Cisco's Express Forwarding (CEF) technology for IP is a scalable, distributed, layer 3 switching solution designed to meet the future performance requirements of. Cisco Express Forwarding (CEF) is advanced Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and. Warning and Disclaimer. This book is designed to provide information about Cisco Express Forwarding (CEF). Every effort has been made to make this book as.
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Cisco Express Forwarding (CEF) is advanced, Layer 3 IP switching technology. CEF optimizes Feature History for Cisco Express Forwarding. Cisco Express Forwarding is an advanced Layer 3 IP switching technology. It optimizes Cisco Express Forwarding Adjacency Tables Overview, page This document explains what Cisco Express Forwarding is, and how it is implemented in the Cisco Series Internet Router.
It means that packets can be forwarded without any noticeable delay. Everything that I explain about the multilayer switches from now on also applies to routers. The multilayer switch can do the same but is also able to route between VLANS and filter on layer 3 or 4 using access-lists. Forwarding on layer 2 is based on the destination MAC address. Forwarding on layer 3 is based on the destination IP address.
It also covers the enhanced CEF structure and general troubleshooting. The second part of the book provides case studies that focus on the common topics that have been problematic for customers and those supporting Cisco networks.
Full of practical examples and configurations, this book draws on years of experience to help you keep your Cisco networks running efficiently. Nakia has been with Cisco for more than six years, previously serving as a technical leader for the Architecture TAC team.
Russ primarily works in the development of new features and design architectures for routing protocols.
Understanding Broadcast and Collision Domains 5. Understanding Per-Destination Load Sharing Catalyst with a Supervisor 3BXL Site Multihomed to Different PEs Get unlimited day access to over 30, books about UX design, leadership, project management, teams, agile development, analytics, core programming, and so much more. All rights reserved. Join Sign In. View Larger Image. Description Sample Content Updates. Copyright Dimensions: Networking Covers: Routing and Switching Because per-destination load balancing depends on the statistical distribution of traffic, load sharing becomes more effective as the number of sourcedestination pairs increase.
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 which could cause reordering of packets.
For this reason, per-packet load balancing is inappropriate for certain types of data traffic, such as VoIP, that depend on packets arriving at the destination in sequence. Use per-packet load balancing to help ensure that a path for a single sourcedestination pair does not become overloaded. Each session see the table above is assigned to an active path. The session-to-path assignment is done using a hash function that takes the source and destination IP addresses and, in recent releases of Cisco IOS, a unique hash ID that randomizes the assignment across the end-to-end path.
Active paths are assigned internally to several of 16 hash buckets. The path-to-bucket assignment varies with the type of load balancing and the number of active paths. The result of the hash function is used to pick one of the enabled buckets, and thus which path to use for the session. For all sessions being forwarded by the router, each active path carries the same number of sessions.
Let's look at an example of these Cisco Express Forwarding internals.
Use the maximum-paths command to reduce to two the number of active paths for the destination prefix. RouterB config router ospf 1 RouterB config-router maximum-paths 2 2.
The simple case is for an even number of paths. The 16 buckets are evenly filled with the active paths. If 16 isn't divisible by the number of active paths, the last few buckets that represent the remainder are disabled.
The following table shows how the hash buckets look for two and three active paths. Notice how Cisco Express Forwarding has removed hash bucket 16 and how the three serial links are assigned evenly to hash buckets 1 through Page 10 of 16 next hop , Serial1 switched through the prefix Load distribution: refcount 1! Note: Although they select a single path for a destination, the Cisco Express Forwarding perdestination and fast switching mechanisms differ in how they select that path.
Cisco Express Forwarding considers both the source and destination IP addresses, while fast switching considers only the destination IP address. Confirm that Cisco Express Forwarding is enabled globally on the router.
Confirm per-packet or per-destination switching is enabled on the particular outbound interfaces. The default is per-destination. Page 12 of 16 via , Serial3, 0 dependencies next hop , Serial3 via , Serial2, 0 dependencies next hop , Serial2 via , Serial1, 0 dependencies next hop , Serial1 switched through the prefix tmstats: external internal Load distribution: refcount 1 Hash OK Interface Address Packets 1 Y Serial3 point2point 0 2 Y Serial2 point2point 0 3 Y Serial1 point2point 0 4 Y Serial3 point2point 0 5 Y Serial2 point2point 0 6 Y Serial1 point2point 0 7 Y Serial3 point2point 0 8 Y Serial2 point2point 0 9 Y Serial1 point2point 0 10 Y Serial3 point2point 0 11 Y Serial2 point2point 0 12 Y Serial1 point2point 0 13 Y Serial3 point2point 0 14 Y Serial2 point2point 0 15 Y Serial1 point2point 0 Note: The loadshare table above shows load distribution , and traffic share is 1 for each route.
This means an equal cost per-destination load sharing among three equal cost routes. Check Cisco Express Forwarding adjacencies. Page 13 of Confirm the expected Cisco Express Forwarding load-balancing mechanism is configured on all the outbound interfaces.
Enable hash bucket accounting to collect statistics to better understand Cisco Express Forwarding 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. Use the following command: router config ip cef accounting load-balance-hash Verify the packet flow by observing the values under the Packet field.
RouterB show ip cef internal [ If you use ping to generate traffic, ensure that ping packets must transit the Cisco Express Forwarding-switched router.
In other words, ping packets must enter from one Cisco Express Forwarding-switched interface, be Cisco Express Forwardingswitched and leave from another Cisco Express Forwarding-switched interface. Note: Per-destination load sharing becomes more effective as the number of source-destination pairs increases. While sending traffic to the prefix, capture several outputs of the show interface command.
Analyze the "txload" and "rxload" values. Some interfaces display a single "load" value, which considers both transmit and receive. Page 14 of 16 distribution in the number of packets, parallel links may exhibit a slightly uneven rate depending on the packet size. Add the keyword internal to display which hash bucket is used. If the output appears uneven, consider the following: The number of unique source and destination address pairs or sessions traversing the parallel links.
The number and size of packets in each session. Does one session have a large number of packets? Per-destination load balancing depends on the statistical distribution of traffic and becomes more effective as the number of source-destination pairs increase.
Cisco Express Forwarding then matches newly arriving packets to an adjacency and corresponding hash bucket. The selected bucket may or may not be the same as the one used previously. The following steps describe how Cisco Express Forwarding load-balancing information updates after a change in the number of active paths to a destination prefix.
Assume a destination prefix is reachable through two paths.
Path 1 is inactive, and path 2 is active and carrying all traffic. When path 1 is available again, it prompts IP routing reconvergence processes.
Page 15 of Cisco Express Forwarding now balances the load across both paths and does not preserve existing flows across path 2. Doing so would leave path 1 unused. In other words, Cisco Express Forwarding does not consider that it can forward packets for a session on a valid path and may select a new path for a flow depending on which hash bucket it selects. Load Sharing Information Due for Deletion After a change in the routing table, Cisco Express Forwarding deletes and then rebuilds the load balancing structures that assign active paths to the hash buckets.
Specifically, Cisco Express Forwarding now allocates an FIB entry with space for the maximum possible active paths for this route. Known Issues Cisco Express Forwarding load balancing is not equal on four paths. For further details, refer to CSCdm registered customers only. In Cisco IOS versions earlier than This situation occurs when the destination prefix is recursive and the router is load-sharing to the next hop.