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New Member

Bypassing hub ... solution should be easy

I have a cisco 1841. This has two L3 fast ethernet ports on it.

fa0/0 is admin down (not in use).

fa0/1 used for the internal IP of the private LAN.

The router also has a serial up link on it but that is not relevant in this case.

fa0/1 connects to a small unmanaged netgear hub which two PCs then connect to.

Both PCs are statically configured to use fa0/1 as their default gateway.

I wish to bypass the hub and connect both PCs directly to the router without having to reconfigure the PCs and put them on different subnets.

My initial thought was to configure both fa0/0 and fa0/1 as layer 2 switchports on vlan 1 and create an SVI with the IP address that fa0/1 used to have.

But upon logging onto the router I cannot enter the "switchport" command to push the port into layer 2. I am unsure if it is not possible or the command is different for this IOS.

Can anyone help?

Everyone's tags (4)
1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Bypassing hub ... solution should be easy

Hello Steven,

There is a way of doing this but it is not a recommended practice. In any case, you may be interested in trying it out - but beware that all switching performed this way must be processed by the CPU of the router, and it will be very easy to overload it with larger amounts of traffic.

The physical ports on your 1841 indeed can not be directly configured as switchports. They are routed ports only. However, what we can do is to create a software bridge over them that will perform the switching function. The physical interfaces will not be configured with any IP addresses, and instead, the IP address including all remaining IP settings will be moved to a virtual BVI (Bridged Virtual Interface) in much the same way like with SVIs.

The configuration should then be as follows:

bridge irb

bridge 1 protocol ieee

bridge 1 route ip

bridge 1 bridge ip

!

interface Fa0/0

  no ip address

  bridge-group 1

  no shutdown

!

interface Fa0/1

  no ip address

  bridge-group 1

  no shutdown

!

interface BVI1

  ip address X.X.X.X M.M.M.M

  ...

  ...

Once again, this switching will be performed via CPU and may cause significant loads - depending on the amount of traffic between your two PCs, you may notice a slowdown or packet losses.

A better solution would be to obtain a HWIC-4ESW switching module with 4 switchports - but yeah, that's quite expensive. The aforementioned approach with bridging might just do the trick.

Don't forget that the Fa0/0 and Fa0/1 will be running STP and will need up to 30 seconds to become Forwarding after connecting a PC.

Best regards,

Peter

2 REPLIES
Cisco Employee

Re: Bypassing hub ... solution should be easy

Hello Steven,

There is a way of doing this but it is not a recommended practice. In any case, you may be interested in trying it out - but beware that all switching performed this way must be processed by the CPU of the router, and it will be very easy to overload it with larger amounts of traffic.

The physical ports on your 1841 indeed can not be directly configured as switchports. They are routed ports only. However, what we can do is to create a software bridge over them that will perform the switching function. The physical interfaces will not be configured with any IP addresses, and instead, the IP address including all remaining IP settings will be moved to a virtual BVI (Bridged Virtual Interface) in much the same way like with SVIs.

The configuration should then be as follows:

bridge irb

bridge 1 protocol ieee

bridge 1 route ip

bridge 1 bridge ip

!

interface Fa0/0

  no ip address

  bridge-group 1

  no shutdown

!

interface Fa0/1

  no ip address

  bridge-group 1

  no shutdown

!

interface BVI1

  ip address X.X.X.X M.M.M.M

  ...

  ...

Once again, this switching will be performed via CPU and may cause significant loads - depending on the amount of traffic between your two PCs, you may notice a slowdown or packet losses.

A better solution would be to obtain a HWIC-4ESW switching module with 4 switchports - but yeah, that's quite expensive. The aforementioned approach with bridging might just do the trick.

Don't forget that the Fa0/0 and Fa0/1 will be running STP and will need up to 30 seconds to become Forwarding after connecting a PC.

Best regards,

Peter

New Member

Bypassing hub ... solution should be easy

Perfect. Thank you very much

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