ATOM on dot1q sub interfaces

Answered Question
Sep 24th, 2009

Hello, networkers!

Long time no see ;-)

Straight on question now. Imagine a MPLS network with the following topology:

A B C D E

(X) --- (X) --- (X) --- (X) --- (X)

CE PE P PE CE

Router A & E are customer's routers.

Router B & D are PE routers

Let's say that we have created MPLS ATOM using Xconnect in between routers B and D. They are both using FastEthernet interfaces with sub-interfaces configured on. Router D is configured to RouterE in this way:

interface FastEthernet0/0.15

description ** RouterD->RouterE **

encapsulation dot1Q 15

no cdp enable

xconnect 2.2.2.2 666 encapsulation mpls

on the other end, router B is configured as follow:

interface FastEthernet0/0.26

description ** RouterB->RouterA **

encapsulation dot1Q 26

no cdp enable

xconnect 1.1.1.1 666 encapsulation mpls

end

Where 1.1.1.1 is RouterD loopback and 2.2.2.2 is Router B lo0.

What do you think about that scenario? Should it work with this configuration when the dot1q vlans differs? In my opinion this shouldn't work as expected as long as MPLS is doing just transparent transport of entire L2 frame (instead of using internetworking on IP level)

Can anyone, please explain how does Cisco handle this? I remember that I've read somewhere during my CCIE journey that there are different types of AtOM VC's which can either carry the dot1q tag or not.

Thank you in advance!

Kind regards,

Dani Petrov

P.p. I tried it in a few different configurations and the results are very interesting but please first share your thoughts ;-)

I have this problem too.
0 votes
Correct Answer by Laurent Aubert about 7 years 2 months ago

Hi,

You can't force the vc-type and don't need to.

To summarize:

- switchport trunk mode and subinterfaces will always pop the outer tag

- EVC interfaces do nothing by default.

On top of that vc-type 4 will add a service-delimiter tag to the frame received from the AC. It's the responsibility of the egress router to know what to do with this tag (rewrite or remove it).

GSR and 7200 will negotiate a vc-type 4 if the AC is a subinterface. 7600 will always negotiate a vc-type 5 except if the peer wants a vc-type 4.

HTH

Laurent.

Correct Answer by Laurent Aubert about 7 years 2 months ago

Hi Danail,

With your configuration, PE's will negotiate vc-type 4 (VLAN mode). It means each transmitted frames must have a service-delimiter tag which is usually local significant. That's why this tag is re-written or removed at the egress PE to reflect its local configuration.

With VC-type 5 (port mode), the service-delimiter tag is removed.

Please check chapter 4.4.1 of RFC 4448

HTH

Laurent.

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Overall Rating: 4.3 (3 ratings)
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shivlu jain Thu, 09/24/2009 - 20:59

Hi

This scenario will work because when the sub interfaces are point to point interfaces for your CE, when the traffic comes to the dot1q information lost but the vcid information remain be common between the PE. In your case 666 is unique and xconnect will come up. So the config will work. No issues at all.

regards

shivlu jain

danailpetrov Thu, 09/24/2009 - 22:49

Hello,

I'm not quite sure that the information is "lost" as you said. Furthermore, the information about d1q tag is carried throughout the MPLS network, but it looks like the newer versions of IOS automatically rewrite or just remove the dot1q tag from the frame. I just can't understand from which versions of IOS this feature is implemented.

I can provide you with a few snippets to see the difference.

Kind regards,

Danail Petrov

Correct Answer
Laurent Aubert Fri, 09/25/2009 - 18:52

Hi Danail,

With your configuration, PE's will negotiate vc-type 4 (VLAN mode). It means each transmitted frames must have a service-delimiter tag which is usually local significant. That's why this tag is re-written or removed at the egress PE to reflect its local configuration.

With VC-type 5 (port mode), the service-delimiter tag is removed.

Please check chapter 4.4.1 of RFC 4448

HTH

Laurent.

danailpetrov Fri, 09/25/2009 - 21:41

Laaubert,

thank you for your reply! Do you know any methods to manipulate this behavior in Cisco IOS? I mean the VC-Types and is there any documentation provided by Cisco regarding these techniques? Because this RFC says : "For an Ethernet VLAN PW, VLAN tag rewrite can be achieved by NSP at the egress PE, which is outside the scope of this document"

Thank you once again!

Correct Answer
Laurent Aubert Sun, 09/27/2009 - 13:01

Hi,

You can't force the vc-type and don't need to.

To summarize:

- switchport trunk mode and subinterfaces will always pop the outer tag

- EVC interfaces do nothing by default.

On top of that vc-type 4 will add a service-delimiter tag to the frame received from the AC. It's the responsibility of the egress router to know what to do with this tag (rewrite or remove it).

GSR and 7200 will negotiate a vc-type 4 if the AC is a subinterface. 7600 will always negotiate a vc-type 5 except if the peer wants a vc-type 4.

HTH

Laurent.

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