Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
First Published: May 2, 2005
Last Updated: August 12, 2010
This module contains information about Cisco Express Forwarding and describes the required and optional tasks for configuring a per-destination load-balancing scheme for Cisco Express Forwarding traffic. Load-balancing allows you to optimize resources by distributing traffic over multiple paths.
Note The Cisco ASR 1000 Series Aggregation Services Routers supports only per-destination load-balancing schemes. Cisco ASR 1000 Series Aggregation Services Routers does not support per-packet load-balancing schemes.
Cisco Express Forwarding is an advanced Layer 3 IP switching technology. It optimizes network performance and scalability for all kinds of networks: those that carry small amounts of traffic and those that carry large amounts of traffic in complex patterns, such as the Internet and networks characterized by intensive web-based applications or interactive sessions.
Finding Feature Information
For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic" section.
Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Contents
•Prerequisites for Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
•Information About Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
•How to Configure a Load-Balancing Scheme for Cisco Express Forwarding Traffic
•Configuration Examples for Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
•Additional References
•Feature Information for Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
•Glossary
Prerequisites for Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
Cisco Express Forwarding or distributed Cisco Express Forwarding must be enabled on your switch or router.
Information About Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
•Cisco Platform Support for Central Cisco Express Forwarding and Distributed Cisco Express Forwarding
•Cisco Express Forwarding Load-Balancing Overview
•Per-Destination Load Balancing for Cisco Express Forwarding Traffic
•8-Way Cisco Express Forwarding Per Destination Load Balancing
•Load-Balancing Algorithms for Cisco Express Forwarding Traffic
For links to information about other Cisco Express Forwarding and distributed Cisco Express Forwarding features that you can configure, see the "Additional References" section.
Cisco Platform Support for Central Cisco Express Forwarding and Distributed Cisco Express Forwarding
Cisco Express Forwarding is enabled by default on the Cisco ASR 1000 Series Aggregation Services Routers.
To find out if Cisco Express Forwarding is enabled on your platform, enter the show ip cef command. If Cisco Express Forwarding is enabled, you receive output that looks like this:
Prefix Next Hop Interface
10.2.61.8/24 192.168.100.1 FastEthernet1/0/0
192.168.101.1 FastEthernet2/1/0
If Cisco Express Forwarding is not enabled on your platform, the output for the show ip cef command looks like this:
If Cisco Express Forwarding is not enabled on your platform, use the ip cef command to enable (central) Cisco Express Forwarding or the ip cef distributed command to enable distributed Cisco Express Forwarding.
Cisco Express Forwarding Load-Balancing Overview
Cisco Express Forwarding 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.
Per-Destination Load Balancing for Cisco Express Forwarding Traffic
Per-destination load balancing allows the router to use multiple paths to achieve load sharing across multiple source-destination host pairs. Packets for a given source-destination host pair are guaranteed to take the same path, even if multiple paths are available. Traffic streams destined for different pairs tend to take different paths.
Per-destination load balancing is enabled by default when you enable Cisco Express Forwarding. To use per-destination load balancing, you do not perform any additional tasks once Cisco Express Forwarding is enabled. Per-destination 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 host pairs increases.
You can use per-destination load balancing to ensure that packets for a given host pair arrive in order. All packets intended for a certain host pair are routed over the same link (or links).
Note Per-destination load balancing is the default load-balancing scheme for Cisco Express Forwarding traffic on the Cisco ASR 1000 Series Aggregation Services Routers. No configuration steps are required to enable per-destination load balancing. Additionally, on the Cisco ASR 1000 Series Aggregation Services Routers per-destination load balancing cannot be disabled.
8-Way Cisco Express Forwarding Per Destination Load Balancing
Destination IP prefixes are added to the routing table by routing protocols or static routes. Each path is a valid route to reach the destination prefix. The set of active paths is the set of paths with the best cost. The 8-Way Cisco Express Forwarding Load Balancing feature provides the ability to share the traffic to a destination prefix over up to eight active paths (an increase from the previous support of six active paths). The traffic load among the active paths can be distributed by destination.
Load-Balancing Algorithms for Cisco Express Forwarding Traffic
The following load-balancing algorithms are provided for use with Cisco Express Forwarding traffic. You select a load-balancing algorithm with the ip cef load-sharing algorithm command.
•Original algorithm—The original Cisco Express Forwarding load-balancing algorithm produces distortions in load sharing across multiple routers because the same algorithm was used on every router. Depending on your network environment, you should select either the universal algorithm (default) or the tunnel algorithm instead.
•Universal algorithm—The universal load-balancing algorithm allows each router on the network to make a different load sharing decision for each source-destination address pair, which resolves load-sharing imbalances. The router is set to perform universal load sharing by default.
•Tunnel algorithm—The tunnel algorithm is designed to load balance when only a few source and destination pairs are involved.
•Include-ports algorithm—The include-ports algorithm allows you to use the Layer 4 source and destination ports as part of the load-balancing decision. This method benefits traffic streams running over equal cost paths that are not load shared because the majority of the traffic is between peer addresses that use different port numbers, such as Real-Time Protocol (RTP) streams.
How to Configure a Load-Balancing Scheme for Cisco Express Forwarding Traffic
Note Per-destination load balancing is the default load-balancing scheme for Cisco Express Forwarding traffic on the Cisco ASR 1000 Series Aggregation Services Routers. No configuration steps are required to enable per-destination load balancing. Additionally, on the Cisco ASR 1000 Series Aggregation Services Routers per-destination load balancing cannot be disabled.
Perform the following tasks to fine-tune load balancing for Cisco Express Forwarding:
•Selecting a Cisco Express Forwarding Load-Balancing Algorithm (optional)
•Verifying the Configuration of a Load-Balancing Scheme for Cisco Express Forwarding (optional)
Selecting a Cisco Express Forwarding Load-Balancing Algorithm
Perform one of the following tasks to elect a Cisco Express Forwarding load-balancing algorithm.
•Selecting a Tunnel Load-Balancing Algorithm for Cisco Express Forwarding Traffic (optional)
•Selecting an Include-Ports Layer 4 Load-Balancing Algorithm for Cisco Express Forwarding Traffic (optional)
The router is set to perform universal load sharing by default.
Selecting a Tunnel Load-Balancing Algorithm for Cisco Express Forwarding Traffic
Perform the following task to select a tunnel load-balancing algorithm for Cisco Express Forwarding traffic. Select the tunnel algorithm when your network environment contains only a few source and destination pairs.
SUMMARY STEPS
1. enable
2. configure terminal
3. ip cef load-sharing algorithm {original | tunnel [id] | universal [id] | include-ports {source [id] | [destination] [id] | source [id] destination [id]}}
4. end
DETAILED STEPS
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Step 1 |
enable
Router> enable |
Enables privileged EXEC mode. •Enter your password if prompted. |
Step 2 |
configure terminal
Router# configure terminal |
Enters global configuration mode. |
Step 3 |
ip cef load-sharing algorithm {original | tunnel [id] | universal [id] | include-ports {source [id]| [destination] [id] | source [id] destination [id]}}
Router(config)# ip cef load-sharing algorithm tunnel |
Selects a Cisco Express Forwarding load-balancing algorithm. •The original keyword sets the load-balancing algorithm to the original algorithm, based on a source and destination hash. •The tunnel keyword sets the load-balancing algorithm to one that can be used in tunnel environments or in environments where there are only a few IP source and destination address pairs. •The id argument is a fixed identifier. •The universal keyword sets the load-balancing algorithm to one that uses a source and destination and an ID hash. •The include-ports source keywords set the load-balancing algorithm to one that uses the source port. •The include-ports destination keywords set the load-balancing algorithm to one that uses the destination port. •The include-ports source destination keywords set the load-balancing algorithm to one that uses both source and destination ports. |
Step 4 |
end
Router(config)# end |
Exits to privileged EXEC mode. |
Selecting an Include-Ports Layer 4 Load-Balancing Algorithm for Cisco Express Forwarding Traffic
Perform the following task to select an include-ports load-balancing algorithm for Cisco Express Forwarding traffic. Select the include-port algorithm when your network environment contains traffic running over equal-cost paths that is not load shared because the majority of the traffic is between peer addresses with different port numbers, such as RTP streams.
SUMMARY STEPS
1. enable
2. configure terminal
3. ip cef load-sharing algorithm {original | tunnel [id] | universal [id] | | include-ports {source [id] | [destination] [id] | source [id] destination [id]}}
4. end
DETAILED STEPS
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Step 1 |
enable
Router> enable |
Enables privileged EXEC mode. •Enter your password if prompted. |
Step 2 |
configure terminal
Router# configure terminal |
Enters global configuration mode. |
Step 3 |
ip cef load-sharing algorithm {original | tunnel [id] | universal [id] include-ports {source [id] | [destination] [id] | source [id] destination [id]}}
Router(config)# ip cef load-sharing algorithm include-ports source destination |
Selects a Cisco Express Forwarding load-balancing algorithm. •The original keyword sets the load-balancing algorithm to the original algorithm, based on a source and destination hash. •The tunnel keyword sets the load-balancing algorithm to one that can be used in tunnel environments or in environments where there are only a few IP source and destination address pairs. •The id argument is a fixed identifier. •The universal keyword sets the load-balancing algorithm to one that uses a source and destination and an ID hash. •The include-ports source keyword sets the load-balancing algorithm to one that uses the source port. •The include-ports destination keyword sets the load-balancing algorithm to one that uses the destination port. •The include-ports source destination keyword sets the load-balancing algorithm to one that uses the source and destination ports. |
Step 4 |
end
Router(config)# end |
Exits to privileged mode. |
Verifying the Configuration of a Load-Balancing Scheme for Cisco Express Forwarding
To verify the configuration of a load-balancing scheme, perform the following task.
SUMMARY STEPS
1. enable
2. show cef state
3. show ip cef adjacency drop detail
4. show ipv6 cef detail
5. show cef table
6. disable
DETAILED STEPS
Step 1 enable
Use this command to enter privileged EXEC mode. Enter your password, if prompted. For example:
Step 2 show cef state
Use this command to verify that load-balancing algorithm is configured, as you expect. For example:
dCEF disabled/not running
CEF switching enabled/running
! The load-balancing scheme configured for IPv4 Cisco Express Forwarding traffic, follows:
universal per-destination load sharing algorithm, id A189DD49 ! <== scheme for IPv4
traffic
dCEF disabled/not running
The load-balancing scheme configured for IPv6 Cisco Express Forwarding traffic, follows:
original per-destination load sharing algorithm, id A189DD49
Step 3 show ip cef adjacency drop detail
Use this command to verify that to verify that a load-balancing scheme is configured for Cisco Express Forwarding. For example:
Router# show ip cef adjacency drop detail
IP CEF with switching (Table Version 4), flags=0x0
4 routes, 0 reresolve, 0 unresolved (0 old, 0 new), peak 0
4 leaves, 8 nodes, 8832 bytes, 13 inserts, 9 invalidations
0 load sharing elements, 0 bytes, 0 references
universal per-destination load sharing algorithm, id 00B999CA ! <== Load-sharing scheme
3 CEF resets, 0 revisions of existing leaves
Resolution Timer: Exponential (currently 1s, peak 1s)
refcounts: 533 leaf, 536 node
0 packets, 0 bytes, Precedence routine (0)
via 0.0.0.0, 0 dependencies
Step 4 show ipv6 cef detail
Use this command to verify that a load-balancing scheme is configured for IPv6 Cisco Express Forwarding traffic. For example:
Router# show ipv6 cef detail
IPv6 CEF is enabled and running
20 prefixes (20/0 fwd/non-fwd)
Table id 0, version 20, 0 resets
Database epoch:0 (20 entries at this epoch)
4004::/64, epoch 0, per-destination sharing ! <== Per-destination load-sharing configured
recursive via 172.11.11.1 label 27
nexthop 10.1.1.3 Ethernet0/0 label 16
recursive via 172.30.30.1 label 26
nexthop 10.1.1.3 Ethernet0/0 label 18
Step 5 show cef table
Use this command to verify that load-sharing build characteristics are configured for Cisco Express Forwarding. For example:
Output chain build favors:
operational: convergence-speed
Output chain build characteristics:
Inplace modify ! <== Build characteristic configured for load-sharing
operational for: load-sharing
Collapse ! <== Build characteristic configured for load-sharing
operational for: load-sharing
operational for: recursive-prefix
pool[C/8 bits]: 12 allocated (0 failed), 12480 bytes {1 refcount}
1 active IPv4 table (9 prefixes total) out of a maximum of 10000.
VRF Prefixes Memory Flags
1 active IPv6 table (1 prefix total) out of a maximum of 10000.
VRF Prefixes Memory Flags
VRF Prefixes Memory Flags
Step 6 disable
Use this command to exit to user EXEC mode. For example:
Configuration Examples for Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
This section provides the following examples for configuring a load-balancing scheme for Cisco Express Forwarding traffic:
•Example: Selecting a Cisco Express Forwarding Load-Balancing Algorithm
Example: Selecting a Cisco Express Forwarding Load-Balancing Algorithm
The router is set to perform universal load balancing by default.
The following examples show how to select a different Cisco Express Forwarding load-balancing algorithm:
•Example: Selecting a Tunnel Load-Balancing Algorithm for Cisco Express Forwarding Traffic
•Example: Selecting an Include-Ports Layer 4 Load-Balancing Algorithm for Cisco Express Forwarding Traffic
Example: Selecting a Tunnel Load-Balancing Algorithm for Cisco Express Forwarding Traffic
The following example shows how to select a tunnel load-balancing algorithm for Cisco Express Forwarding:
ip cef load-sharing algorithm tunnel
The following example shows how to disable the tunnel load-balancing algorithm:
no ip cef load-sharing algorithm tunnel
Example: Selecting an Include-Ports Layer 4 Load-Balancing Algorithm for Cisco Express Forwarding Traffic
The following example shows how to select an include-ports Layer 4 load-balancing algorithm for Cisco Express Forwarding traffic:
ip cef load-sharing algorithm include-ports source
This example sets up load sharing that includes the source port in the load-balancing decision.
To disable the include-ports Layer 4 load-balancing algorithm and return to the default universal mode, enter the following commands:
no ip cef load-sharing algorithm
Additional References
Related Documents
Standards
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No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature. |
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MIBs
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No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature. |
To locate and download MIBs for selected platforms, Cisco IOS XE software releases, and feature sets, use Cisco MIB Locator found at the following URL: http://www.cisco.com/go/mibs |
RFCs
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No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature. |
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Technical Assistance
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The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password. |
http://www.cisco.com/cisco/web/support/index.html |
Feature Information for Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
Table 1 lists the features in this module.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note Table 1 lists only the Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.
Table 1 Feature Information for Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
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Feature Configuration Information
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Cisco Express Forwarding Support for Layer 4 Port-Based Load Balancing |
Cisco IOS XE Release 2.1 |
This feature allows Cisco Express Forwarding to include Layer 4 port information in the decision for load sharing on equal cost paths. In Cisco IOS XE Release 2.1, this feature was introduced on the Cisco ASR 1000 Series Aggregation Services Router. The following sections provide information about this feature: •Load-Balancing Algorithms for Cisco Express Forwarding Traffic •Selecting a Cisco Express Forwarding Load-Balancing Algorithm The following commands were modified by this feature: ip cef load-sharing algorithm and show ip cef exact-route. |
Glossary
adjacency—A relationship formed between selected neighboring routers and end nodes for the purpose of exchanging routing information. Adjacency is based upon the use of a common media segment by the routers and nodes involved.
Cisco Express Forwarding—A Layer 3 switching technology. Cisco Express Forwarding can also refer to central Cisco Express Forwarding mode, one of two modes of Cisco Express Forwarding operation. Cisco Express Forwarding enables a Route Processor to perform express forwarding. Distributed Cisco Express Forwarding is the other mode of Cisco Express Forwarding operation.
distributed Cisco Express Forwarding—A mode of Cisco Express Forwarding operation in which line cards maintain identical copies of the forwarding information base (FIB) and adjacency tables. The line cards perform the express forwarding between port adapters; this relieves the Route Processor of involvement in the switching operation.
FIB—forwarding information base. A component of Cisco Express Forwarding that is conceptually similar to a routing table or information base. The router uses the FIB lookup table to make destination-based switching decisions during Cisco Express Forwarding operation. The router maintains a mirror image of the forwarding information in an IP routing table.
LSP—label switched path. A sequence of hops (Router 0...Router n). A packet travels from R0 to Rn by means of label switching mechanisms. An LSP can be chosen dynamically, based on normal routing mechanisms, or you can configure the LSP manually.
prefix—The network address portion of an IP address. A prefix is specified by a network and mask and is generally represented in the format network/mask. The mask indicates which bits are the network bits. For example, 1.0.0.0/16 means that the first 16 bits of the IP address are masked, making them the network bits. The remaining bits are the host bits. In this example, the network number is 10.0.
RIB—Routing Information Base. A central repository of routes that contains Layer 3 reachability.
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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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