switch ports (Layer‑2), and the routed LAN IP lives on interface Vlan1 (SVI). Cisco’s VLAN/DHCP config examples show assigning switchports to VLANs with switchport access vlan …, which is exactly how the FE0–FE3 ports are treated. [cisco.com] [cisco.com]

2) Put the cable modem into Bridge / Passthrough mode (important)

If your modem is doing routing/NAT, you can end up with double NAT and your router may not directly hold the public IP. Bridge/passthrough mode disables the gateway’s routing/NAT so your Cisco can be the edge router. [community….p-link.com], [whatismyip.com], [support.google.com]

Practical tips:

  • Many ISPs bind the public IP to the first MAC address they see. If you swap devices, you often must power-cycle the modem so it learns the Cisco WAN MAC (some “passthrough” implementations explicitly select a device MAC). [owl360it.com]
  • After enabling bridge mode, connect modem → Cisco FE4 and reboot modem once.

3) Information you need from the ISP

Have these ready:

  1. Static public IP (e.g., 203.0.113.10)
  2. Subnet mask/prefix (e.g., /30 = 255.255.255.252)
  3. Default gateway (ISP next hop, often the other IP in your /30)
  4. DNS servers (ISP DNS or public DNS)

Cisco’s own “Information Needed for Configuration” list explicitly calls out DNS and default gateways as required items for Internet setup. [cisco.com]

4) Minimal working Cisco 881 configuration (Static WAN + NAT + DHCP LAN)

This is the most common scenario: one static public IP on FE4, and your inside LAN uses RFC1918 with PAT (NAT overload).

Example addressing (edit to match your environment)

  • WAN (FE4): 203.0.113.10/30, gateway 203.0.113.9
  • LAN (VLAN1): 192.168.10.1/24
  • DHCP pool: 192.168.10.100–192.168.10.199
				
					conf t
!
! ---- Ensure routing is enabled (some default configs show "no ip routing") ----
ip routing
ip cef
!
! ---- WAN: FastEthernet4 is the 881 WAN port ----
interface FastEthernet4
 description WAN-to-CableModem
 ip address 203.0.113.10 255.255.255.252
 ip nat outside
 no cdp enable
 no shutdown
!
! ---- LAN: FE0-FE3 are switchports; the routed LAN IP is on interface Vlan1 ----
interface Vlan1
 description LAN
 ip address 192.168.10.1 255.255.255.0
 ip nat inside
 no shutdown
!
! (Optional but common) Make sure LAN switchports are in VLAN 1 and up
interface FastEthernet0
 switchport access vlan 1
 no shutdown
interface FastEthernet1
 switchport access vlan 1
 no shutdown
interface FastEthernet2
 switchport access vlan 1
 no shutdown
interface FastEthernet3
 switchport access vlan 1
 no shutdown
!
! ---- Default route to ISP gateway ----
ip route 0.0.0.0 0.0.0.0 203.0.113.9
!
! ---- DNS (use ISP DNS or public DNS) ----
ip name-server 1.1.1.1
ip name-server 8.8.8.8
!
! ---- NAT overload (PAT) for inside users ----
ip access-list standard NAT_INSIDE
 permit 192.168.10.0 0.0.0.255
exit
ip nat inside source list NAT_INSIDE interface FastEthernet4 overload
!
! ---- DHCP for LAN clients (optional) ----
ip dhcp excluded-address 192.168.10.1 192.168.10.49
ip dhcp pool LAN_DHCP
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.1
 dns-server 1.1.1.1 8.8.8.8
exit
end
wr mem

Why these commands?

  • Cisco documents show the 881 WAN interface is FE4 and how to set ip address … and no shutdown on it. [cisco.com]
  • NAT configuration is built around:
    1. marking inside/outside interfaces, and
    2. using ip nat inside source … overload for PAT. [cisco.com]
  • DHCP server configuration steps (excluded addresses, pool, network, default-router, dns-server) follow Cisco’s DHCP/VLAN chapter. [cisco.com]

5) If you have a block of static IPs (e.g., /29), not just one

ISPs often provide either:

  • Option A: one static IP to put on FE4, or
  • Option B: a routed block (e.g., /29) routed to your WAN IP.

Common use case: publish an internal server with a public IP

Use static NAT (1:1) or port-forward.

Example: map inside server 192.168.10.50 to public 203.0.113.20

				
					ip nat inside source static 192.168.10.50 203.0.113.20
``

Cisco’s NAT guide includes the concept and syntax of static NAT mappings and explains defining inside/outside first.

Example: forward TCP/443 from public to inside

				
					ip nat inside source static tcp 192.168.10.50 443 203.0.113.20 443

Port redirection / static port NAT is also covered with examples in Cisco’s NAT guide. [cisco.com]

If you tell me what the ISP handed you (single IP vs /29 routed block), I can tailor the cleanest approach.

6) Verification (use these commands)

Check interfaces & addressing

				
					
show ip interface brief
show interface FastEthernet4
show interface Vlan1


show ip route
show ip route 0.0.0.0


ping 203.0.113.9          ! ping ISP gateway
ping 8.8.8.8 source Vlan1 ! test from inside

``

show ip nat translations
show ip nat statistics


show ip dhcp binding
show ip dhcp pool

7) Troubleshooting checklist (most common gotchas)

  1. Modem not truly bridged → Cisco FE4 won’t get/hold the public IP; you may see upstream private ranges or double NAT symptoms. Bridge mode disables NAT/routing on the ISP gateway. [community….p-link.com], [whatismyip.com], [support.google.com]
  2. ISP MAC binding → power-cycle modem after connecting Cisco WAN (common with passthrough/bridge workflows). [owl360it.com]
  3. Wrong default gateway → add/verify ip route 0.0.0.0 0.0.0.0 <ISP-gateway> (don’t point the default route at the interface unless your ISP specifically requires that style).
  4. LAN IP placed on FE0–FE3 → won’t work as expected; use interface Vlan1 for the LAN gateway and treat FE0–FE3 as switch ports. The VLAN examples show switchport VLAN assignment and an SVI approach. [cisco.com]
  5. Forgot ip nat inside / ip nat outside → NAT overload won’t translate. Inside/outside assignment is the first step in Cisco’s NAT deployment steps

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