It is been a while that I read some papers about EVC, flexible matching, … But I have not found a good paper explaining EVC and a way to understanding well except to cisco configuration guide.

 

This figure is exactly what I searched for. This example is simple but enough to understand well how it works.

My misunderstanding was about “ingress” and “symmetric” : I didn’t understand, but now with the figure that it is clear that :

1. encapsulation dot1q 10 : match all frames where a dot1Q tag is present with value 10
2. “rewrite ingress tag pop 1 symmetric” : We pop 1 tag on this frame
3. Give this frame to the MPLS xconnect without any tag

When a frame comes from the xconnect :

1. It arrives without any tag
2. With “symmetric”, we push the 10 dot1q tag
3. put in TX the frame through the GigabitEthernet 0/2

You can ask me : “Ok this is really complicated your configuration, why don’t you use xxxx” ?
Where xxx is :

int gi 0/2
switchport trunk encapsulation dot1q
switchport trunk allow vlan 10
switchport mode trunk

int vl 10
xconnect 192.168.1.1 33 encapsulation mpls

I will reply : “Have you declare your vlan 100 on your switch ? What does it involve for the scalability ?”
=> Yes, you understand well : you are limited by the number of vlans.
=> With EVC : the “encapsulation dot1q 10” is local to the port. So no limitation.
=> Yes it is really great !!!!!

I urge you to read these papers :

• https://supportforums.cisco.com/document/85231/understanding-ethernet-virtual-circuits-evc
• http://ccie-in-3-months.blogspot.fr/2009/06/evc-flexible-frame-matching.html
• http://ccie-in-3-months.blogspot.fr/2009/06/evc-flexible-vlan-tag-rewrite.html
• http://ccie-in-3-months.blogspot.fr/2009/09/evc-flexible-service-mapping.html