Regarding service instance

Question

interface g0/2
      service instance 1 ethernet
        encapsulation dot1q 18
        rewrite ingres tag pop 1 symmetric
        bridge-domain 44
      service instance 2 ethernet
        encapsulation dot1q 66
        rewrite ingress tag pop 1 symmetric
       bridge-domain 44
    !
    interface Vlan44
      ip address 192.168.1.1 255.255.255.0

What is the above config likely to achieve?

Please explain line by line.

My understanding:

If the vlan 18 tagged traffic comes to this interface,it will be placed in service instance 1 .

The vlan18 tag will be popped out Now 'bridge-domain 44' will result in looking at the vlan 44 for the destination mac. If found ,all OK else: If the destination is out the other Service Instance, the frame will be placed on Service Instance 2 and a new VLAN tag(tag 66) will be added

a.Please validate this understanding

b.Using the above,we are making vlan 18 talk to vlan 66 ? Is that the only objective?

c.How on earth, for 2 different vlan tags(vlan tag 18 and vlan tag 66) be using the same ip address(for routing)

ip address 192.168.1.1 255.255.255.0

d.I am also remembering traditional dot1q subinterfacing(router on a stick) method.Using that i could do inter-vlan routing?Why do i need such a complex scenario?

e.If we were to connect a L2 switch to this interface g0/2,and say we have a PC one connected to switch on vlan 18, what would be it's gateway? Similarly , i connect another PC two(vlan 66) to switch,what would be the gateway? How can i make PC one and PC two talk to each other?

Answer 1

If the vlan 18 tagged traffic comes to this interface,it will be placed in service instance 1 .

Correct

The vlan18 tag will be popped out Now 'bridge-domain 44' will result in looking at the vlan 44 for the destination mac. If found ,all OK else: If the destination is out the other Service Instance, the frame will be placed on Service Instance 2 and a new VLAN tag(tag 66) will be added

Close. I confess I'm not 100% sure if the tags need to be explicily translated like:

rewrite ingress tag translate 1-to-1 dot1q 66 symmetric

rather than the way you have it. I would have to lab it up to be sure.

a.Please validate this understanding

b.Using the above,we are making vlan 18 talk to vlan 66 ? Is that the only objective?

You also have a layer three interface, 192.168.1.1

c.How on earth, for 2 different vlan tags(vlan tag 18 and vlan tag 66) be using the same ip address(for routing)

ip address 192.168.1.1 255.255.255.0

Since VLANs are layer 2 and IP is layer 3, they operate independently of each other. VLANs don't know about subnets, and IP doesn't know about VLAN tags.

d.I am also remembering traditional dot1q subinterfacing(router on a stick) method.Using that i could do inter-vlan routing?Why do i need such a complex scenario?

You are not doing routing. Even though there are two VLANs, at layer 3 they are the same subnet. If you have devices on different IP subnets on each VLAN, they will not be able to talk to each other.

e.If we were to connect a L2 switch to this interface g0/2,and say we have a PC one connected to switch on vlan 18, what would be it's gateway? Similarly , i connect another PC two(vlan 66) to switch,what would be the gateway? How can i make PC one and PC two talk to each other?

They would have to be in the same IP subnet, and their gateway would (normally) be 192.168.1.1