Generic Routing Encapsulation
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942
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OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
Alcatel-Lucent
SUMMARY
GRE tunnels are a flexible and powerful tool on any Router for offering a VPN
service. Contexts and interfaces are used in combination with GRE tunneling to
create a VPN service complete with private addressing, routing, user
authentication, debugging, and logging.
• GRE tunnels may also be used by providers who wish to offer a VPN service
before transitioning to MPLS.
• GRE protocol is defined in RFC-2784
• Provides a means of encapsulating IP and non IP packets inside GRE header and
transport the payload over the GRE tunnel.
• GRE protocol header size (minimum without any options) is 4 bytes.
• GRE header format is as follows:
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| Reserved0 = 0 (13 bits) | Ver=0 (2 bits) | Protocol (16bits) |
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• GRE uses the ethernet protocol identifiers (from RFC-1700) to identify the type of
protocol packet that is being tunnelled.
• GRE packet is encapsulated using an outer IP header.
• Outer IP header’s protocol value = 47
• Support for GRE keepalive feature to monitor tunnel state.
ALCATEL-LUCENT SPECIFIC OVERVIEW
• The source IP address of the tunnel must be of either a loopback interface or one
of the physical interfaces.
Note: Non IP Packets are not supported in the standard release. But it is available as a part
of the component upgrade.
• By default, when a tunnel is configured for a destination address, the mode is
GRE.
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