Generic Routing Encapsulation Support For L2Vpn; Ethernet Over Mpls Forwarding Using Gre Tunnels - Cisco CRS Configuration Manual

Generic Routing Encapsulation Support for L2VPN

• Currently, preferred tunnel path configuration applies only to MPLS encapsulation.
Note
• The fallback enable option is supported.
Generic Routing Encapsulation Support for L2VPN
Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate many types of packets to
enable data transmission using a tunnel.

Ethernet over MPLS Forwarding Using GRE Tunnels

This section describes the working of the Ethernet over MPLS (EoMPLS) over GRE tunnels. The following
description assumes that the GRE tunnels connect two PE routers, and Layer 2 circuits exist between the PE
and CE routers.
Ingress of Encapsulation Router
To enable L2VPN forwarding over GRE tunnels, the targeted Label Distribution Protocol (LDP) neighbor
session (established over the GRE tunnel interface) enables exchanging virtual circuit labels between the PE
routers. LDP also installs an implicit null label to be used across the GRE tunnels. The implicit null label is
a label with special semantics that an LDP can bind to an address prefix. The Layer 2 forwarding tracks the
GRE tunnel or addresses, depending on the GRE tunnel.
Egress of Encapsulation Router
On the egress side of the encapsulation PE router, the following events occur:
• The Layer 2 packet passes through the regular L2VPN processing based on the local label imposed by
• The Layer 2 packet is encapsulated with the GRE header and outer IP header.
• The packet is transmitted based on the outer IP header information.
Ingress of Decapsulation Router
When the decapsulation router (remote PE) receives the GRE encapsulated Layer 2 packet, the following
events occur:
• The packet is processed based on the outer IP header information.
• The packet is decapsulated to retrieve the inner LDP packet.
Further processing of the packet is based on the Layer 2 payload. The packet is handed over to the egress line
card (that hosts the Layer 2 circuit) based on the VC label forwarding information programmed by the Layer
2 FIB.
Egress of Decapsulation Router
On the egress side of the decapsulation router, the following events occur:
Cisco IOS XR Virtual Private Network Configuration Guide for the Cisco CRS Router, Release 6.1.x
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the ingress side.
Implementing Point to Point Layer 2 services