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Generic Routing Encapsulation (GRE) and IP-in-IP (IPIP) are two rather similar tunneling mechanisms which are often confused. In this article, we'll examine the operation of each, how they differ, and when each should be used.

IPIP

IP-in-IP encapsulation is exactly what it sounds like: one IP packet encapsulated inside another. The protocol fieldof the outer header is set to 4 for IPv4 or 41 for IPv6.

ipip_encapsulation.png

Here's a sample capture of IPv4-in-IPv4, and here's IPv6-in-IPv4. All combinations of IPv4/IPv6 encapsulation are technically possible, though not all may be supported on a given platform. Three IPIP encapsulation methods are possible on IOS: IPv4/IPv4, IPv6/IPv6, and IPv6/IPv4.

Router(config)# interface tun0
Router(config-if)# tunnel mode ?
  aurp    AURP TunnelTalk AppleTalk encapsulation
  cayman  Cayman TunnelTalk AppleTalk encapsulation
  dvmrp   DVMRP multicast tunnel
  eon     EON compatible CLNS tunnel
  gre     generic route encapsulation protocol
  ipip    IP over IP encapsulation
  ipsec   IPSec tunnel encapsulation
  iptalk  Apple IPTalk encapsulation
  ipv6    Generic packet tunneling in IPv6
  ipv6ip  IPv6 over IP encapsulation
  mpls    MPLS encapsulations
  nos     IP over IP encapsulation (KA9Q/NOS compatible)
  rbscp   RBSCP in IP tunnel

GRE

GRE (defined in RFC 2784 and updated by RFC 2890) goes a step further than IP-in-IP, adding an additional header of its own between the inside and outside IP headers.

gre_encapsulation.png

The GRE header is variable in length, from 4 to 16 bytes, depending on which optional features have been enabled.

gre_header.png

  • C, K, and S: Bit flags which are set to one if the checksum, key, and sequence number fields are present, respectively
  • Ver: GRE version number (zero)
  • Protocol: Ethertype of the encapsulated protocol
  • Checksum: Packet checksum (optional)
  • Key: Tunnel key (optional)
  • Sequence Number: GRE sequence number (optional)

Here's a sample capture of GRE in action. Note that GRE can theoretically encapsulate any layer three protocol with a valid Ether type, unlike IPIP, which can only encapsulate IP.

GRE can be encapsulated by either IPv4 or IPv6 on IOS. (The multipoint option is used for Dynamic Multipoint VPN (DMVPN).)

Router(config)# interface tun0
Router(config-if)# tunnel mode gre ?
  ip          over IP
  ipv6        over IPv6
  multipoint  over IP (multipoint)

By default, only a minimal header of four bytes will be included. Additional GRE options can be turned on and off independently of one another:

Router(config-if)# tunnel ?
  bandwidth           Set tunnel bandwidth informational parameter
  checksum            enable end to end checksumming of packets
  destination         destination of tunnel
  flow                flow options
  key                 security or selector key
  mode                tunnel encapsulation method
  mpls                MPLS tunnel commands
  path-mtu-discovery  Enable Path MTU Discovery on tunnel
  protection          Enable tunnel protection
  rbscp               Set tunnel RBSCP parameters
  route-via           Select subset of routes for tunnel transport
  sequence-datagrams  drop datagrams arriving out of order
  source              source of tunnel packets
  tos                 set type of service byte
  ttl                 set time to live
  udlr                associate tunnel with unidirectional interface
  vrf                 set tunnel vrf membership

To summarize, GRE can:

  • Encapsulate any layer three protocol (versus just IP)
  • Add an additional checksum (which isn't useful for TCP/IPv4)
  • Specify a tunnel key
  • Enforce packet sequencing

Of course, these features come at a cost of additional overhead; in cases where the extra capabilities of GRE aren't needed, IPIP will do just fine.

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