1. OVC ID
A string of at most 45 bytes that uniquely identifies the OVC within a given CEN.
2. OVC type
This can be Point-to-Point or Multipoint-to-Multipoint. An OVC that can associate two or more OVC End Points is defined to have OVC Type of Multipoint-to-Multipoint. An OVC that associates exactly two OVC End Points is defined to have OVC Type of Point-to-Point, and can be considered a special case of the Multipoint-to-Multipoint OVC Type. Note that because an OVC can associate more than one OVC End Point at a given ENNI, the type of an OVC is not determined by the number of External Interfaces supported by the OVC.
3. OVC End Point List
A list of OVC End Point Identifiers.
4. Maximum Number of UNI OVC End Points
The Maximum Number of UNI OVC End Points is the upper bound on the number of OVC End Points that are at different UNIs that can be associated by an OVC. This must be an integer greater or equal to 0.
5. Maximum Number of ENNI OVC End Points
A list of OVC End Point Identifiers. The Maximum Number of ENNI OVC End Points is the upper bound on the number of OVC End Points that are at an ENNI that can be associated by an OVC. This must be an integer greater or equal to 1.
6. OVC Maximum Transmission Unit Size
The OVC MTU size in bytes. The OVC MTU must be at least 1526 bytes and it is recommended to be at least 2000 bytes. It must be less than or equal to the ENNI MTU size. When an ENNI frame is larger than the OVC MTU that is mapped to an OVC, then the frame must be discarded. It may or may not consume tokens from the ingress BWP.
Note that when an EVC that spans multiple CENs is set with a specific MTU size, then each OVC that is part of the EVC should have its MTU set to at least the MTU size of the EVC.
7. CE-VLAN ID preservation
OVC CE-VLAN ID Preservation is used to achieve EVC CE-VLAN ID Preservation that is a key property of the EPL and EP-LAN as specified in MEF 6.1. In order to achieve EVC CE-VLAN ID Preservation, all OVCs that are part of this EVC must have their OVC CE-VLAN ID Preservation set to Yes. Similar to EVC CE-VLAN ID Preservation, the handling of untagged and priority tagged frames can be a challenge. Therefore, there is a distinction between the two cases, which is analogous to the all-to-one bundling case for EVC CE-VLAN ID Preservation.
The first table at right describes the relationship between an ingress frame at an External Interface and the corresponding egress frame at an External Interface for the case where all OVC end points associated with an OVC map ALL CE-VLAN IDs to this OVC. OVC CE-VLAN ID (equivalent to All-to-One Bundling). Preservation is required in this case.
The second table describes the relationship between ingress frame at External Interface to the egress frame at the External Interface for the case where all OVC end points associated with an OVC map some but NOT ALL CE-VLAN IDs to this OVC.
| Ingress Interface | Ingress Frame Format | Egress Interface | Egress Frame Format |
|---|---|---|---|
| UNI | Untagged | UNI | Untagged |
| UNI | Untagged | ENNI | S-Tag only |
| UNI | C-Tagged | UNI | C-Tagged with VLAN ID value of the C-Tag equal to that of the ingress Service Frame at the UNI |
| UNI | C-Tagged | ENNI | S-Tag and C-Tag with the VLAN ID value in the CTag equal to the VLAN ID value in the C-Tag at the UNI |
| ENNI | S-Tag and C-Tag | UNI | C-Tagged with the VLAN ID value of the C-Tag equal to that of the VLAN ID value in the C-Tag of the ingress frame at the ENNI |
| ENNI | S-Tag only | UNI | Untagged |
| ENNI | S-Tag and C-Tag | ENNI | S-Tag and C-Tag with the VLAN ID value in the CTag equal to the VLAN ID value in the C-Tag at the ingress ENNI. |
| ENNI | S-Tag only | ENNI | S-Tag only |
| Ingress Interface | Ingress Frame Format | Egress Interface | Egress Frame Format |
|---|---|---|---|
| UNI | C-Tagged with VLAN ID value in the range 1, … , 4094 | UNI | C-Tagged with VLAN ID value equal to that of the ingress Service Frame at the UNI |
| UNI | C-Tagged with VLAN ID value in the range 1, … , 4094 | ENNI | S-Tag and C-Tag with the VLAN ID value in the C-Tag equal to the VLAN ID value in the C-Tag at the UNI |
| ENNI | S-Tag and C-Tag with VLAN ID value in the CTag in the range 1, … , 4094 | UNI | Tagged with the VLAN ID value of the C-Tag equal to that of the C-Tag of the ingress frame at the ENNI |
| ENNI | S-Tag and C-Tag with VLAN ID value in the CTag in the range 1, … , 4094 | ENNI | S-Tag and C-Tag with the VLAN ID value in the C-Tag equal to the VLAN ID value in the C-Tag at the ingress ENNI. |
(*) The S-tag value is defined in the OVC end point map and can take any value, regardless of the value of CE-VLAN ID.
8. CE-VLAN CoS preservation
OVC CE-VLAN CoS Preservation is used to achieve EVC CE-VLAN CoS Preservation. In order to achieve EVC CE-VLAN CoS Preservation, all OVCs that are part of this EVC must have their OVC CE-VLAN CoS Preservation set to Yes. The mechanism specified keeps the original PCP value in both C-tag and S-tag. Note that this means that color forwarding should be done using DEI bit in the S-tag. The table at right. specifies the required mapping for implementing OVC CE-VLAN CoS Preservation.
| Ingress Interface | Ingress Frame Format | Egress Interface | Egress Frame Format |
|---|---|---|---|
| UNI | C-Tagged | UNI | C-Tagged with PCP value equal to that of ingress Service Frame |
| UNI | C-Tagged | ENNI | S-Tag and C-Tag with the PCP value in the C-Tag equal to the PCP value in the C-Tag at the UNI |
| ENNI | S-Tag and C-Tag | UNI | C-Tagged with the PCP value of the tag equal to that of the C-Tag of the ingress frame at the ENNI |
| ENNI | S-Tag and C-Tag | ENNI | S-Tag and C-Tag with the PCP value in the C-Tag equal to the PCP value in the C-Tag of the ingress frame at the ingress ENNI. |
9. S-VLAN ID & CoS preservation
S-VLAN ID Preservation and S-VLAN CoS Preservation apply between two ENNIs connected by an OVC. This attribute does NOT affect ENNI to UNI frame exchange. Preservation means that the value of S-VLAN ID and/or S-VLAN CoS at one ENNI must be equal to the value at a different ENNI connected by the OVC.
The following rules apply:
- When an OVC has the S-VLAN ID Preservation attribute with a value of Yes, it must associate at most one OVC End Point located at a given ENNI.
- When an OVC has the S-VLAN ID Preservation attribute with a value of No, an egress ENNI Frame mapped to an OVC End Point resulting from an ingress ENNI Frame mapped to a different OVC End Point MUST have an S-VLAN ID value that has a one-to-one association with the S-VLAN ID of the ingress service frame.
Note that S-VLAN ID preservation cannot be achived when hairpin switching is enabled.
The benefit of using S-VLAN ID preservation is to allow simpler end-to-end service provisioning and service monitoring for cases where an EVC spans several CENs. In such a case, the SP may require S-VLAN ID preservation from the CEN operators in order to easily define the mapping rules of EVC to OVC.
10. Color forwarding
Color Forwarding describes the relationship between the color on an ingress frame into the Operator Network and the color of the resulting egress ENNI Frame. When Color Forwarding is Yes, the OVC cannot “promote” a frame from Yellow to Green. Promoting a frame from Yellow to Green could have an undesired impact on the EVC performance. The newly promoted Green frames are now competing with equal rights for resources as frames marked Green at the ingress UNI. For this reason, this attribute is useful to prevent such behavior. When color forwarding is set, an ingress frame at ENNI declared yellow by an ingress BWP must have its color marked in the header, using PCP bits or DEI bit. Note that this attribute does not describe Color marking of an egress Service Frame at a UNI because a method for such marking is not specified in MEF 23.
11. Frame Delivery
Similar to EVCs, OVCs also have three attributes to govern frame delivery rules. These are:
- Unicast Frame Delivery
- Multicast Frame Delivery
- Broadcast Frame Delivery
Each is independent from the others and can take two values:
- Deliver unconditionally – This means that assuming that the frame has a valid FCS/CRC and assuming that the ingress BWP declared the frame color as Green or Yellow, it should be passed to the OVC.
- Delivery Conditionally – Must specify the condition.
An example of such a condition is that the destination MAC address is known by the Operator CEN to be “at” the OVC End Point.
In some E-LAN scenarios, the SP may want to limit the consumed bandwidth over an OVC (for example tunnel to a UNI) and thus would like to avoid flooding for unknown addresses.
12. SLS and Performance Metrics
The Service Level Specification (SLS) consists of definitions of delivery performance attributes for frames between External Interfaces, e.g. delay, loss. For each performance attribute, the SLS also contains a quantitative objective. When the frame delivery performance level meets or exceeds the objective for each performance attribute, the SLS is said to be met. The SLS that applies to a given ENNI or Service Frame is based on the Class of Service Identifier for the frame. When an ENNI Frame is not sufficiently compliant with an ingress Bandwidth Profile that applies to it, the SLS does not apply to the frame. Details of ENNI and OVC SLS are expected in future versions of MEF 26.x or MEF 23.x.