Vpnv4 Forwarding Using Gre Tunnels; Ingress Of Encapsulation Router; Egress Of Encapsulation Router; Ingress Of Decapsulation Router - Cisco CRS Configuration Manual
Ios xr virtual private network
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Implementing MPLS Layer 3 VPNs
• Carrier Supporting Carrier (CsC) or Inter-AS is not supported.
• GRE-based L3VPN does not interwork with MPLS or IP VPNs.
• GRE tunnel is supported only as a core link(PE-PE, PE-P, P-P, P-PE). A PE-CE (edge) link is not
• VPNv6 forwarding using GRE tunnels is not supported.
VPNv4 Forwarding Using GRE Tunnels
This section describes the working of VPNv4 forwarding over GRE tunnels. The following description assumes
that GRE is used only as a core link between the encapsulation and decapsulation provider edge (PE) routers
that are connected to one or more customer edge (CE) routers.
Ingress of Encapsulation Router
On receiving prefixes from the CE routers, Border Gateway Protocol (BGP) assigns the VPN label to the
prefixes that need to be exported. These VPN prefixes are then forwarded to the Forwarding Information Base
(FIB) using the Route Information Base (RIB) or the label switched database (LSD). The FIB then populates
the prefix in the appropriate VRF table. The FIB also populates the label in the global label table. Using BGP,
the prefixes are then relayed to the remote PE router (decapsulation router).
Egress of Encapsulation Router
The forwarding behavior on egress of the encapsulation PE router is similar to the MPLS VPN label imposition.
Regardless of whether the VPN label imposition is performed on the ingress or egress side, the GRE tunnel
forwards a packet that has an associated label. This labeled packet is then encapsulated with a GRE header
and forwarded based on the IP header.
Ingress of Decapsulation Router
The decapsulation PE router learns the VPN prefixes and label information from the remote encapsulation
PE router using BGP. The next-hop information for the VPN prefix is the address of the GRE tunnel interface
connecting the two PE routers. BGP downloads these prefixes to the RIB. The RIB downloads the routes to
the FIB and the FIB installs the routes in the hardware.
Egress of Decapsulation Router
The egress forwarding behavior on the decapsulation PE router is similar to VPN disposition and forwarding,
based on the protocol type of the inner payload.
Carrier Supporting Carrier Support for L3VPN
This section provides conceptual information about MPLS VPN Carrier Supporting Carrier (CSC) functionality
and includes the following topics:
•
supported.
CSC Prerequisites
Cisco IOS XR Virtual Private Network Configuration Guide for the Cisco CRS Router, Release 6.1.x
Carrier Supporting Carrier Support for L3VPN
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