Bridging VLAN traffic over T1 between two Cisco 1721?

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Apr 3rd, 2007
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This is my first use of the forum I have two 1721 routers with a T1 between. I would like to connect them to a Cat6509 and forward a VLAN over to the remote end. Is this possible with a 1721? There is a dhcp pool on the 6509 that would/should assign the IP for the workstations on the VLAN.

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paolo bevilacqua Tue, 04/03/2007 - 13:47
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Yes you can do bridging one the T1 however I do strongly recommend use routing instead.

There is simply no reason to use bridging, the 1721 can perfectly do DHCP server and you will have a nicely subnetted network, easy to configure, maintain and diagnose!

Hope this helps, please rate all useful posts using the scrollbox below!

d-mark Tue, 04/03/2007 - 22:25
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When you use routing, you can also use something like:

interface fastethernet0

ip helper-address


to forward the dhcp request form the LAN of your 1721 to your dhcp server.



desai.jaideep Tue, 04/03/2007 - 23:13
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There are two cases:

(1)If you want to extend a single VLAN onto your remote sites. In that case, implement routing instead.

Then you can forward the DHCP requests to the remote subnets using

"ip helper-address " command.

(2)If you have more then one VLANs to extend to your remote sites, then through bridging this can be implented.

just give following commands :

no ip routing

bridge-group 10 protocol ieee (in global)

bridge-group 10 (in interfaces)

........But in this case, is your 6509 also working in L2 mode?

As one of my friend have quoted the L2 traffic can eventually kill your T1.B'coz you can consider a good amount of non-required data transfer(broadcasts) on L2 links.

The choice is all urs buddy.

Pls rate helpfull posts.



coughlind Wed, 04/04/2007 - 10:59
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The root bridge and default gateway for the VLAN 85 is on the CAT 6509 that the near end 1721 will be connected to. Should the port on the 6509 be a router port as well as on each interface of the 1721s? Or, could I make a dot1Q trunk between the 1721 and the Cat6509 and use ip unnumbered address on the T1 interfaces?

desai.jaideep Wed, 04/04/2007 - 20:47
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Now the picture's getting more clearer.

My suggestions :

(1)If you are not using Vlan 85 anywhere in your CAT6509 except for this remote location then you can safely implement routing.

And if you are implement routing then you can enable DHCP server on remote 1721.Then you need to create the port on 6509 as router port and implement routing at all locations.

(2) In case you are using VLAN 85 in your 6509, then the only way to extend the vlan to remote location is through bridging.

On both the routers, you can configure bridging as I have mentioned earlier.No need for giving WAN IPs on both of them. Just a IP from same pool should be given on your ethernet interfaces for management purpose.

This topology will just work like you directly connect a non-manageable switch to one of your 6509 and the consolidated traffic goes out as VLAN 85.In this case you do not have to enable the port 0n 6509 at router port. Let it be in vlan 85 only.

All the PCs on that white switch will reach the DHCP server located on ur 6509.

Though its always the requirement which drives the implementation, but the best practices says that you generally do not implement bridging on low speed links.If you are having say 100 PCs on each locations, the your link will choke up.

Hope I have answered your queries. If you still have any doubts then you are welcomed to revert back.



coughlind Thu, 04/05/2007 - 05:24
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JD, thanks for the info, to everyone actually. Yes, VLAN 85 is passed throughout the 6509 and to other edge/access switches. So I would have the 1721 connect into an access port on the 6509 then configure tranparent bridging accross from E0 on router A and Serial0 and then the same bridge group on both interfaces on router B. Then, if I connect E0 on router B into an access port into a stand alone switch, that will bridge VLAN 85 and any dhcp request on that VLAN will be passed back to the dhcp server pool.

We should only be looking at 6 - 10 PCs connecting onto that VLAN at the remote site. Unfortunately this is a Frac-T1.

desai.jaideep Thu, 04/05/2007 - 08:52
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Thats what exactly I had in my mind.

Go ahead with the implementation.You should be able to get through.




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