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Type 4 & 5 Pseudowires


I'm currently working on hashing out the details regarding pseudowire types in order to up train some of my technicians.  I work in an environment where the majority of our circuits operate using MPLS and L2TP encapsulation for L2VPN circuits.  Over the past few months I have been reviewing a good amount of documentation, white papers, case studies and production networks regarding PW Types.  It seems like every time I "think" that I have a firm comprehension of the subject matter, I read something else that places a twist on things.  I have been using Cisco Press L2VPN Architectures (great book by the way!) and there are a few things I'm trying to clear up.



PW type ONLY dictates how traffic/vlan adjustments occur during the PW forwarding.  Operates independtly of the EFP vlan rewrite options.

Ethernet - TYPE 5 (0x0005)


RAW mode

No modification is performed on the frame before placed on the pseudowire unless rewrite option is present on the EFP

Default when xconnect is built on physical port for IOS but is default for XR-IOS regardless if subinterface is used.

Ethernet Vlan - TYPE 4 (0x0004)



Dummy pushed on ingress before placed on the pseudowire.

Default for IOS when built on subinterface but must be defined on XR-IOS by use of pw-class transport mode command.

There seems to a few things that change regarding when using subinterfaces vs Service Instances and the default operations on IOS vs XR.  I have a few questions regarding the above information:

1 - I have read that the pw-type 4 dummy vlan is only implied when the a frame has no 802.1Q tag when being placed on the pseudowire while I have seen other case studies where the dummy vlan is pushed on regardless. I believe that it boils down to a service delimiting tag being present or not but how does the router determine what is service delimiting?  When exactly is the dummy vlan imposed and does this differ when using service instances where the pseudowire is built on an EVC (Service Instance)?

2 - When an EVC is tied to a bridge domain and traffic is destined outside the bridge via a standard layer 2 trunk port, I understand that the trunk has a push pop operation using the bridge domain ID as the vlan even though the command is not present?

3 - Interworking, I understand this is used to join different types of VCs and comes in 2 flavors (Ethernet(L2) and Bridged (L3).  Ethernet passes the traffic through unmodified like a VC type 5 while bridged mode removes the 802.1Q tags?

4 - XR-IOS uses a default type 5 while type 5 or 4 default is dependant on IOS by using physical port or subinterface?

Any insight to get some clarity would be greatly appreciated.