BGP on a vlan interface with tagged packets on none trunk link

[paul amaral]

Hi, I have an issue that I'm hoping someone here can clarify this for me.  This is an unusual issue for us so im not going into too much detail why i'm trying to see if this is possible.

Currently we use Level 3 as a bgp upstream that terminates on a routed interface, doing ebgp multihop. Is it possible to terminate this BGP connection on a routed vlan interface assuming Level 3 can tag the packets coming back to us over a trunk. Is there any negatives or concerns i should be aware of, i think its possible just not sure what the negatives are as i never did this before with BGP routes. I'm assuming the important thing is that tag exists on packets to me that correspond to the correct vlan on my end and any tagged packets from me to them makes it to their interface after which point the tags are dropped by them once the packet hits other intra AS hops and eventually the internet ?

TIA, paul

[Jon Marshall]

Paul

I have never done this with BGP although obviously have with IGPs but I can't see any reason why it shouldn't work.

Just as a side note does it need to be a trunk ie. is the requirement simply to use an SVI rather than you need it to be a trunk link ?

That aside if the link was a trunk link then yes either -

1) the specific vlan is the native vlan in which case no tag is added. Both ends of the trunk need to agree on the native vlan

or

2) it is a tagged vlan and yes the tags would only be relevant on the trunk link ie. they are not preserved beyond that.

Either way I believe it will work fine.

Jon

[paul amaral]

thanks john,

this is weird setup as this is going to be on a comcast ENS connection, basically lan service over internet. The thing is i have more customers than just Level 3 which i need to keep separated. We have Level 3 xconnected to comcast which is directly connected to us, this connection to us will be a trunk. So my thinking is if i tag Level 3 packets from me to level 3 will be tagged to the dot1q interface at level 3 but not beyond to their core correct ? packets from the internet to level 3 then over to me will be tagged only when it reaches level 3 dot1q interface which then goes over the trunk.

I guess im just concerned that if i can even get Level 3 to tag packets its not going to break level 3 BGP/transit.

paul

[Jon Marshall · Accepted solution]

Paul

Just as a side note to this.

You don't say what device you are connecting to at the other end eg. a L3 routed port with subinterfaces or a switch which could be running SVIs or could simply be used to connect to a L3 device.

Either way if there is a switch obviously you need to be aware that by using an SVI and L2 connection from your end L2 protocols are now sent across that link as well, the primary one to think about being STP.

This isn't a BGP or L3 issue it is just a general concern about extending vlans between you and the provider.

It may not be an issue but it is something to be aware of.

Jon

[paul amaral]

I ended up getting this working a few days after my original post.

When Level 3 started to tag the packets I assumed that i could take the port connected to them and set it up as a dot1q interface. That turned out not to be the case on the cisco 6500 it complained because i use VTP server mode

"Command rejected: VLAN 300 cannot be allocated. VLANs 1-1005 are VTP VLANsVTP mode is client or server and must be changed to Transparent/Off to use VLANs 1-1005"

My second attempt was just to configure an L2/L3 vlan and assign port gig6/2(current Level3 port) as a switchport on Vlan 300 this also did not work because the switch STP blocked the port right away.

"Mar 19 22:24:07: %SPANTREE-SP-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk GigabitEthernet6/2 VLAN300."

to get it to work i configured port gig6/2 as a trunk, even though the other side is not trunking.

interface GigabitEthernet6/2
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 300-399
 switchport trunk pruning vlan 300-399
 switchport mode trunk
 speed nonegotiate
 no cdp enable
end

after disabling STP for vlan 300 i got ip/l2 connectivity and it started to work. I now that STP is there to prevent loops but given the fact that I'm not going to have another link to Level 3 its perfectly safe to disable STP on that vlan.