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,

[Jon Marshall]

The subinterface numbers do not need to match ie. they are local to the router.

However you may want to match them up simply for ease of administration/understanding when you are troubleshooting.

Jon

[4everlearning]

Thank you Jon, what do you mean if the subinterfaces local to the router? the connectivity will be done between 2 routers PE and CE, so the subinterfaces can be any numbers from both sides and does not need to match, right ?

[Jon Marshall]

I am embarrassed to say this (and your other question) has got me somewhat unsure as to whether or not you need encapsulation. It's something I should know but at the moment simply cannot remember ever setting up subinterfaces without encapsulation.

Apologies for that. Perhaps someone else can confirm.

However the answer to this and your other question still stands ie.

a subinterface with the same number as an SVI should work fine. If you do need to use encapsulation on the subinterface it cannot be vlan 10 because that is already in use. It would need to be an unused vlan purely for that link.

and

the subinterface numbers do not need to be the same on either end because that information is not passed between the router. That is what I meant when I said they are only locally significant.

Jon

[Richard Burts]

I must admit that I do not really understand parts of the original post. When it says "sub interfaces that are serving different services" I am not sure what that really means. Some clarification would really help.

However part of the discussion is quite clear. The original post suggests using 1/0/0.10 and 2/0/0.20. John is quite correct in saying that .10 and .20 are local to the router and do not need to be the same between the routers.

To go a little bit deeper into the discussion we should start by remembering that when we configure these subinterfaces on router interfaces we are dealing with trunking between the router and a connected switch. And when we do the .10 subinterface there should be a command for dot1q encapsulation under the subinterface which specifies the vlan number for the vlan associated with this subinterface. This vlan number is what needs to match between the routers. For example you could have 1/0/0.10 associated with vlan 30 and 2/0/0.20 associated with vlan 30 and it would work just fine. But if 1/0/0.10 is associated with vlan 10 (which is a usual way of doing it) and if 2/0/0.20 is associated with vlan 20 then it will not work for BGP peering.

HTH

Rick

[Jon Marshall]

Rick

Agree with all you say.

Can you put me out of my misery on this simple question though.

If the routers are connected directly ie. no switch in between and you use subinterfaces do you still need dot1q encapsulation ?

I assume you do as the tag tells the receiving router which subinterface the traffic should be sent to.

I was thinking that each subinterface could have a different mac address and therefore you wouldn't need tagging to identify the subinterface the traffic was destined for but as far as I know the subinterfaces all use the same mac address as the main interface.

It really is something I should know but either I never did or I have forgotten it :-)

Jon

[4everlearning]

Thanks Rick and John for your answers.

Rick,

My case is exactly as John's question, both routers will be connected directly to each others without any switch in between with 10G interface that is divided to many sub interfaces serving different services (each subinterface will be associated with different VRF from the PE side).  Now in this case is encapsulation needed? or should the interfaces numbers match in any case for this scenario? 

I ask because according to friend of mine he tried to do this scenario with a Juniper CE connected to a Cisco PE without adding encapsulation command manually on the juniper from the juniper side (this command could have been added from cisco side but i'm not sure since the guy was working on juniper) ,and said that when he used different sub interface number on the juniper router the ebgp did not establish but when he used the same number the ebgp was established!! does that make any sense? would the encapsulation be enabled by default? 

and if so, let's say you have a requirement to establish new ebgp connectivity (L3) with vlan 10 , but then you discovered that this vlan is configured internally on your router for an existing network (different subnet) and has an svi (irb) would it be fine to proceed? or the subinterface (vlan) number for ebgp peering should be unique.  

Thanks,

[Richard Burts]

While I have some experience with Juniper equipment I am no expert on it and will not attempt to address the Juniper side. But I believe that the Cisco side is pretty clear. If you configure a subinterface on an Ethernet on Cisco then the router will expect the encapsulation command under the subinterface. If the encapsulation command is missing from the subinterface I believe that the subinterface will not pass traffic.

On Cisco the subinterface number does not need to be the same as the VLAN number (though this is a common practice). So on Cisco the subinterface numbers do not need to be the same between peers, but the VLAN number which is used in the tag for the frame does need to match.

HTH

Rick

[4everlearning]

Thanks Rick, now it's clear from cisco side. just one last question assuming both the CEs and PEs are cisco devices. 

if the CEs already have vlan 10 configured and had SVI configured for it for X subnet. 

Now for some requirements, the subinterfaces with the eBGP should be configured with vlan 10 as well but this will be with a point to point IPs from a new subnet. 

Now would adding the encapsulation dot1Q for vlan 10 for eBGP peering affect the existing VLAN 10 or the SVI ? in other words can vlan 10 be used for ebgp subinterface encapsulation or should a new vlan be used?

Thanks,

[Richard Burts]

I do not know how to answer this question because it is not clear what the context of the question is. When you ask about configuring subinterfaces I assumed that we were talking about a router (where Ethernet subinterfaces are supported). Now the question includes VLANs and SVIs which sounds like a layer 3 switch. And as far as I know Ethernet subinterfaces are not supported on layer 3 switches. So can you clarify whether we are dealing with a router or with a layer 3 switch?

HTH

Rick

[Jon Marshall]

Rick

It's linked to this question -

https://supportforums.cisco.com/discussion/12373741/svi-and-subinterface

I assumed that the switch was a 6500 to be honest as these do support subinterfaces on a L3 port.

As you can see my answer was that you definitely couldn't use the same IP subnet and I don't think you can use the same vlan for both an SVI and a subinterface although you may know differently about that second point.

Jon

[4everlearning]

This is right, we can assume that the devices are 6500 or Cisco asr where both bridging and L3 subinterfaces are supported 

[Jon Marshall]

Thanks for confirming.

Can I just ask though, is this just for your learning or are you attempting to set this up.

I ask because you can just use any unused vlan so I'm not sure why you would want to use the same vlan number that is already in use.

Same goes for the interface number actually.

Why not just pick numbers that are not already being used ?

Is it because you see a possible advantage to using the same number or are you just curious ?

Jon

[4everlearning]

Jon, 

The reason is, i received a design to implement and after checking the configuration i noticed that those vlans exist. now changing the design will take take so i was curious if proceeding with this setup will cause any problem. otherwise i'll just ask to have the design changed even if it will delay the implementation. 

Thanks,

[Jon Marshall]

No problem, just wanted to check.

I have a strong feeling the 6500 will not like using the same vlan number for the encapsulation if it is already in use but can't be 100% sure as I don't have the equipment to test with.

As I said before the actual interface number shouldn't make any difference as far as I know.

Jon