David’s Cisco Networking Blog

Nov 16 2007   5:39PM GMT

Bridged ports on a Cisco 851 & 871 series routers

David Davis David Davis Profile: David Davis

Recently I had a question from a Cisco router admin and I suspect that many of you out there could benefit from the answer. This admin had a Cisco 871W and was having trouble creating VLANs.

I believe that the confusion on this topic comes from the fact that the Cisco 851 & 871 series routers have 4 LAN Ethernet ports + 1 Additional Ethernet. The 4 LAN port (Fa0 to Fa3) are intended to be your local LAN switch ports to your, say, 4 devices on your LAN. The 1 Additional Ethernet  port (Fa4) is intended to be connected to something like a DSL modem or cable modem.

These 4 special internal LAN ports on the Cisco 851 & 871 are special and don’t work like other ports on other Cisco routers or switches with 4+ Ethernet ports. These 4 LAN ports on the 851 & 871 cannot be turned into VLANs and cannot be configured individually.

According to the 871 specs, you can create up to 4 VLANs with the 871w. These can include wireless VLANs.

I think the problem that is coming up here is that FastEthernet ports 0-3 are all in a bridge. You cannot create a subinterface on Fa0-3 but you can on Fa4.

Here is how I created a subinterface for a VLAN off of Fa4:

Router#sh ip int brie
Interface                  IP-Address      OK? Method Status  Protocol
FastEthernet0              unassigned      YES unset  up       down
FastEthernet1              unassigned      YES unset  up        down
FastEthernet2              unassigned      YES unset  up        down
FastEthernet3              unassigned      YES unset  up        down
FastEthernet4              unassigned      YES unset  administratively        down    down
FastEthernet4.1            unassigned      YES unset  administratively        down    down
Dot11Radio0                unassigned      YES TFTP   administratively        down     down
Vlan1                      unassigned              YES unset  up                down
Virtual-Dot11Radio0        unassigned      YES TFTP   administratively   down    down

Still, not all of us are going to want to have a VLAN only on Fa4. So the answer is, as I understand it (haven’t tried it) to use the bridge group function. You put certain Fa ports in a certain bridge group, then put that bridge group in a VLAN.

Please see this Cisco doc for more information

And take a look at George Ou’s Examples of configuring VLANs on the 871.

He offers a downloadable Excel template. Now, if I recall he is configuring wireless VLANs but the same principles should apply.


So if you are trying this yourself, I think this knowledge and those configs will give you examples of what you are looking to do.

Personal Website: HappyRouter.com – home of Cisco how-to articles & videos
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