{"id":1062,"date":"2017-02-07T23:29:13","date_gmt":"2017-02-07T23:29:13","guid":{"rendered":"https:\/\/fir3netwp.gmsrrpobkbd.com\/2017\/02\/07\/what-is-cef-cisco-express-forwarding\/"},"modified":"2021-07-31T17:02:05","modified_gmt":"2021-07-31T17:02:05","slug":"what-is-cef-cisco-express-forwarding","status":"publish","type":"post","link":"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html","title":{"rendered":"What is CEF (Cisco Express Forwarding)?"},"content":{"rendered":"<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_65 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<p class=\"ez-toc-title\">Table of Contents<\/p>\n<label for=\"ez-toc-cssicon-toggle-item-6626da87b02e1\" class=\"ez-toc-cssicon-toggle-label\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/label><input type=\"checkbox\"  id=\"ez-toc-cssicon-toggle-item-6626da87b02e1\"  aria-label=\"Toggle\" \/><nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Introduction\" title=\"Introduction\">Introduction<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#History\" title=\"History\">History<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Process-Switching\" title=\"Process-Switching\">Process-Switching<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Fast-Switching\" title=\"Fast-Switching\">Fast-Switching<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Components\" title=\"Components\">Components<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#FIB\" title=\"FIB\">FIB<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Adjacency_Table\" title=\"Adjacency Table\">Adjacency Table<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Loadbalancing\" title=\"Loadbalancing\">Loadbalancing<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-9\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Polarization\" title=\"Polarization\">Polarization<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-10\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Commands\" title=\"Commands\">Commands<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-11\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Additional_Links\" title=\"Additional Links\">Additional Links<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-12\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\/#Sources\" title=\"Sources\">Sources<\/a><\/li><\/ul><\/nav><\/div>\n<h2><span class=\"ez-toc-section\" id=\"Introduction\"><\/span>Introduction<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Cisco Express Forwarding (CEF) is a packet-switching technique used within Cisco routers. The main purpose of CEF is to optimize the forwarding of packets and increase the packet switching speed.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"History\"><\/span>History<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Prior to CEF there were 2 methods for packet-switching &#8211; <strong>Process- Switching<\/strong> and <strong>Fast-Switching<\/strong>.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"Process-Switching\"><\/span>Process-Switching<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>The first method, process-switching is the oldest and slowest. In short the CPU is involved in every forwarding decision.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"Fast-Switching\"><\/span>Fast-Switching<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>With fast-switching, the CPU is still used to determine the destination, but only for the initial packet. This information is stored with a fast-switching cache. Subsequent packets are then switched using the cache rather then CPU.<br \/>\nHowever, the problem with fast-switching is that the cache is built on-demand and the first packet is always process switched. This means, in the event of the router receiving a high volume of traffic to destinations not yet in cache, the CPU will still be consumed and switching performance affected.<\/p>\n<p>To overcome the problems with process-switching and fast-switching CEF was created.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Components\"><\/span>Components<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>CEF is built around 2 main components &#8211; the <strong>Forwarding Information Base (FIB)<\/strong> and the <strong>Adjacency Table<\/strong>.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"FIB\"><\/span>FIB<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>The FIB is an optimized version of the routing table (RIB).<br \/>\nThe FIB contains destination reachability information as well as next hop information. This information is then used by the router to make forwarding decisions. The FIB is organized as a multiway trie (Figure1) which allows for very efficient and easy lookups<sup>[1]<\/sup>.<\/p>\n<p><picture><source srcset=\"https:\/\/www.fir3net.com\/wp-content\/uploads\/2017\/02\/www.ciscopress.com_content_images_chap6_9781587143960_elementLinks_06fig02.avif 360w,https:\/\/www.fir3net.com\/wp-content\/uploads\/2017\/02\/www.ciscopress.com_content_images_chap6_9781587143960_elementLinks_06fig02-263x300.avif 263w\" sizes=\"(max-width: 360px) 100vw, 360px\" type=\"image\/avif\"><source srcset=\"https:\/\/www.fir3net.com\/wp-content\/uploads\/2017\/02\/www.ciscopress.com_content_images_chap6_9781587143960_elementLinks_06fig02.webp 360w,https:\/\/www.fir3net.com\/wp-content\/uploads\/2017\/02\/www.ciscopress.com_content_images_chap6_9781587143960_elementLinks_06fig02-263x300.webp 263w\" sizes=\"(max-width: 360px) 100vw, 360px\" type=\"image\/webp\"><img src=\"https:\/\/www.fir3net.com\/wp-content\/uploads\/2017\/02\/www.ciscopress.com_content_images_chap6_9781587143960_elementLinks_06fig02.jpg\" height=\"410\" width=\"360\" srcset=\"https:\/\/www.fir3net.com\/wp-content\/uploads\/2017\/02\/www.ciscopress.com_content_images_chap6_9781587143960_elementLinks_06fig02.jpg 360w, https:\/\/www.fir3net.com\/wp-content\/uploads\/2017\/02\/www.ciscopress.com_content_images_chap6_9781587143960_elementLinks_06fig02-263x300.jpg 263w\" sizes=\"(max-width: 360px) 100vw, 360px\" class=\" size-full wp-image-1061 sp-no-webp\" alt=\"FIB Tree\" loading=\"lazy\" decoding=\"async\" style=\"display: block; margin-left: auto; margin-right: auto;\"  > <\/picture><\/p>\n<p style=\"text-align: center;\">Figure1 &#8211; source <a href=\"http:\/\/www.ciscopress.com\/articles\/article.asp?p=2244117&amp;seqNum=2\">http:\/\/www.ciscopress.com\/articles\/article.asp?p=2244117&amp;seqNum=2<\/a><\/p>\n<h3><span class=\"ez-toc-section\" id=\"Adjacency_Table\"><\/span>Adjacency Table<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>The adjacency table maintains layer 2 or switching information linked to a particular FIB entry, avoiding the need for an ARP request for each table lookup<sup>[2]<\/sup>.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Loadbalancing\"><\/span>Loadbalancing<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>CEF provides 2 methods for loadbalancing traffic over multiple links. They are,<\/p>\n<ul>\n<li><strong>Per packet<\/strong> &#8211; As the name suggests, additionally weights can also be assigned to an interfaces. This allows you to send more packets over one link then another. Useful for unequal links.<\/li>\n<li><strong>Per destination<\/strong> &#8211; Also known as per session. Packets are loadbalanced based on the source and destination addresses.<\/li>\n<\/ul>\n<h2><span class=\"ez-toc-section\" id=\"Polarization\"><\/span>Polarization<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Polarization is a term given when traffic is sent over a single link, even though multiple links are available. An example would be traffic from multiple sources being proxied and using per destination loadbalancing.<\/p>\n<p>To avoid this you can include additional attributes to your CEF hashing options. Here are the command options,<\/p>\n<table class=\"table table-condensed\">\n<tbody>\n<tr>\n<th>Command<\/th>\n<th>Description<\/th>\n<\/tr>\n<tr>\n<td>mls ip cef load-sharing full<\/td>\n<td>\u00a0Layer 4 only (src\/dest ports)<\/td>\n<\/tr>\n<tr>\n<td>mls ip cef load-sharing simple<\/td>\n<td>\u00a0Layer 3 only (src\/dst ip)<\/td>\n<\/tr>\n<tr>\n<td>mls ip cef load-sharing full simple<\/td>\n<td>\u00a0Layer 3 and 4<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2><span class=\"ez-toc-section\" id=\"Commands\"><\/span>Commands<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Here are some useful commands for verifying CEF,<\/p>\n<table class=\"table table-condensed\">\n<tbody>\n<tr>\n<th>Command<\/th>\n<th>Description<\/th>\n<\/tr>\n<tr>\n<td>show ip cef<\/td>\n<td>Show CEF table<\/td>\n<\/tr>\n<tr>\n<td>show ip cef [address] [detail]<\/td>\n<td>Show CEF entry within table for a given address<\/td>\n<\/tr>\n<tr>\n<td>show ip cef exact-route [source] [destination]<\/td>\n<td>Show CEF entry within table for source and destination address<\/td>\n<\/tr>\n<tr>\n<td>show cef interface<\/td>\n<td>Show CEF options enabled on each interface<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2><span class=\"ez-toc-section\" id=\"Additional_Links\"><\/span>Additional Links<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p><a href=\"http:\/\/blog.ipspace.net\/2013\/02\/process-fast-and-cef-switching-and.html\">http:\/\/blog.ipspace.net\/2013\/02\/process-fast-and-cef-switching-and.html<\/a><br \/>\n<a href=\"https:\/\/howdoesinternetwork.com\/2015\/routing-cef\">https:\/\/howdoesinternetwork.com\/2015\/routing-cef<\/a><\/p>\n<h2><span class=\"ez-toc-section\" id=\"Sources\"><\/span>Sources<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>[1] <a href=\"https:\/\/www.petri.com\/cisco-express-forwarding-cef-overview\" class=\"broken_link\">https:\/\/www.petri.com\/cisco-express-forwarding-cef-overview<\/a><br \/>\n[2] <a href=\"https:\/\/en.wikipedia.org\/wiki\/Cisco_Express_Forwarding\">https:\/\/en.wikipedia.org\/wiki\/Cisco_Express_Forwarding<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Introduction Cisco Express Forwarding (CEF) is a packet-switching technique used within Cisco routers. The main purpose of CEF is to optimize the forwarding of packets and increase the packet switching speed. History Prior to CEF there were 2 methods for packet-switching &#8211; Process- Switching and Fast-Switching. Process-Switching The first method, process-switching is the oldest and &#8230; <a title=\"What is CEF (Cisco Express Forwarding)?\" class=\"read-more\" href=\"https:\/\/www.fir3net.com\/Routers\/Cisco\/what-is-cef-cisco-express-forwarding.html\" aria-label=\"Read more about What is CEF (Cisco Express Forwarding)?\">Read more<\/a><\/p>\n","protected":false},"author":2,"featured_media":1060,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[12],"tags":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v22.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>What is CEF (Cisco Express Forwarding)? - Fir3net<\/title>\n<meta name=\"description\" content=\"Introduction Cisco Express Forwarding (CEF) is a packet-switching technique used within Cisco routers. 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