Generic routing encapsulation (GRE) is atunneling protocol developed byCisco Systems that canencapsulate a wide variety ofnetwork layerprotocols inside virtualpoint-to-point links or point-to-multipoint links over anInternet Protocol network.^[2]

• In conjunction withPPTP to createVPNs.
• In conjunction withIPsecVPNs to allow passing of routing information between connected networks.
• Inmobility management protocols.
• InA8/A10 interfaces to encapsulate IP data to/from Packet Control Function (PCF).
• Linux andBSD can establish ad-hoc IP over GRE tunnels which are interoperable with Cisco equipment.
• Distributed denial of service (DDoS) protected appliance to an unprotectedendpoint.

Based on the principles of protocol layering in OSI, protocol encapsulation, not specifically GRE, breaks the layering order. It may be viewed as a separator between two different protocol stacks, one acting as a carrier for another.

GRE packets that are encapsulated withinIP directly, use IP protocoltype 47 in the IPv4 header'sProtocol field^[3] or theIPv6 header'sNext Header field.^[4]

For performance reasons, GRE can also be encapsulated in UDP packets.^[5] Better throughput may be achieved by usingEqual-cost multi-path routing.

The extended version of the GREpacket header^[6] is represented below:

Extended GRE header format

Offsets	Octet	0								1								2								3
Octet	Bit	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
0	0	C		K	S	Reserved 0									Version			Protocol Type
4	32	Checksum (optional)																Reserved 1 (optional)
8	64	Key (optional)
12	96	Sequence Number (optional)

C (1 bit)

Checksum bit. Set to 1 if a checksum is present.

K (1 bit)

Key bit. Set to 1 if a key is present.

S (1 bit)

Sequence number bit. Set to 1 if a sequence number is present.

Reserved 0 (9 bits)

Reserved bits; set to 0.

Version (3 bits)

GRE Version number; set to 0.

Protocol Type (16 bits)

Indicates theether protocol type of the encapsulated payload. (ForIPv4, this would be hex 0800.)

Checksum (16 bits)

Present if theC bit is set; contains the checksum for the GRE header and payload.

Reserved 1 (16 bits)

Present if theC bit is set; is set to 0.

Key (32 bits)

Present if theK bit is set; contains an application-specific key value.

Sequence Number (32 bits)

Present if theS bit is set; contains a sequence number for the GRE packet.

A standard GREpacket header structure^[7] is represented in the diagram below.

Standard GRE header format

Offsets	Octet	0								1								2								3
Octet	Bit	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
0	0	C	Reserved 0												Version			Protocol Type
4	32	Checksum (optional)																Reserved 1 (optional)

C (1 bit)

Checksum bit. Set to 1 if a checksum is present.

Reserved 0 (12 bits)

Reserved bits; set to 0.

Version (3 bits)

GRE Version number; set to 0.

Protocol Type (16 bits)

Indicates theether protocol type of the encapsulated payload. (ForIPv4, this would behexadecimal 0x0800; forIPv6, it would be 0x86DD.^[4])

Checksum (16 bits)

Present if theC bit is set; contains the checksum for the GRE header and payload.

Reserved 1 (16 bits)

Present if theC bit is set; its contents is set to 0.

The newer structure superseded the original structure:^[1]

Original GRE header format

Offsets	Octet	0								1								2								3
Octet	Bit	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
0	0	C	R	K	S	s	Recur			Flags					Version			Protocol Type
4	32	Checksum (optional)																Offset (optional)
8	64	Key (optional)
12	96	Sequence Number (optional)
16	128	Routing (optional, variable length)

The original GRE RFC defined further fields in the packet header which became obsolete in the current standard:

C (1 bit)

Checksum bit. Set to 1 if a checksum is present.

R (1 bit)

Routing Bit. Set to 1 if Routing and Offset information are present.

K (1 bit)

Key bit. Set to 1 if a key is present.

S (1 bit)

Sequence number bit. Set to 1 if a sequence number is present.

s (1 bit)

Strict source route bit.

Recur (3 bits)

Recursion control bits.

Flags (5 bits)

Reserved for future use, set to 0.

Version (3 bits)

Set to 0.

Protocol Type (16 bits)

Indicates theether protocol type of the encapsulated payload.

Checksum (16 bits)

Present if theC bit is set; contains the checksum for the GRE header and payload.

Offset (16 bits)

Present ifR bit orC bit is set; contains valid information, only ifR bit is set. An offset field indicating the offset within theRouting field to the active source route entry.

Key (32 bits)

Present if theK bit is set; contains an application-specific key value.

Sequence Number (32 bits)

Present if theS bit is set; contains a sequence number for the GRE packet.

Routing (variable)

Present ifR bit is set; contains a list of source route entries, therefore is of variable length.

ThePoint-to-Point Tunneling Protocol (PPTP)^[8] uses a variant GRE packet header structure, represented below. PPTP creates a GRE tunnel through which the PPTP GRE packets are sent.

PPTP GRE header format

Offsets	Octet	0								1								2								3
Octet	Bit	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
0	0	C	R	K	S	s	Recur			A	Flags				Version			Protocol Type
4	32	Key Payload Length																Key Call ID
8	64	Sequence Number (optional)
12	96	Acknowledgement Number (optional)

C (1 bit)

Checksum bit. For PPTP GRE packets, this is set to 0.

R (1 bit)

Routing bit. For PPTP GRE packets, this is set to 0.

K (1 bit)

Key bit. For PPTP GRE packets, this is set to 1. (All PPTP GRE packets carry a key.)

S (1 bit)

Sequence number bit. Set to 1 if a sequence number is supplied, indicating a PPTP GRE data packet.

s (1 bit)

Strict source route bit. For PPTP GRE packets, this is set to 0.

Recur (3 bits)

Recursion control bits. For PPTP GRE packets, these are set to 0.

A (1 bit)

Acknowledgment number present. Set to 1 if an acknowledgment number is supplied, indicating a PPTP GRE acknowledgment packet.

Flags (4 bits)

Flag bits. For PPTP GRE packets, these are set to 0.

Version (3 bits)

GRE Version number. For PPTP GRE packets, this is set to 1.

Protocol Type (16 bits)

For PPTP GRE packets, this is set to hex 880B.

Key Payload Length (16 bits)

Contains the size of the payload, not including the GRE header.

Key Call ID (16 bits)

Contains the Peer's Call ID for the session to which the packet belongs.

Sequence Number (32 bits)

Present if the S bit is set; contains the GRE payload sequence number.

Acknowledgement Number (32 bits)

Present if the A bit is set; contains the sequence number of the highest GRE payload packet received by the sender.

• RFC 1701:Generic Routing Encapsulation (GRE) (informational)
• RFC 1702:Generic Routing Encapsulation over IPv4 networks (informational)
• RFC 2637:Point to Point Tunneling Protocol (informational)
• RFC 2784:Generic Routing Encapsulation (GRE) (proposed standard, updated by RFC 2890)
• RFC 2890:Key and Sequence Number Extensions to GRE (proposed standard)
• RFC 8086:GRE-in-UDP Encapsulation (proposed standard)

• Network Virtualization using Generic Routing Encapsulation - carries L2 packets over GRE
• GPRS Tunnelling Protocol - GTP-U is similar to GRE and used in cellular networks

Wikimedia Commons has media related toGeneric Routing Encapsulation.

• Generic Routing Encapsulation, Subprotocol homepage at Cisco
• Generic Routing EncapsulationArchived 2018-12-29 at theWayback Machine, Entry in Cisco DocWiki (formerly known as the "Internetworking Technology Handbook")

• Tunneling protocols
• Cisco Systems

• Articles with short description
• Short description is different from Wikidata
• Commons category link from Wikidata
• Webarchive template wayback links