Chapter 7: Understanding CEF in an MPLS VPN Environment

Cisco Press

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Engine 3 only supports per-destination load sharing as an ingress card. For IP packets, the line card hardware can do a hash on the source and destination IP addresses to determine the egress interface. If an Engine 3 port is a VRF interface, the line card's software instead of the hardware does the load-sharing hash. Therefore, the software selects the egress interface on a round-robin basis and programs the hardware in this case.

Originally, on a Cisco Catalyst 6500 with a SUP720, load sharing was available only in the IP-to-label and the label-to-IP directions for MPLS VPN. On a Catalyst 6500 with a SUP720 or a Cisco 7600 router that serves as a P router, the mpls load-balance per-label command allows use of the incoming label for load sharing. With this command, load sharing occurs based on the topmost incoming label in the stack that the packet arrives with. So, for example, if the outgoing path is an equal-cost path, load sharing will occur using the top IGP label with this feature enabled.


This chapter reviews some of common CEF issues seen across various Cisco platforms. However, this is not an exhaustive examination of CEF troubleshooting in an MPLS VPN environment across all Cisco platforms. This chapter demonstrates some key elements that you should examine when troubleshooting a difficult MPLS VPN problem regarding CEF. It also illustrates that sometimes the basic issue is that the routes are not in the routing table in the beginning. Hence, CEF or the LFIB will not know about the multiple paths. This chapter also introduces some of the BGP and MPLS features to allow multiple paths to be known through CEF. However, many features are available to gain the desired results.


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Copyright © 2007 Pearson Education. All rights reserved.


Copyright © 2007 IDG Communications, Inc.

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