Configure two routers in bridge mode

[net buzz]

Hi, i need to configure two Cisco routers in bridge mode as they interconnect two LANs that are in the same subnet. The two routers are connected by a fiber optic link. Can you please indicate me how to set the routers in bridge mode.

[Giuseppe Larosa]

Hello Alvin,

this kind of configuration is thought to make routers to bridge between them.

you need to keep this scenario:

RA ----- RB

if you put a switch in the middle several facts can happen.

By the way if you had two switches with fiber optic link you can connect them together without using bridging on routers.

I guess (I hope) that you have only one switch in site B. (no switch on site A)

You need to connect the switch downstream RB interface in bridging mode.

Verify how the switch port is configured depending on some settings it can react negatively when it receives an IEE STP BPDU on its port.

Or the port can be configured to be a trunk port.

feel free to post the configuration of the switch and the way you connect RB and the switch (what ports you use)

Hope to help

Giuseppe

[net buzz]

Dear Guiseppe,

I am still having problems with the deployment of the routers. Please refer to attached topology.

From router A, I am able to ping router B and the server. Similarly from router B, I am able to ping router A and the devices in LAN A.

But as soon as I connect the uplink (red line) from the HP switch 2626 to the D-Link switch, I cannot ping router B and the server from router A anymore. From router B also, I am also unable to ping router A and the devices in LAN A.

As soon as I disconnect the uplink, I am able to ping both sides again.

Below is the configuration of the HP switch:

Running configuration:

; J4900B Configuration Editor; Created on release #H.08.83

hostname "ProCurve Switch 2626"

mirror-port 19

snmp-server community "public" Unrestricted

vlan 1

name "DEFAULT_VLAN"

untagged 1-26

ip address 172.18.14.1 255.255.252.0

exit

interface 21

monitor

exit

So, is there some modification that I need to do to the routers configurations.

Please also refer to the second message that I will send.

Thanks for your help.

Alvin

[net buzz]

Dear Guiseppe,

Actually there are two Cisco 878 SHDSL routers (Router C and D) being used to provide the link between LAN A and LAN B. Refer to the attached topology. The fiber optic routers are meant to replace those two routers.

What is strange is that these two routers are also configured in bridge mode and they are working fine.

Find attached the configuration of the two routers C and D.

With routers C and D, the devices in both LANs A and B are accessible.

Referring to the configuration of these two routers, is there some modifications that needs to be done to the fiber optic routers?

Best regards,

Alvin

[Giuseppe Larosa]

Hello Alvin,

when you connect the fiber uplink between the D-link unmanaged switch and the HP switch the SHDSL is still connected ?

I see something I think it could be wrong in routerD config:

no spanning-tree vlan 1

I would fix this I'm not sure on the effects of this command.

May you schedule a maintanance window and can you try to connect the fiber uplink after having shutted the SHDSL link ?

I also recommend to verify that STP is running correctly on all devices.

Being a multi-vendor scenario you need to verify that all devices are able to understand and talk 802.1D (protocol ieee in IRB config).

The guidelines are:

all devices have to be running 802.1D

all devices have to agree on root bridge-id.

if two different devices are claiming to be the root bridge something wrong is happening.

I suspect that a loop forms when you connect the fiber cable :

there is the SHDSL link and the fiber cable.

I'm starting to think that you should think to redesign the network for using different subnets so that you can use IP routing.

Also I would move the D-link unmanaged switch after the hp switch and if possible I would connect directly the router to the hp switch.

Hope to help

Giuseppe

[net buzz]

Hello Giuseppe,

The SHDSL link is working fine with the present configuration of routers C and D. Servers and PCs are accessible on both sides.

The uplink between the HP switch and the D-Link switch is actually a UTP cable. It is when plugging this UTP cable in the D-Link switch that there is no more connection between the two fiber optic routers. And yes when connecting this uplink the connection between the SHDSL routers is still up.

Also, the use of different subnets will not be possible according to the client. We will have to stick with this single subnet.

Also, I may not have mentioned it before but this very strange issue occurred last week. I disconnected the SHDSL routers (C and D) from LANs A and B and I then connected the fiber optic routers (A and B). The link went up and I was able to access the servers at each end. I even connected the UTP uplink from the HP switch and it worked. The client was able to work for like 12 hours and then suddenly the link went down. I had to revert back to the SHDSL routers. Since then, each time I connect the UTP uplink even after having removed the SHDSL routers there is no communication between the two fiber optic routers. Could this be indicating that there is some kind of loop?

Also will the use of the following command help:

bridge-group 1 spanning disabled

Please find attached spanning tree for routers A,B (Fiber) and C,D (SHDSL).

Regards,

Alvin

[net buzz]

Dear Giuseppe,

I have done some testing after disconnecting the two SHDSL routers from the network at both locations A and B.

So far the bridge mode configuration of the fiber optic routers A and B are working out fine. But the loss of connection occurred again when I connected the UTP uplink from existing HP switch found in LAN B to the unmanaged D-Link switch on which router B is connected. When this UTP uplink is connected, I am not able to ping router A from router B. Similarly from router A, I can no longer ping router B.

Removing this UTP uplink connecting the HP switch to the D-Link switch restores the connection between routers A and B.

Find below the configuration of the HP switch:

ProCurve Switch 2626# sh run

Running configuration:

; J4900B Configuration Editor; Created on release #H.08.83

hostname "ProCurve Switch 2626"

mirror-port 19

snmp-server community "public" Unrestricted

vlan 1

name "DEFAULT_VLAN"

untagged 1-26

ip address 172.18.14.1 255.255.252.0

exit

interface 21

monitor

exit

ProCurve Switch 2626#

Based on the above configuration, is there some modifications that needs to be done to the routers A and B configurations or to the HP switch configuration so that there is no connection interruption when the UTP uplink is connected?

Best regards,

Alvin

[Giuseppe Larosa]

Hello Alvin,

the problem here is to have an unmanaged and unmanageable device on the path: it is the D-link switch that is not able to work well with the other devices.

You should find a way to avoid to have it in the data path.

In my opinion that kind of device is acceptable only at home.

When we speak of a network for a business even a small business first basic requirement is to have manageable devices (and I don't say it has to be of a specific brand/vendor) so that you can know how they are configured and how they perceive the other devices.

Hope to help

Giuseppe

[net buzz]

Dear Guiseppe,

The bridge mode configuration was working fine but in the end I had to shift to routing because the routers were having a conflict with a Cisco switch already present in the network. This was because of the spanning tree protocol being used in the bridge mode configuration.

Thanks for all your help.

Regards,

Alvin

[a.alekseev]

What is the sense to use two routers for doing this?