This feature enables the termination of MPLS-over-GRE and IP-over-GRE packets on destination IP addresses from a user-configured subnet. SR OS supports processing received GRE encapsulated packets concurrently when the destination address in the outer IPv4 header matches the system interface address (exact match) and when it matches an address on the user-configured GRE termination subnet (longest prefix match).
RFC 2890 specifies the following format for the GRE header:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| |K|S| Reserved0 | Ver | Protocol Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum (optional) | Reserved1 (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All the fields of the GRE encapsulation in RFC 2890 are optional except for the base header (first 4 bytes). The C, K, and S flags are used to indicate if the header includes the optional fields of Checksum (plus Reserved field), Key, and Sequence Number. SR OS can process packets received with the base 4-byte header or with the 8-byte header which includes the Key field. In other words, packets with the flags set to {C=0, K=0/1, S=0}. Any other GRE header setting results in the packet being dropped.
When originating a GRE encapsulated packet, SR OS supports the following header formats:
The 4-byte base header {C=0, K=0, S=0} in the IP-over-GRE feature using a Port Cross Connect (PXC) port (see GRE tunnel overview).
The 4-byte base header {C=0, K=0, S=0} in the IP-over-GRE feature using the Multiservice Integrated Service Adapter.
See Section 4.1, IP Tunnel Overview, of the 7450 ESS, 7750 SR, and VSR Multiservice ISA and ESA Guide.
The 4-byte base header {C=0, K=0, S=0} in the MPLS-over-GRE tunnel and SDP.
See 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 2 Services and EVPN Guide, the 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 3 Services Guide: IES and VPRN, and the 7450 ESS, 7750 SR, 7950 XRS, and VSR Services Overview Guide.
The 8-byte header which includes the Key field {C=0, K=1, S=0} in the filter-based GRE tunneling feature (see Configuring filter-based GRE tunneling).