Vlan Across a Layer 3 P-T-P

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Oct 20th, 2009
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I'm trying to come up with a way to take a layer 2 vlan (81) that is on network A and make it available on Network C. Each network being a PtP to one another:

A-B-C. Is such a thing possible? It seems it would be some what simple to do, but not sure where to look. What I need to do is be able to stick devices from network C in vlan 80 originating on Network A and have them not know any difference? ANY help is greatly appreciated.

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raun.williams Tue, 10/20/2009 - 10:57
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I'm sorry, instead of layer 3 point to point, I should have said routed point-to-point connections. How can I carry a vlan from network A to network C over the routed connections?

Lucien Avramov Tue, 10/20/2009 - 11:08
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You can enable ip routing if you have an image supporting it.

Then you can use a routing protocol between a,b,c.

iyde Wed, 10/21/2009 - 08:33
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If I understand you correctly, then you want VLAN81 and it's associated IP subnet from site A to be present at site C, right?

If this is correct, then you should be able to use pseudowire with l2tpv3 on your routers a and c to establish what you want.


raun.williams Wed, 10/21/2009 - 08:45
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Yup, you got it. That's what I need. Is pseudowire available on 3750's and 6509's? We don't use true routers on site to site as it's all a private network. What about vlan bridging?

Giuseppe Larosa Wed, 10/21/2009 - 08:52
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Hello Raun,

in your case change the link in a L2 trunk on both sides with previous IP subnet used on point to point link associated to a new vlan.

Add vlan 81 and eventually other vlans on the trunk.


switchport mode trunk

switchport nonegotiate

switchport trunk allowed vlan 81,X

where X is the new vlan for L3 traffic, 81 the vlan to be extended

to avoid problems with different VTP domains

L2TPv3 may be supported on 6509 but it shouldn't be on 3750


note that Lucien had suggested this on its first post, providing details about devices involved help to make the best choice.

A L2 trunk between two lan switches is easy and efficient.

Hope to help


raun.williams Wed, 10/21/2009 - 11:08
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I suppose I consider this doing fancy stuff myself as we're talking about links between major campuses for a hospital system. I was looking for an add-on config versus's a rebuild, something to go along with our current configuration.


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