Implementing Cisco Express ForwardingCisco Express Forwarding (CEF) is an advanced, Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns, such as the Internet, on networks characterized by intensive web-based applications, or interactive sessions. CEF is an inherent feature and the users need not perform any configuration to enable it. If required, the users can change the default route purge delay and static routes. Cisco NCS 5500 Series Routers supports only single stage forwarding. Show
ComponentsCisco IOS XR software CEF always operates in CEF mode with two distinct components:
The following features are supported for CEF on Cisco IOS XR software:
CEF Benefits
The following CEF forwarding tables are maintained in Cisco IOS XR software:
Verifying CEFTo view the details of the IPv4 or IPv6 CEF tables, use the following commands:
Displays the IPv4 or IPv6 CEF table. The next hop and forwarding interface are displayed for each prefix. The output of the show cef command varies by location.
Displays the details of the IPv4 or IPv6 CEF table.
Displays detailed adjacency information, including Layer 2 information for each interface. The output of the show adjacency command varies by location. Unicast Reverse Path ForwardingConfiguration of Unicast IPv4 and IPv6 Reverse Path Forwarding (uRPF) enables a router to verify the reachability of the source address in packets being forwarded. Configuring uRPF, both strict and loose modes, helps to mitigate problems caused by the introduction of spoofed IP source addresses into a network. Configuration of uRPF discards IP packets that lack a verifiable IP source address after a reverse lookup in the CEF table. When strict uRPF is enabled, the source address of the packet is checked in the FIB. If the packet is received on the same interface that would be used to forward the traffic to the source of the packet, the packet passes the check and is further processed. Otherwise, the packet is dropped. Configure strict uRPF only where there is natural or configured symmetry. Internal interfaces are likely to have a routing asymmetry, that is, multiple routes to the source of a packet. Therefore, you should not implement strict uRPF on interfaces that are internal to the network. Implementation of strict mode uRPF requires maintenance of a uRPF interfaces list for the prefixes. The list contains only the interfaces configured with strict mode uRPF. The interfaces are provided by the prefix path. The uRPF interface list is shared among the prefixes wherever possible. When loose uRPF is enabled, the source address of the packet is checked in the FIB. If the source address exists and matches a valid forwarding entry, the packet passes the check and is further processed. Otherwise, the packet is dropped.
Loose and strict uRPF supports two options: allow self-ping and allow default. The allow self-ping option allows the source of the packet to ping itself. The allow default option allows the lookup result to match a default routing entry. When the allow default option is enabled with the strict mode of the uRPF, the packet is processed further only if it arrives through the default interface. Restrictions Consider the following restrictions when you configure uRPF:
 In Figure 1, uRPF is enabled on Router R2 and R2 has the following FIB table entries:
R2 allows the packet from Host 1 to be routed because Host 1 subnet is available in R2's FIB table. However, R2 does not allow Host 3 to be routed because Host 3 subnet is not available in R2's FIB table. If strict uRPF is enabled on R2, then the source address 10.1.1.1/24 should be reachable through the same interface from which it is received. If loose uRPF is enabled on R2, then it is not mandatory that the source address 10.1.1.1/24 be reachable through the same interface from which it is received. The only criteria for a packet to be forwarded is that the host address should be present in R2's FIB table. Configure Unicast Reverse Path ForwardingConfiguring Unicast Reverse Path Forwarding (uRPF) enables a router to verify the reachability of the source address of packets being forwarded. If the source IP address is not valid, the packet is discarded. This capability can limit the appearance of spoofed addresses in a network. To configure uRPF on a hardware module, use the following steps:
ConfigurationVerificationUse the following command to check the uRPF status: Per-Flow Load BalancingThe system inherently supports the 7-tuple hash algorithm. Load balancing describes the functionality in a router that distributes packets across multiple links based on Layer 3 (network layer) and Layer 4 (transport layer) routing information. If the router discovers multiple paths to a destination, the routing table is updated with multiple entries for that destination. Per-flow load balancing performs these functions:
Load balancing decisions are taken based on a packet header, type of load balancing, type of scenario and platform specifics as follows:
The following tables includes detailed list of options, list of scenarios, and headers fields to specify how ECMP or LAG load balancing is done. Note:
Per-Destination Load BalancingPer destination load balancing is used for packets that transit over a recursive MPLS path (for example, learned through BGP 3107). Per-destination load balancing means the router distributes the packets based on the destination of the route. Given two paths to the same network, all packets for destination1 on that network go over the first path, all packets for destination2 on that network go over the second path, and so on. This preserves packet order, with potential unequal usage of the links. If one host receives the majority of the traffic all packets use one link, which leaves bandwidth on other links unused. A larger number of destination addresses leads to more equally used links. Configuring Static RouteRouters forward packets using either route information from route table entries that you manually configure or the route information that is calculated using dynamic routing algorithms. Static routes, which define explicit paths between two routers, cannot be automatically updated; you must manually reconfigure static routes when network changes occur. Static routes use less bandwidth than dynamic routes. Use static routes where network traffic is predictable and where the network design is simple. You should not use static routes in large, constantly changing networks because static routes cannot react to network changes. Most networks use dynamic routes to communicate between routers but might have one or two static routes configured for special cases. Static routes are also useful for specifying a gateway of last resort (a default router to which all unroutable packets are sent). Configuration ExampleCreate a static route between Router A and B over a HundredGigE interface. The destination IP address is 203.0.1.2/32 and the next hop address is 1.0.0.2. Running ConfigurationVerificationVerify that the Next Hop Flags fields indicate COMPLETE for accurate functioning of the configuration. The database, such as LPM, EXT-TCAM, and LEM, in which a prefix is updated is also provided through the output. Therefore, you can efficiently manage your network resources because you can understand the scaling of prefixes. You can also understand why a particular IP address configuration for a device fails and thereby debug easily. Associated Commands
BGP Attributes DownloadThe BGP Attributes Download feature enables you to display the installed BGP attributes in CEF.
VerificationAssociated Commands
Proactive Address Resolution Protocol and Neighbor DiscoveryWhen CEF installs a route for which there is no layer 2 adjacency information, CEF creates an incomplete layer 3 next-hop and programs it on the hardware. Because of this incomplete programming, the first packet will be forwarded to the software forwarding path. The software forwarding in turn strips off the layer 2 header from the packet and forwards it to ARP (Address Resolution Protocol) or ND (Neighbor Discovery) in order to resolve the layer 2 adjacency information. In such a packet, if there is feature specific information present in the layer 2 header, the software forwarding path fails to strip off the layer 2 header completely and thus ARP or ND is unable to resolve the missing layer 2 adjacency information and thereby this results in traffic being dropped. Proactive ARP and ND feature solves the above problem by ensuring that CEF proactively triggers ARP or ND in order to resolve the missing layer 2 adjacency information, retrying every 15 seconds until the next-hop information is resolved. Thus, when you configure a static route which has an incomplete next-hop information, this feature automatically triggers ARP or ND resolution. ConfigurationRunning ConfigWhat is used to pre populate the adjacency table on Cisco devices that use CEF to process?This is accomplished with the command R1(config)#ipnat inside source list 1 pool NAT-POOL2.
What is used to pre populate the adjacency table on Cisco devices that use CEF to process packets the FIB the DSP the routing table the ARP table navigation bar?The adjacency table is pre-populatedby the ARP table and the FIB is pre-populated by the routing table.
What is adjacency table in CEF?Adjacency tables—Network nodes in the network are said to be adjacent if they can reach each other with a single hop across a link layer. In addition to the FIB, CEF uses adjacency tables to store Layer 2 addressing information. The adjacency table maintains Layer 2 addresses for all FIB entries.
What is FIB and adjacency table?FIB is basically mirror of RIB so assume it contains mirror of routing table. The FIB maintains next-hop address information based on the information in the IP routing table. The other part of the process is the adjacency table, the adjacency table maintains L2 next hop addresses for all FIB entries.
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What is used to pre populate the adjacency table on Cisco devices that use CEF to process packets the FIB the routing table the DSP the ARP table?
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