Sending Mpls Labels With Routes; Generic Routing Encapsulation Support For L3Vpn; Gre Restriction For L3Vpn - Cisco CRS Configuration Manual

Generic Routing Encapsulation Support for L3VPN

• Open messages—After a router establishes a TCP connection with a neighboring router, the routers
• Update messages—When a router has a new, changed, or broken route, it sends an update message to
• Keepalive messages—Routers exchange keepalive messages to determine if a neighboring router is still
• Notification messages—When a router detects an error, it sends a notification message.

Sending MPLS Labels with Routes

When BGP (eBGP and iBGP) distributes a route, it can also distribute an MPLS label that is mapped to that
route. The MPLS label mapping information for the route is carried in the BGP update message that contains
the information about the route. If the next hop is not changed, the label is preserved.
When you issue the show bgp neighbors ip-address command on both BGP routers, the routers advertise to
each other that they can then send MPLS labels with the routes. If the routers successfully negotiate their
ability to send MPLS labels, the routers add MPLS labels to all outgoing BGP updates.
Generic Routing Encapsulation Support for L3VPN
Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate many types of packets to
enable data transmission using a tunnel. The GRE tunneling protocol enables:
• High assurance Internet Protocol encryptor (HAIPE) devices for encryption over the public Internet and
• Service providers (that do not run MPLS in their core network) to provide VPN services along with the
GRE is used with IP to create a virtual point-to-point link to routers at remote points in a network. For detailed
information about configuring GRE tunnel interfaces, see the <module-name> module of the Cisco IOS XR
Interfaces and Hardware Components Configuration Guide.
For a PE to PE (core) link, enable LDP (with implicit null) on the GRE interfaces for L3VPN.
Note

GRE Restriction for L3VPN

The following restrictions are applicable to L3VPN forwarding over GRE:
Cisco IOS XR Virtual Private Network Configuration Guide for the Cisco CRS Router, Release 6.1.x
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exchange open messages. This message contains the number of the autonomous system to which the
router belongs and the IP address of the router that sent the message.
the neighboring router. This message contains the NLRI, which lists the IP addresses of the usable routes.
The update message includes any routes that are no longer usable. The update message also includes
path attributes and the lengths of both the usable and unusable paths. Labels for VPN-IPv4 routes are
encoded in the update message, as specified in RFC 2858. The labels for the IPv4 routes are encoded
in the update message, as specified in RFC 3107.
available to exchange routing information. The router sends these messages at regular intervals. (Sixty
seconds is the default for Cisco routers.) The keepalive message does not contain routing data; it contains
only a message header.
nonsecure connections.
security services.
Implementing MPLS Layer 3 VPNs