eBGP connectivity

[4everlearning]

Hi Guys,

a Quick question, let's say we are connecting 2 routers (PE and CE) with eBGP over an sub interfaces that are serving different services.... should the sub interface number match on each router for each service ? or that would not effect the eBGP peering??

For example, would it work if im configuring 1/0/0.10 for X service on the PE and 2/0/0.20 for the same service on the CE??

Thanks,

[Richard Burts]

Interesting that this is a 6500. I do not have experience with this feature on that platform. So I am certainly not authoritative on the question at this point. So my response is more what I think ought to be the case than knowing that it is so.

In the beginning of this question, and in the related question, the original poster says that he will not be doing the encapsulation command under the subinterface. On a router I am confident that this would not work. On the 6500 I am not sure but I believe that it is probably still the case. And if it is not required on the 6500 I do not know when the 6500 receives a frame on that interface how it would decide which subinterface to give it to without having a vlan tag.

So my advice to the original poster is to ask for the design to be changed. If you have a test environment where you could try it out that would be interesting. But without being able to test I am guessing that the odds of encountering problems with this design are pretty high.

As an afterthought: if the design does not call for encapsulation under the subinterface I believe that this is a problem. But if you add the encapsulation command and specify a vlan number that is not being used, then you ought to be able to use 1/0/0.10 without problem.

HTH

Rick

[4everlearning]

Thanks rick, I checked again and you are right the encapsulation command has to be configured for this setup under the subinterface. However, since this is an L3 link (ebgp point to point) i was told that the encapsulation vlan i would use under the interface would be local significant to the interface itself and it would be stripped out and would not be related to the actual existing l2 vlan or the bridging done with the same vlan number. Also i was told since the encapsulation vlan (10 for example) is local significant to the interface, i can use the same number on my interfaces who connect to different networks without facing any issue. 

is that right ? or the encapsulation vlan created on the L3 subinterface will not be locally significant to that interface and will somehow communicate with the l2 vlan and bridging or svi configured on the router??

[Jon Marshall]

I did a bit of searching and found this -

http://www.thenetworker.co.uk/blog/?p=36

it is saying that prior to the sup2T you could not use the same vlan number on different interfaces which matches with what I suspected was the case.

However it then says with the sup2T you can now reuse the same vlan number.

I can't really comment on that as I haven't used that supervisor.

Jon

[4everlearning]

Thanks Jon. This is very interesting. so it seems like in that sup2T it works as i was told which is that the encapsulation vlan on the l3 interface will be locally signigicant to the interface and you can use any number regardless if the number is configured internally on an SVI or on an L2 vlan or even on a different interface?

this document was released on 2011 so maybe by now most most new releases work this way.

[Jon Marshall]

you can use any number regardless if the number is configured internally on an SVI or on an L2 vlan or even on a different interface?

As long as you have a sup2T then yes. Before that vlan usage was global on the 6500 which meant if you allocate that vlan to one interface you can't then allocate it to another as well.

So if you have a sup720, for example, I don't believe you can use the same vlan number on different interfaces.

Like I say though, I haven't used the sup2T so can't say for sure it will work but that document does show an example of using the same vlan number on multiple interfaces so I suspect it would.

Jon

[4everlearning]

Very interesting! but it seems like if the SUP does not support this feature it would not allow me to create the same vlan number to begin with as mentioned in the document. 

(Command rejected: VLAN 3 already in use by interface Vlan3)

However, if the router allows you to create the vlan more than once then it means that the feature is supported in the router and there is no risk of loops or effect on l2 vlan or svi with same vlan number. 

[4everlearning]

I just found this cisco document for a general L3 Interface Configuration:

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/15-0SY/configuration/guide/15_0_sy_swcg/layer3.html

And the document mentions that:

"The VLAN IDs used for Layer 2 VLANs and Layer 3 VLAN interfaces are separate from any VLAN IDs configured on Layer 3 subinterfaces. You can configure the same VLAN ID on a Layer 2 VLAN or Layer 3 VLAN interface and on a Layer 3 subinterface."

So it seems like the default behaviour is that it is fine to use the same number between L3 subinterface and L2 vlans and svis.

However, the re-use vlan document for sup2T was just for re-using the same vlan number on subinterfaces under different L3 interfaces. 

[Richard Burts]

I do not have an authoritative answer to this question. Certainly from a layer 2 perspective this segment (and the vlan on it) are isolated and layer 2 frames from this vlan will not be forwarded to other interfaces. In that sense we can surely say that the vlan is isolated.

But I am concerned that if the 6500 sees vlan 10 on  1/0/0.10 and has vlan 10 on other interfaces that there might be some interaction.

It might work fine or it might cause a problem. I am not sure which it will turn out to be. But if it were me I would choose some different vlan number to use in the encapsulation under the subinterface.

Another aspect of the situation occurs to me. Do you control both devices for this connection (PE and CE) or does someone else control one of them? Both devices need to specify the same vlan number for this to work. If you control both devices it will be easy to implement which ever vlan number you decide to use. If someone else controls one side then you will need to work with them to determine what the vlan number will be.

HTH

Rick