A Router Into a Switch

zoro_2009

Hello,

I have an 1800 series router, it has 2 interfaces attached to it, what I would like to do is to make it behave just like an ordinary switch, and since the "switchport" command didn't recognized on those interfaces configuration I couldn't figure out what to do !

Thanks alot for any clarification !:)

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JeanM

What are you trying to accomplish?

Jollycork

A router is a router. Same thing with a switch. A switch is a switch [with the exception of a layer 3 switch which can perform routing functions].

So on a router, what's the interfaces? the same question can be asked about a switch. What are the interfaces on a switch [leaving out trunk ports and a switch that's layer 3]?

Magmadragoon

I assume you have a 1841 router which has two fast ethernet ports 0/0 & 0/1. To connect them to a switch you use a port on the switch but you must turn the connection on by assigning one of the ports an IP address and using the no shutdown command or use the connection as a trunk link.

Switchport mode does not work on a router because a router is doing layer 3 work.
Switchport mode works on switches that do layer 2 or layer 2 + layer 3 switches.

zoro_2009

Thanks for the replies, I'm little newbie !

Basically what I'm trying to accomplish here is:

Router (Providing Internet and performing NAT)
|
|
1840 Router (Acting like a simple switch)
|
|
Switch
|
|
Clients

So, the 1840 has nothing necessary to do except for the basic switching !
The reason why I am doing this is that I don't have a switch but instead I have an 1840 router free, so thought transforming that router into a switch !

Magmadragoon wrote: »

I assume you have a 1841 router which has two fast ethernet ports 0/0 & 0/1. To connect them to a switch you use a port on the switch but you must turn the connection on by assigning one of the ports an IP address and using the no shutdown command or use the connection as a trunk link.

Switchport mode does not work on a router because a router is doing layer 3 work.
Switchport mode works on switches that do layer 2 or layer 2 + layer 3 switches.

I used to add 16SW module into 3750 router in GNS3, than do the "switchport" on those interfaces, that made 'em "switchable", Doesn't the same apply here ?

Thanks

Magmadragoon

I think you are talking about the NM-16ESW Cisco 16 port module in GNS3

zoro_2009

Magmadragoon wrote: »

I think you are talking about the NM-16ESW Cisco 16 port module in GNS3

Yes thats the one !

Magmadragoon

I might not be correct but for a 1840 router you will only be able to do routing. The 3700 in GNS3 with the NM-16ESW has autosensing ports that will tell the router it has a limited switch capability. The only switch capability add-on for the 1800 series is the 4-port HWIC add on.

For example, In my home lab I have a home ISP router that is in bridge mode, connected to my 1841 router in which does dhcp at the router and is connected to my 2950 Switches.

theodoxa

You can configure NAT and DHCP on the 1841. This won't make it a switch, but it will allow your clients to reach the internet through it.

R1(config)# interface FastEthernet 0/0
R1(config-if)# ip address dhcp
R1(config-if)# ip nat outside
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface FastEthernet 0/1
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1(config-if)# ip nat inside
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# ip dhcp excluded-address 192.168.10.1
R1(config)# ip dhcp pool INSIDE
R1(dhcp-config)# network 192.168.10.0 /24
R1(dhcp-config)# default-router 192.168.10.1
R1(dhcp-config)# dns-server 8.8.8.8 8.8.4.4
R1(dhcp-config)# exit
R1(config)# ip route 0.0.0.0 0.0.0.0 FastEthernet 0/0
R1(config)# access-list 10 permit 192.168.10.0 0.0.0.255
R1(config)# ip nat inside source list 10 interface FastEthernet 0/0 overload

zoro_2009

Thanks for the replies !

theodoxa wrote: »

You can configure NAT and DHCP on the 1841. This won't make it a switch, but it will allow your clients to reach the internet through it.

R1(config)# interface FastEthernet 0/0
R1(config-if)# ip address dhcp
R1(config-if)# ip nat outside
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface FastEthernet 0/1
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1(config-if)# ip nat inside
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# ip dhcp excluded-address 192.168.10.1
R1(config)# ip dhcp pool INSIDE
R1(dhcp-config)# network 192.168.10.0 /24
R1(dhcp-config)# default-router 192.168.10.1
R1(dhcp-config)# dns-server 8.8.8.8 8.8.4.4
R1(dhcp-config)# exit
R1(config)# ip route 0.0.0.0 0.0.0.0 FastEthernet 0/0
R1(config)# access-list 10 permit 192.168.10.0 0.0.0.255
R1(config)# ip nat inside source list 10 interface FastEthernet 0/0 overload

I see you are configuring the DHCP, Isn't this a configuration of a router with a DHCP enabled !
All I want is to make the router a transparent equipment for the users(switch), so the users will get directly the Internet routers address as the default gateway (NOT the 1840 router)

awitt11

You need to be more clear about what you are trying to accomplish. Acting as a 'simple switch' or bridge is not what the 1841 was designed for. It could be set up as a proxy or zone-based IOS firewall, but that might be more configuration than you are looking for here. You showed a switch in your diagram but then said you don't have a switch....

zoro_2009

awitt11 wrote: »

You need to be more clear about what you are trying to accomplish. Acting as a 'simple switch' or bridge is not what the 1841 was designed for. It could be set up as a proxy or zone-based IOS firewall, but that might be more configuration than you are looking for here. You showed a switch in your diagram but then said you don't have a switch....

That switch is already used and cannot be replaced !
So the only way is to use the 1840 router as a switch !

What I am doing is transforming the 1840 router to a basic switch (layer 2 switch) so it can be transparent equipement in the network (i.e a switch)

If this cannot be done, how can I transform the ports in a switchable manner (one Trunk and the other for clients) !

deth1k

if you want router to do just switching then you need to disable routing:

no ip routing

and configure IRB

bridge irb
!
bridge 1 protocol ieee
!
int fa0/0
bridge-group 1
!
int fa0/1
bridge-group 1

gorebrush

deth1k wrote: »

if you want router to do just switching then you need to disable routing:

no ip routing

and configure IRB

bridge irb
!
bridge 1 protocol ieee
!
int fa0/0
bridge-group 1
!
int fa0/1
bridge-group 1

I'd have thought that ought to do it.

Though, quite why you'd need to turn a two port device into a switch beats me... Unless you've got some epic application for it.

Magmadragoon

gorebrush wrote: »

I'd have thought that ought to do it.

Though, quite why you'd need to turn a two port device into a switch beats me... Unless you've got some epic application for it.

I thought I was the only one confused with the OP request. It would be much easier to use a switch.

Jollycork

But [shrug] guess if one wants to use a router for a switch I've seen stranger configs.

JeanM

I think the OP is wanting to use this router as a L2 switch because it's got a switch module installed in it...maybe the switch he's got is out of ports and he wants to add this 1800 series with the switch module installed to the existing switch (trunk) in order to provide more switch ports?

Not sure....but why not just get the right component for the job? IE. another switch.

theodoxa

JeanM wrote: »

I think the OP is wanting to use this router as a L2 switch because it's got a switch module installed in it...maybe the switch he's got is out of ports and he wants to add this 1800 series with the switch module installed to the existing switch (trunk) in order to provide more switch ports?

Not sure....but why not just get the right component for the job? IE. another switch.

If he was a switch module, then those ports (but not FastEthernet 0/0 or 0/1 as these are Layer 3 ports) can be configured just like a switch. Only caveat is that he has to use only the switch ports (the ones on the module). Connect one to his existing switch and the other(s) to his clients.

theodoxa

zoro_2009 wrote: »

I see you are configuring the DHCP, Isn't this a configuration of a router with a DHCP enabled !

Yes. That is the configuration to setup the router as a router that can be used behind an existing router.

All I want is to make the router a transparent equipment for the users(switch), so the users will get directly the Internet routers address as the default gateway (NOT the 1840 router)

Is there a reason you want to use the 1841 as a switch? Does it have a switch module (HWIC-4ESW) installed? The only reason I know of for a 2 port switch (bridge) was to connect two hubs together, but divide them into separate collision domains. This might have been done 10 or 20 years ago, but not anymore. Switch ports are cheap enough now that nobody uses Hubs, except for Troubleshooting when they don't know how to configure SPAN.

JeanM

theodoxa - check his reply #7 , but then if he was trying to have more switch ports, wouldn't you connect the 1841 with it's switch ports to the existing switch and not the other way around , reply #5 with diagram...

zoro_2009

deth1k wrote: »

if you want router to do just switching then you need to disable routing:

no ip routing

and configure IRB

bridge irb
!
bridge 1 protocol ieee
!
int fa0/0
bridge-group 1
!
int fa0/1
bridge-group 1

That did the trick, thank you very much !

Guys please understand, I don't have a free switch, in the other hand I have this spare router in my hands, so I thought why not, after all this is just temporary and for tests purposes only !

Thanks !

ebohlman

theodoxa wrote: »

The only reason I know of for a 2 port switch (bridge) was to connect two hubs together, but divide them into separate collision domains. This might have been done 10 or 20 years ago, but not anymore. Switch ports are cheap enough now that nobody uses Hubs, except for Troubleshooting when they don't know how to configure SPAN.

Admittedly not very common cases, but at least two other reasons:

1) Connecting two devices that would otherwise require a cable longer than the per-segment maximum (100M for copper).
2) Connecting two hosts when you don't have a crossover cable handly.