GRE format

C

Specifies whether there is a checksum in the header

If set to 1, both the checksum and reserved1 fields must be present

In the network egress (transmit) direction, the C bit is always set to 0; therefore, the checksum and reserved1 fields are omitted from the header. The GRE header is therefore always 4 bytes (32 bits) in the network egress direction.

In the network ingress direction, the C bit validity is checked. If it is set to a non-zero value, the GRE packet is discarded and the IP discards counter is increased.

Reserved0

Indicates whether the header contains optional fields

The first 5 bits of the field are always set to 0 and bits 6 to 12 are reserved for future use and also set to 0

Ver

Always set to 000 for GRE

At network ingress, if a GRE packet is received with the version field set to any value other than 000, the packet is discarded and the IP discards counter is increased

Protocol type

Specifies the protocol type of the original payload packet—identical to Ethertype with the only supported option being MPLS unicast (0x8847)

Checksum (optional)

Not applicable

Reserved1 (optional)

Not applicable

Eth

The Layer 2 transport header

The only Layer 2 protocol supported is Ethernet

MTU size depends on the encapsulation type (14 bytes for null encapsulation and 18 bytes for dot1q encapsulation)

The Ethertype is always set to IP (0x800)

IP

Indicates the transport protocol

IPv4 is the transport protocol for GRE-MPLS

GRE

Indicates the encapsulation protocol

MPLS service header

The MPLS service label identifies the service and the specific service element being transported

For VLL and VPWS services, the label references the pseudowire that was statically configured, or signaled via T-LDP or BGP signaling

For VPLS services, the label references a particular VPLS pseudowire that was signaled via T-LDP or BGP signaling to allow the end-to-end VPLS service

For VPRN services, the label references either a spoke SDP pseudowire associated with the VPRN, or an MP-BGP advertised route that has been signaled via BGP to allow the end-to-end VPRN service

CW for VLL and VPWS

The pseudowire Control word (CW) is a 32-bit (4-byte) field that is inserted between the VC label and the Layer 2 frame

For more information about the Control word, see "Pseudo-wire Control word" in the 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 2 Services and EVPN Guide.

Services payload

The services payload is the payload of the service being encapsulated

For VLL, VPWS, and VPLS, this is a Layer 2 frame with either null or with dot.1q encapsulation

For VPRN, this is a Layer 3 IPv4 or IPv6 packet without Layer 2 information