Figure 30: GRE Encapsulation
In the new IP packet, new payload is similar to the original IP packet. Additionally, the new IP header (New
IP Hdr) is similar to the tunnel IP header which in turn is similar to the transport header.
Note
When a GRE tunnel endpoint decapsulates a GRE packet, it further forwards the packet based on the payload
type. For example, if the payload is a labeled packet then the packet is forwarded based on the virtual circuit
(VC) label or the VPN label for L2VPN and L3VPN respectively.
L2VPN over GRE Restrictions
Some of the restrictions that you must consider while configuring L2VPN over GRE:
• For VPLS flow-based load balancing scenario, the GRE tunnel is pinned down to outgoing path based
on tunnel source or destination cyclic redundancy check (CRC). Unicast and flood traffic always takes
the same physical path for a given GRE tunnel.
• Ingress attachment circuit must be an ASR 9000 Enhanced Ethernet Line Card for L2VPN over GRE.
Additionally, GRE tunnel destination should be reachable only on an ASR 9000 Enhanced Ethernet Line
Card.
• The L2VPN over GRE feature is not supported on the ASR 9000 Ethernet Line Card or Cisco ASR 9000
Series SPA Interface Processor-700 line cards as the ingress attachment circuit and GRE destination is
reachable over GRE.
• Pseudowire over TE over GRE scenario is not supported.
• Preferred Path Limitations:
• When you configure GRE as a preferred path, egress features are not supported under the GRE
tunnel (Egress ACL).
• VCCV ping or traceroute are not supported for preferred path.
• Preferred path is supported only for pseudowires configured in a provider egde (PE) to PE topology.
GRE Deployment Scenarios
In an L2VPN network, you can deploy GRE in the following scenarios:
• Configuring GRE tunnel between provider edge (PE) to PE routers
• Configuring GRE tunnel between P to P routers
• Configuring GRE tunnel between P to PE routers
The following diagrams depict the various scenarios:
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
225
Implementing Multipoint Layer 2 Services
L2VPN over GRE Restrictions
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