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Cisco Express Forwarding (CEF) is 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.
Although you can use CEF in any part of a network, it is designed for high-performance, highly resilient Layer 3 IP backbone switching.
CEF offers these benefits:
Cisco Express Forwarding (CEF) Layer 3 switching technology. CEF can also refer to central CEF mode, one of the two modes of CEF operation that enables a route processor to perform express forwarding.
Distributed CEF (dCEF) One of two modes of CEF operation that enables line cards to perform the express forwarding between port adapters.
Forwarding Information Base (FIB) A component of CEF. It is the lookup table the router uses to make destination-based switching decisions during CEF operation. It maintains a mirror image of the forwarding information stored in the IP routing table.
Gigabit Switch Router (GSR) Also called the Cisco 12000 series router.
line card-A general term for an interface processor that can be used in a line of Cisco products. For example, a VIP is a line card for the Cisco 7500 series router.
prefix-Indicates the network address portion of an IP address is the network versus the host. It is a combination of network and mask. The mask indicates which bits are the network bits. The prefix is in the format network/mask. For example, 10.0.0.0/16 means that the first 16 bits of the IP address are masked, making them the network bits and the remaining bits the host bits. In this example, the network number is 10.0.
route processor-A general term for a processor module on either the Cisco 7000 or Cisco 7500 router.
Route Processor (RP) Processor module on the Cisco 7000 series routers that contains the CPU, system software, and most of the memory components that are used in the router.
Route Switch Processor (RSP) Processor module used in the Cisco 7500 series routers that integrates the functions of the RP and the Switch Processor (SP). The SP acts as the administrator for all data bus activities on a Cisco 7000 series router.
Versatile Interface Processor (VIP) Interface card used in Cisco 7000 and Cisco 7500 series routers. The VIP provides multilayer switching and runs the Cisco IOS software. The most recent version of VIP is the VIP2.
Inter Process Communication (IPC) The mechanism that ensures synchronization of FIBs and adjacency tables on the RSP and line cards during dCEF mode.
Consider these items when implementing CEF in your network:
This feature is supported on these platforms:
CEF is supported on second-generation Versatile Interface Processor models VIP2-20, VIP2-40, and VIP2-50.
Depending on the router platform you are using for CEF, the exact hardware performing the switching might differ. Exactly where the switching occurs in the router depends on the router model and the hardware installed in the router. For example, on the Cisco 12000 series, all switching occurs on the line cards. On the Cisco 7500 series, packets can be switched concurrently by the RSP and VIP line cards. Each interface card runs its own express forwarding engine and maintains its own exact copy of the FIB table. Each card, independently, is able to forward packets, reducing the number of packets centrally switched by the route processor.
For the purpose of simplicity and consistency, discussions throughout this document use route processor to indicate the central processor and line card to indicate the interface processor or other line card.
For information about specific hardware requirements and compatibility, refer to the Cisco Product Catalog or the installation and configuration guide for the type of router you are using for CEF switching.
None
Cisco Express Forwarding (CEF) is 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.
Although you can use CEF in any part of a network, it is designed for high-performance, highly resilient Layer 3 IP backbone switching. For example, Figure 1 shows CEF being run on Cisco 12000 series Gigabit Switch Routers (GSRs) at aggregation points at the core of a network where traffic levels are dense and performance is critical.
In a typical high-capacity internet service provider environment, Cisco 12012 GSRs as aggregation devices at the core of the network support links to Cisco 7500 series routers or other feeder devices. CEF in these platforms at the network core provides the performance and scalability needed to respond to continued growth and steadily increasing network traffic. CEF is a distributed switching mechanism that scales linearly with the number of interface cards and bandwidth installed in the router.
Information conventionally stored in a route cache is stored in several data structures for CEF switching. The data structures provide optimized lookup for efficient packet forwarding. The two main components of CEF operation are the
CEF uses a Forwarding Information Base (FIB) to make IP destination prefix-based switching decisions. The FIB is conceptually similar to a routing table or information base. It maintains a mirror image of the forwarding information contained in the IP routing table. When routing or topology changes occur in the network, the IP routing table is updated, and those changes are reflected in the FIB. The FIB maintains next-hop address information based on the information in the IP routing table.
Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for route cache maintenance that is associated with earlier switching paths such as fast switching and optimum switching.
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 prepend Layer 2 addressing information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.
The adjacency table is populated as adjacencies are discovered. Each time an adjacency entry is created (such as through the ARP protocol), a link-layer header for that adjacent node is precomputed and stored in the adjacency table. Once a route is determined, it points to a next hop and corresponding adjacency entry. It is subsequently used for encapsulation during CEF switching of packets.
A route might have several paths to a destination prefix, such as when a router is configured for simultaneous load balancing and redundancy. For each resolved path, a pointer is added for the adjacency corresponding to the next-hop interface for that path. This mechanism is used for load balancing across several paths.
In addition to adjacencies associated with next-hop interfaces (host-route adjacencies), other types of adjacencies are used to expedite switching when certain exception conditions exist. When the prefix is defined, prefixes requiring exception processing are cached with one of the special adjacencies listed in Table 1.
| This adjacency type... | Receives this processing... |
|---|---|
Null adjacency | Packets destined for a Null0 interface are dropped. This can be used as an effective form of access filtering. |
Glean adjacency | When a router is connected directly to several hosts, the FIB table on the router maintains a prefix for the subnet rather than for the individual host prefixes. The subnet prefix point to a glean adjacency. When packets need to be forwarded to a specific host, the adjacency database is gleaned for the specific prefix. |
Punt adjacency | Features that require special handling or features that are not yet supported in conjunction with CEF switching paths are forwarded to the next switching layer for handling. Features that are not supported are forwarded to the next higher switching level. |
Discard adjacency | Packets are discarded. This type of adjacency occurs only on the Cisco 12000 series routers. |
Drop adjacency | Packets are dropped, but the prefix is checked. |
When a link-layer header is prepended to packets, FIB requires the prepend to point to an adjacency corresponding to the next hop. If an adjacency was created by FIB and not discovered through a mechanism, such as ARP, the Layer 2 addressing information is not known and the adjacency is considered incomplete. Once the Layer 2 information is known, the packet is forwarded to the route processor, and the adjacency is determined through ARP.
CEF currently supports ATM/AAL5snap, ATM/AAL5mux, ATM/AAL5nlpid, Frame Relay, Ethernet, FDDI, PPP, HDLC, and tunnels.
CEF can be enabled in one of two modes:
When CEF mode is enabled, the CEF FIB and adjacency tables reside on the route processor, and the route processor performs the express forwarding. You can use CEF mode when line cards are not available for CEF switching or when you need to use features not compatible with distributed CEF switching.
Figure 2 shows the relationship between the routing table, FIB, and adjacency table during CEF mode.
Figure 2 shows Cisco Catalyst switches forwarding traffic from workgroup LANs to a Cisco 7500 series router on the enterprise backbone running Cisco Express Forwarding. The route processor performs the express forwarding.
When distributed CEF (dCEF) is enabled, line cards, such as VIP line cards or Gigabit Switch Router (GSR) line cards, maintain an identical copy of the FIB and adjacency tables. The line cards perform the express forwarding between port adapters, relieving the RSP of involvement in the switching operation.
dCEF uses an Inter Process Communication (IPC) mechanism to ensure synchronization of FIBs and adjacency tables on the route processor and line cards.
Figure 3 shows the relationship between the route processor and line cards when dCEF mode is active.
In this Cisco 12000 series router the line cards perform the switching. In other routers where you can mix various types of cards in the same router, it is possible that not all of the cards you are using support CEF. When a line card that does not support CEF receives a packet, the line card forwards the packet to the next higher switching layer (the route processor) or forwards the packet to the next hop for processing. This structure allows legacy interface processors to exist in the router with newer interface processors.
1 . The Cisco 12000 series Gigabit Switch Routers operate only distributed CEF (dCEF mode).
2 . Distributed CEF switching cannot be configured on the same VIP card as distributed fast switching.
3 . Distributed CEF is not supported on Cisco 7200 series routers.
In addition to configuring CEF and dCEF, you can also configure these features:
This section describes the required and optional tasks for configuring CEF. The first task is required; all other tasks are optional.
Enable CEF when your router has interface processors that do not support CEF.
To enable or disable CEF, perform one of the following tasks in global configuration mode:
| Task | Command |
|---|---|
ip cef switch | |
Disable standard CEF operation | no ip cef switch |
Enable dCEF when you want your line cards to perform express forwarding so that the route processor can handle routing protocols or switch packets from legacy interface processors.
To enable or disable dCEF operation, perform one of the following tasks in global configuration mode:
| Task | Command |
|---|---|
ip cef distributed switch | |
Disable dCEF operation. | no ip cef distributed switch |
When you enable CEF or dCEF globally, all interfaces that support CEF are enabled by default. If you want to turn off CEF or dCEF on a particular interface, you can do so.
You might want to disable CEF or dCEF on a particular interface because that interface is configured with a feature that CEF or dCEF does not support. For example, policy routing and CEF cannot be used together. You might want one interface to support policy routing while the other interfaces support CEF. In this case, you would enable CEF globally, but disable CEF on the interface configured for policy routing, enabling all but one interface to express forward.
To disable CEF or dCEF on an interface, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
Disable CEF operation on the interface. |
When you disable CEF or dCEF, Cisco IOS software switches packets using the next fastest switching path. In the case of dCEF, the next fastest switching path is CEF on the route processor.
If you have disabled CEF or dCEF operation on an interface and want to reenable it, you can do so by using the ip route-cache cef command from interface configuration mode.
CEF load balancing is based on a combination of source and destination packet information; it allows you to optimize resources by distributing traffic over multiple paths for transferring data to a destination. You can configure load balancing on a per-destination or per-packet basis. Load balancing decisions are made on the outbound interface. When you configure load balancing, configure it on outbound interfaces.
These sections describe how to configure each type of load balancing:
Per-destination load balancing allows the router to use multiple paths to achieve load sharing. Packets for a given source-destination host pair are guaranteed to take the same path, even if multiple paths are available. Traffic destined for different pairs tend to take different paths. Per-destination load balancing is enabled by default when you enable CEF, and is the load balancing method of choice for most situations.
Because per-destination load balancing depends on the statistical distribution of traffic, load sharing becomes more effective as the number of source-destination pairs increase.
You can use per-destination load balancing to ensure that packets for a given host pair arrive in order. All packets for a certain host pair are routed over the same link (or links).
Per-destination load balancing is enabled by default when you enable CEF. To use per-destination load balancing, you do not perform any additional tasks once you enable CEF.
Typically, you would disable per-destination load balancing when you want to enable per-packet load balancing.
To disable per-destination load balancing, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
Per-packet load balancing allows the router to send successive data packets over paths without regard to individual hosts or user sessions. It uses the round-robin method to determine which path each packet takes to the destination. Per-packet load balancing ensures balancing over multiple links.
Path utilization with per-packet load balancing is good, but packets for a given source-destination host pair might take different paths. Per-packet load balancing could introduce reordering of packets. This type of load balancing would be inappropriate for certain types of data traffic (such as voice traffic over IP) that depend on packets arriving at the destination in sequence.
Use per-packet load balancing to help ensure that a path for a single source-destination pair does not get overloaded. If the bulk of the data passing through parallel links is for a single pair, per-destination load balancing will overload a single link while other links have very little traffic. Enabling per-packet load balancing allows you to use alternate paths to the same busy destination.
To enable per-packet load balancing, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
You might want to collect statistics to better understand CEF patterns in your network. For example, you might want to collect information such as the number of packets and bytes switched to a destination or the number of packets switched through a destination.
You can configure network accounting for CEF by performing these optional tasks:
To collect network accounting information for CEF, perform the following tasks in global configuration mode:
When you enable network accounting for CEF from global configuration mode, accounting information is collected at the route processor when CEF mode is enabled. When network accounting is enabled for distributed CEF (dCEF), information is collected at the line cards.
You can then view the collected accounting information. To do so, perform the following task in EXEC mode:
| Task | Command |
|---|---|
Display the collected accounting information. |
CEF supports distributed tunnel switching, such as GRE tunnels. Distributed tunnel switching is enabled automatically when you enable CEF or dCEF. You do not perform any additional tasks to enable distributed tunnel switching once you enable CEF or dCEF.
This section documents new or modified commands. All other commands used with this feature are documented in the Cisco IOS Release 11.2 command references.
To clear the Cisco Express Forwarding (CEF) adjacency table, use the clear adjacency EXEC command.
clear adjacencyThis command has no arguments or keywords.
EXEC
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
When you issue this command, entries in the adjacency table that resides on the route processor are removed and then repopulated. During repopulation, Layer 2 next hop information is reevaluated.
With dCEF mode, the adjacency tables that reside on line cards are always synchronized to the adjacency table that resides on the route processor. Therefore, clearing the adjacency table on the route processor using the clear adjacency command also clears the adjacency tables on the line cards; all changes are propagated to the line cards.
To clear Cisco Express Forwarding (CEF) information from line cards, use the clear cef linecard EXEC command.
clear cef linecard [slot-number] [adjacency | interface | prefix]
slot-number | (Optional) Line card slot number to clear. When you omit this argument, all line card slots are cleared. |
adjacency | (Optional) Clears line card adjacency tables and rebuilds adjacency for the specified line card. |
interface | (Optional) Clears line card interface information and re-creates the interface information for the specified line card. |
prefix | (Optional) Clears line card prefix tables and starts rebuilding the FIB table. |
EXEC
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
This command is available only on routers with line cards. This command clears CEF information only on the line cards; CEF information on the route processor is not affected.
Once you clear CEF information from line cards, the corresponding information from the route processor is propagated to the line cards. The Inter Process Communication (IPC) ensures that CEF information on the route processor matches the CEF information on the line cards.
To clear Cisco Express Forwarding (CEF) counters by resetting the packet and byte count to zero (0), use the clear ip cef prefix-statistics EXEC command.
clear ip cef {network [mask] | *} prefix-statistics
network | Clears counters for a FIB entry specified by network. |
mask | (Optional) Clears counters for a FIB entry specified by network and mask. |
* | Clears counters for all FIB entries. |
EXEC
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
To enable Cisco Express Forwarding (CEF) on the route processor card, use the ip cef global configuration command. To disable CEF, use the no form of this command.
ip cef [distributed] switch
distributed | (Optional) Enables distributed CEF (dCEF) operation. Distributes CEF information to line cards. Line cards perform express forwarding. |
| On this platform... | The default is... |
|---|---|
Cisco 7000 series equipped with RSP7000 | CEF is not enabled. |
Cisco 7200 series | CEF is not enabled. |
Cisco 7500 series | CEF is not enabled. |
Cisco 12000 series Gigabit Switch Router | Distributed CEF is enabled. |
Global configuration
This command first appeared in Cisco IOS Release 11.1 CC.
This command is not available on the Cisco 12000 series GSR because that router series operates only in distributed CEF mode.
CEF is advanced Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with dynamic, topologically dispersed traffic patterns, such as those associated with Web-based applications and interactive sessions.
The following example enables standard CEF operation:
ip cef switch
The following example enables dCEF operation:
ip cef distributed switch
To enable network accounting of Cisco Express Forwarding (CEF), use the ip cef accounting global configuration command. To disable network accounting of CEF, use the no form of this command.
ip cef accounting [per-prefix] [non-recursive]
per-prefix | (Optional) Enables the collection of the number of packets and bytes express forwarded to a destination (or prefix). |
non-recursive | (Optional) Enables accounting through non-recursive prefixes. For prefixes with directly connected next hops, enables the collection of the number of packets and bytes express forwarded through a prefix. |
Accounting is disabled by default.
Global configuration
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
You might want to collect statistics to better understand CEF patterns in your network.
When you enable network accounting for CEF from global configuration mode, accounting information is collected at the route processor when CEF mode is enabled and at the line cards when dCEF mode is enabled.
You can then view the collected accounting information using the show ip cef command.
To enable load balancing for Cisco Express Forwarding (CEF), use the ip load-sharing interface configuration command. To disable load balancing for CEF, use the no form of this command.
ip load-sharing [per-packet] [per-destination]
per-packet | (Optional) Enables per-packet load balancing on the interface. |
per-destination | (Optional) Enables per-destination load balancing on the interface. |
Per-destination load balancing is enabled by default when you enable CEF.
Interface configuration
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
Per-packet load balancing allows the router to send data packets over successive equal-cost paths without regard to individual end hosts or user sessions. Path utilization is good, but packets destined for a given end host might take different paths and might arrive out of order.
Per-destination load balancing allows the router to use multiple, equal-cost paths to achieve load sharing. Packets for a given end host are guaranteed to take the same path, even if multiple, equal-cost paths are available. Traffic for different end hosts tend to take different paths.
The following example enables per-packet load balancing:
interface E0 ip load-sharing per-packet
The following example enables per-destination load balancing:
interface E0 ip load-sharing per-destination
This command has no arguments or keywords.
When standard CEF or dCEF operation is enabled globally, all interfaces that support CEF are enabled by default.
Interface configuration
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
Cisco Express Forwarding (CEF) is advanced Layer 3 switching technology for IP. CEF optimizes network performance and scalability for networks with dynamic, topologically dispersed traffic patterns, such as those associated with Web-based applications and interactive type sessions.
Because all interfaces that support CEF or dCEF are enabled by default when you enable standard CEF or dCEF operation globally, you use the no form of the command to turn off CEF operation on a particular interface.
You might want to disable CEF or dCEF on a particular interface because that interface is configured with a feature that CEF or dCEF does not support. For example, policy routing and CEF cannot be used together. You might want one interface to support policy routing while the other interfaces support CEF. In this case, you would turn on CEF globally, but turn off CEF on the interface configured for policy routing, enabling all but one interface to express forward.
When you disable CEF or dCEF, Cisco IOS software switches packets using the next fastest switching path. In the case of dCEF, the next fastest switching path is CEF on the route processor.
If you have disabled CEF or dCEF operation on an interface and want to reenable it, you can do so by using the ip route-cache cef command in interface configuration mode.
The following example enables CEF operation on the router (globally), but turns off CEF operation on Ethernet interface 0:
ip cef switch interface e0 no ip route-cache cef
The following example enables dCEF operation on the router (globally), but turns off CEF operation on Ethernet interface 0:
ip cef distributed switch interface e0 no ip route-cache cef
The following example reenables dCEF operation on Ethernet interface 0:
ip cef distributed switch interface e0 ip route-cache cef
interface
ip cef
To display Cisco Express Forwarding (CEF) adjacency table information, use the show adjacency EXEC command.
show adjacency [detail]
detail | (Optional) Displays detailed adjacency information, including Layer 2 information. |
EXEC
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
This command is available only on routers that have RP cards.
The following is sample output from the show adjacency detail command:
Router# show adjacency detail
Protocol Interface Address IP Tunnel0 point2point(3) (incomplete) 0 packets, 0 bytes FIB 00:02:45 IP Ethernet1/0/0 192.168.177.15(6) 0 packets, 0 bytes 0060837BEFA0 Protocol Interface Address 0060836FA7000800 ARP 03:59:44 igrp 622 00:04:14 IP Ethernet0/0 192.168.233.88(5) 0 packets, 0 bytes 0060837BEFA0 0060836FA7000800 ARP 03:59:36 IP FastEthernet2/0/0 172.16.1.106 (11) (incomplete) 0 packets, 0 bytes IP FastEthernet2/0/0 172.26.1.106 (11) (incomplete) 0 packets, 0 bytes
Table 2 describes the fields shown in the output.
| Field | Meaning |
|---|---|
Protocol | The routing protocol configured on the interface. |
Interface | The type of interface configured. |
Address | The address of the interface. |
Routing protocol | The method by which the adjacency was learned. |
Adjacent next hop | The MAC address of the adjacent router. |
Time stamp | The time left before the adjacency rolls out of the adjacency table. Once it rolls out, a packet must use the same next hop to the destination. |
To display which packets the line cards dropped or to display which packets were not express forwarded, use the show cef EXEC command.
show cef [drop] | [not-cef-switched]
drop | (Optional) Displays which packets were dropped by each line card. |
not-cef-switched | (Optional) Displays which packets were sent to a different switching path. |
EXEC
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
This command is available only on routers that have RP cards.
A line card might drop packets due to encapsulation failure, no route information, or no adjacency information.
A packet is sent to a different switching path because Cisco Express Forwarding (CEF) does not support the encapsulation or feature, the packet is destined for the router, or the packet has IP options, such as time stamp and record route. IP options are process switched.
The following is sample output from the show cef drop command:
Router# show cef drop
CEF Drop Statistics Slot Encap_fail Unresolved Unsupported No_route No_adj ChksumErr RP 4 89 0 4 0 0 1 0 0 0 0 0 0 2 0 0 5 0 0 5
Table 3 describes the fields shown in the output.
| Field | Meaning |
|---|---|
Slot | The slot number on which the packets were received. |
Encap_fail | Indicates the number of packets dropped after the limit was reached for incomplete packets with no adjacency route. |
Unresolved | Indicates the number of packets dropped because the route for the prefix was not resolved. |
Unsupported | Indicates the number of packets received for which the adjacency route information was dropped due to unsupported features. |
No_route | No route definition is included in the prefix table. |
No_adj | The prefix is resolved, but the adjacent route is not indicated. |
ChksumErr | Indicates the number of packets received with a checksum error. |
The following is sample output from the show cef not-cef-switched command:
Router# show cef not-cef-switched
CEF Packets passed on to next switching layer Slot No_adj No_encap Unsuppted Redirect Receive Bad_ttl Options Access RP 0 0 0 0 91584 0 0 0 1 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0
Table 4 describes the fields shown in the output.
| Field | Meaning |
|---|---|
No_adj | Indicates the number of packets sent to the line card to ARP for the adjacent route. |
No_encap | Number of encapsulated packets received. |
Unsupported Redirect | Number of packets with unsupported features and redirected to another switching layer or location for processing. |
show cef interface
show cef linecard
To display Cisco Express Forwarding (CEF) related interface information, use the show cef interface EXEC command.
show cef interface type number [detail]
type number | Interface type and number about which to display CEF-related information. |
detail | (Optional) Displays detailed CEF information for the specified interface type and number. |
EXEC
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
This command is available on routers that have RP cards and line cards.
The detail command displays more CEF-related information for the specified interface.
You can use this command to show the CEF state on an individual interface.
The following is sample output from the show cef interface detail command for Ethernet interface 0:
Router# show cef interface E0 detail
Ethernet1/0/0 is up (if_number 6) Internet address is 172.19.177.20/24 ICMP redirects are always sent Per-packet load balancing is disabled Inbound access list is 10 Outbound access list is not set Hardware idb is Ethernet1/0/0 Fast switching type 1, interface type 5 IP Distributed CEF switching enabled IP Feature CEF switching turbo vector Fast flags 0x4. ifindex 5(5) Slot 1 Slot unit 0 VC -1 Hardware transmit queue ptr 0x48001A00 (0x48001A00) >- debugging purposes Transmit limit accumulator 0x48001A02 (0x48001A02) IP MTU 1500
Table 5 describes the fields shown in the output.
| Field | Meaning |
|---|---|
interface type number is {up | down} | Indicates status of the interface. |
Internet address | Internet address of the interface |
ICMP packets are {always sent | never sent} | Indicates how packet forwarding is configured. |
Per-packet load balancing | Status of load balancing in use on the interface (enabled or disabled). |
Inbound access list {# | Not set} | Number of access lists defined for the interface. |
Outbound access list | Number of access lists defined for the interface. |
Hardware idb is type number | Interface type and number configured. |
Fast switching type | Used for troubleshooting; indicates switching mode in use. |
IP Distributed CEF switching {enabled | disabled} | Indicates the switching path used. |
Slot n Slot unit n | The slot number. |
Hardware transmit queue | Indicates the number of packets in the transmit queue. |
Transmit limit accumulator | Indicates the maximum number of packets allowed in the transmit queue. |
IP MTU | The value of the MTU size set on the interface. |
To display Cisco Express Forwarding (CEF) related interface information by line card, use the show cef linecard EXEC command.
show cef linecard [slot-number] [detail]
slot-number | (Optional) Slot number containing the line card about which to display CEF-related information. When you omit this argument, information about all line cards is displayed. |
detail | (Optional) Displays detailed CEF information for the specified line card. |
EXEC
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
This command is available only on routers that have RP cards.
When you omit the slot-number argument, information about all line cards is displayed. When you omit the slot-number argument and include the detail keyword, detailed information is displayed for all linecards. When you omit all keywords and arguments, the show cef linecard command displays important information about all line cards in table format.
The following is sample output from the show cef linecard command. The command displays information for the line cards.
Router# show cef linecard
CEF table version 115705, 45877 routes Slot CEF-ver MsgSent XdrSent Seq MaxSeq LowQ HighQ Flags 1 238 668 9641 616 616 0 0 up, sync 2 238 683 10782 619 629 0 0 up, sync
Table 6 describes the fields shown in the output.
| Field | Meaning |
|---|---|
CEF table version | The FIB table version. |
XdrSent | IPC information elements (xdrs) packed into IPC messages sent from the RP to the line card. |
MsgSent | Number of IPC messages sent. |
Seq | Sequence number for the line card. |
MaxSeq | Maximum sequence expected by the line card. |
LowQ/HighQ | Number of xdr elements in LowQ and HighQ. |
Flags | Indicates the status of the line card. Possible states are
|
The following is sample output from the show cef linecard detail command for the line card in slot number 2:
Router# show cef linecard 2 detail
CEF line card slot number 2, status up, sync, disabled line card CEF version number 238 Sequence number 616, Maximum sequence number expected 616 Send failed 0, Out Of Sequence 0 line card CEF reset 2, reloaded 2 92299/15/91 prefix/adjacency/interface elements queued 49641 elements packed in 668 messages(1341286 bytes) sent 0/0 xdr elements in LowQ/HighQ Input packets 0, bytes 0 <--- line card stats Output packets 0, bytes 0, drops 0
To display entries in the FIB that are unresolved or to display a summary of the FIB, use this form of the show ip cef EXEC command:
show ip cef [unresolved | summary]To display specific entries in the FIB based on IP address information, use this form of the show ip cef EXEC command:
show ip cef [network [mask [longer-prefix]]] [detail]To display specific entries in the FIB based on interface information, use this form of the show ip cef EXEC command:
show ip cef [type number] [detail]
unresolved | (Optional) Displays unresolved FIB entries. |
summary | (Optional) Displays a summary of the FIB. |
network | (Optional) Displays the FIB entry for the specified destination network. |
mask | (Optional) Displays the FIB entry for the specified destination network and mask. |
longer-prefix | (Optional) Displays FIB entries for all more specific destinations. |
detail | (Optional) Displays detailed FIB entry information. |
type number | (Optional) Interface type and number for which to display FIB entries. |
EXEC
This command first appeared to support the Cisco 12012 Gigabit Switch Router in Cisco IOS Release 11.2 GS, and first appeared with multiple platform support in Cisco IOS Release 11.1 CC.
The show ip cef command without any keywords or arguments shows a brief display of all FIB entries.
The show ip cef detail command shows detailed FIB entry information for all FIB entries.
The show ip cef internal command shows FIB load-sharing information for all FIB entries.
The following is sample output from the show ip cef unresolved command:
Router# show ip cef unresolved
IP Distributed CEF with switching (Table Version 136632) 45776 routes, 13 unresolved routes (0 old, 13 new) 45776 leaves, 2868 nodes, 8441480 bytes, 136632 inserts, 90856 invalidations 1 load sharing elements, 208 bytes, 1 references 1 CEF resets, 1 revisions of existing leaves refcounts: 527292 leaf, 465617 node 148.214.0.0/16, version 136622 0 packets, 0 bytes via 171.69.233.56, 0 dependencies, recursive unresolved 148.215.0.0/16, version 136623 0 packets, 0 bytes via 171.69.233.56, 0 dependencies, recursive unresolved 148.218.0.0/16, version 136624 0 packets, 0 bytes
The following is sample output from the show ip cef summary command:
Router# show ip cef summary
IP Distributed CEF with switching (Table Version 135165) 45788 routes, 0 reresolve, 4 unresolved routes (0 old, 4 new) 45788 leaves, 2868 nodes, 8442864 bytes, 135165 inserts, 89377 invalidations 0 load sharing elements, 0 bytes, 0 references 1 CEF resets, 0 revisions of existing leaves refcounts: 527870 leaf, 466167 node
The following is sample output from the show ip cef internal command; it shows load-sharing details for multiple paths to a prefix:
Router# show ip cef 192.168.1.0 internal
192.168.1.0/24, version 135490, per-destination sharing 0 packets, 0 bytes via 172.19.233.50, 0 dependencies, recursive <-- possible path 1 info traffic share 1, current path next hop 172.19.233.50, Ethernet0/0 via 172.19.233.50/32 valid adjacency via 172.19.233.49, 0 dependencies, recursive <-- possible path 2 info traffic share 1 next hop 172.19.233.49, Ethernet0/0 via 172.19.233.49/32 valid adjacency 0 packets, 0 bytes switched through the prefix Load distribution: 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 (refcount 1) ^ |.. how the load is distributed among the possible paths Hash OK Interface Address Packets 1 Y Ethernet0/0 172.19.233.50 0 2 Y Ethernet0/0 172.19.233.49 0 3 Y Ethernet0/0 172.19.233.50 0 4 Y Ethernet0/0 172.19.233.49 0 5 Y Ethernet0/0 172.19.233.50 0 6 Y Ethernet0/0 172.19.233.49 0 7 Y Ethernet0/0 172.19.233.50 0 8 Y Ethernet0/0 172.19.233.49 0 9 Y Ethernet0/0 172.19.233.50 0 10 Y Ethernet0/0 172.19.233.49 0 11 Y Ethernet0/0 172.19.233.50 0 12 Y Ethernet0/0 172.19.233.49 0 13 Y Ethernet0/0 172.19.233.50 0 14 Y Ethernet0/0 172.19.233.49 0 15 Y Ethernet0/0 172.19.233.50 0 16 Y Ethernet0/0 172.19.233.49 0
The following is sample output from the show ip cef detail command for Ethernet interface 0. It shows all the prefixes resolving through adjacency pointing to next hop Ethernet interface 0/0 and next-hop interface IP address 172.19.233.33.
Router# show ip cef e0/0 172.19.233.33 detail
IP Distributed CEF with switching (Table Version 136808) 45800 routes, 8 unresolved routes (0 old, 8 new) 45800 leaves, 2868 nodes, 8444360 bytes, 136808 inserts, 91008 invalidations 1 load sharing elements, 208 bytes, 1 references 1 CEF resets, 1 revisions of existing leaves refcounts: 527343 leaf, 465638 node 172.19.233.33/32, version 7417, cached adjacency 172.19.233.33 0 packets, 0 bytes, Adjacency-prefix via 172.19.233.33, Ethernet0/0, 0 dependencies next hop 172.19.233.33, Ethernet0/0 valid cached adjacency
The debug ip cef command is available for troubleshooting Cisco Express Forwarding (CEF) and distributed CEF.
Use the debug ip cef EXEC command for troubleshooting CEF.
debug ip cef {drops [access-list] | receive [access-list] | events [access-list] | prefix-ipc [access-list] | table [access-list]}and
debug ip cef {ipc | interface-ipc}
drops | Records dropped packets. |
access-list | Controls collection of debugging information from specified lists |
receive | Records packets that are not switched using information from the FIB table, but are received and sent to the next switching layer |
events | Records general CEF events. |
prefix-ipc | Records updates related to IP prefix information. Possible updates include: · Debugging of IP routing updates in a line card · Reloading of a line card with a new table · Route update from the route processor to the line card has exceeded the maximum number of routes · Control messages related to FIB table prefixes |
table | Produces a table showing events related to the FIB table. Possible types of events include: · Routing updates that populate the FIB table · Flushing of the FIB table · Adding or removing of entries to the FIB table · Table reloading process |
ipc | Records information related to IPC in CEF. Possible types of events include: · Transmission status of IPC messages · Status of buffer space for ipc messages · IPC messages received out of sequence · Status of resequenced messages · Throttle requests sent from a line card to the route processor |
interface- ipc | Records IPC updates related to interfaces. Possible reporting includes an interface coming up or going down, updates to fibhwidb, fibidb, and so forth. |
For information about the Cisco 12000 series router, refer to the Cisco 12012 Gigabit Router Installation and Configuration Guide (DOC-12012GSR-ICG=). For information about the Cisco 7500 series router, refer to the Cisco 7500 Series Installation and Configuration Guide (DOC-7500-ICG=).
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