Re: ??? Cisco Express Forwarding ??? [7:74794]
in CEF is the whole routing table held in a cache? If so what is the diffrence between this and the routing table held in RAM? Is the cache faster than the regular RAM in the router? There are few excellent documents about this on our favourite website. Watch for wrap. [Cisco IOS Switching Paths] http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fswtch_c/swprt1/ [How to Choose the Best Router Switching Path for Your Network] http://www.cisco.com/en/US/tech/tk827/tk831/technologies_white_paper09186a00800a62d9.shtml Marko. Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=74805t=74794 -- **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
RE: ??? Cisco Express Forwarding ??? [7:74794]
Steve, There are a few reasons why a lookup through the CEF table is faster than a lookup in the IP routing table. A lookup in the IP routing table is done top down until a match is found, much like how an access-list is processed. The problem, however, is that the IP table is not in any specific order, therefore, the worst case lookup for a route is directly proportional to how many prefixes exist in the IP routing table. The CEF table, on the other hand, takes a maximum of four lookups before a match is found. CEF uses four data structures, each with 256 children, with each child having 256 children, etc. This gives us a maximum entry size of 2^32 (all IP address space). These structures are divided as follows: Root -0.0.0.0 -1.0.0.0 -2.0.0.0 .. -255.0.0.0 Suppose we're doing a lookup on the prefix 1.2.3.4. First we find the 1st child under the root (1.0.0.0) Root -1.0.0.0 --1.0.0.0 --1.1.0.0 --1.2.0.0 --... --1.255.0.0 Under the child 1.0.0.0, we now find the 2nd child (1.2.0.0). Next, we find the 3rd child under 1.2.0.0 (1.2.3.0), and finally the fourth child under 1.2.3.0, (1.2.3.4). Our final lookup is now as follows: Root -1.0.0.0 --1.2.0.0 ---1.2.3.0 1.2.3.4 As you can see, no matter which prefix we are doing a lookup on, we have to do a maximum of 4 lookups in order to find it, unlike the normal IP routing table, where our worst case lookup time is proportional to the amount of prefixes in the table. The next reason that CEF is faster than a normal lookup is the adjacency table. Every time a lookup is done in the IP routing table, an addition lookup (recursive lookup) must be done to find the outgoing interface for the next hop IP address. In the case of CEF, this lookup is already done for you in the adjacency table. The adjacency table provides us with the outgoing interface, and the destination layer 2 address that must be encapsulated in order to send the packet out said interface. Lastly, the main advantage of CEF is that the above mentioned lookups are done *before* any traffic is sent. In the case of the other caching mechanisms, a cached entry is not created until the first packet in the flow is fast-switched. This follows the paradigm of route once, switch many. CEF on the other hand is just switch many, since the routing lookup is already performed. HTH, Brian McGahan, CCIE #8593 [EMAIL PROTECTED] Internetwork Expert, Inc. http://www.InternetworkExpert.com Toll Free: 877-334-8987 Direct: 708-362-1418 (Outside the US and Canada) -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Steven Aiello Sent: Thursday, September 04, 2003 10:06 AM To: [EMAIL PROTECTED] Subject: ??? Cisco Express Forwarding ??? [7:74794] Another question, in CEF is the whole routing table held in a cache? If so what is the diffrence between this and the routing table held in RAM? Is the cache faster than the regular RAM in the router? Thanks, Steve **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=74811t=74794 -- **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
RE: ??? Cisco Express Forwarding ??? [7:74794]
Just for the sake of clarity: cache in this context doesn't refer to a faster-than-usual memory. The route cache is in the exact same RAM as the routing table. For more details, see the documents Marko mentioned. Thanks, Zsombor Steven Aiello wrote: Another question, in CEF is the whole routing table held in a cache? If so what is the diffrence between this and the routing table held in RAM? Is the cache faster than the regular RAM in the router? Thanks, Steve Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=74821t=74794 -- **Please support GroupStudy by purchasing from the GroupStudy Store: http://shop.groupstudy.com FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html
Re: Cisco Express forwarding and Memory Requirements --- Please [7:8894]
Couple of items: CEF is a very low overhead process and, in terms of CPU and memory, you should be fine. I am not as certain that your intentions to use the MAC address of the router as a filter point will work. Typically you use an IP extended ACL for CAR. Also review how routers filter outbound traffic. --- Hamid wrote: Hi I have a C2600 router with 32 MB of memory connected to my backbone. This router should share the bandwidth among three Cisco routers connected through the LAN (Fast-Ethernet ports). For example, a 3 Mbps bandwidth should be shared between these routers so the traffic going through each of these routers should be limited to 1 Mbps. I wanted to use CAR using the MAC address of the Fast-Ethernet ports to limit the bandwidth for each router, but I had to enable CEF on the Ethernet interface. I not sure what performance impacts would CEF cause on the C2600 router and I was wondering if the C2600 router could handle this. I am not sure if I am using the best solution, so it would be appreciated if I could have your advice. Thanks In advance Hamid [EMAIL PROTECTED] = Robert Padjen __ Do You Yahoo!? Spot the hottest trends in music, movies, and more. http://buzz.yahoo.com/ Message Posted at: http://www.groupstudy.com/form/read.php?f=7i=8894t=8894 -- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Re: Cisco Express Forwarding
Hi! I would really recommend you get the Inside Cisco IOS architecture book, it gives a lot of information on the different switching modes of cisco... In resume, CEF builds the "switching table" in configuration time (instead on the first packet of a flow, as fast-switching does). To hold the switching information it uses a "mtrie" instead of a mtree, which allows to reference external tables instead of pointing to the destinations in the mtree structure (as fs,distributed sw do) This external tables are updated every time new information is adquired (for example, a arp resolution, or cache time out) so the amount of information that needs to be changed overtime is a lot less than in other switching modes. In previous modes, it was necessary sometimes to invalidate the complete cache in some cases, now only specific entries are changed. It was designed to support very big caches (it was tested on backbone routers), so it should be quite ok for most applications. Just take care if you enable it in a working router, because probably you will need to restart it to get it working stable again (I did test on a 3640 + ipsec +12.1.3, and it needed a restart). Other difference is that the switching is done per source and destination and not per destination (as Fsw) so the loadbalancing of packets over parallel links is better. Regards. Bal Sandhu escribió: Guys, does anyone have any information on CEF ? How can I measure the improvements upon its implementation ? cheers, Bal Sandhu e-mail : mailto:[EMAIL PROTECTED] http://www.madge.com **NOTE: New CCNA/CCDA List has been formed. For more information go to http://www.groupstudy.com/list/Associates.html _ UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html FAQ, list archives, and subscription info: http://www.groupstudy.com Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] -- --- Javier Contreras Albesa Standard Trainer PRO IN Training S.L. PROfessional Information Networks World Trade Center, Moll de Barcelona S/N Edif Sur, Planta 4 Phone: (+34) 93-5088850 E-mail: [EMAIL PROTECTED] Fax: (+34) 93-5088860 Internet: http:// www.proin.com SHAPING THE FUTURE - BE PART OF IT! **NOTE: New CCNA/CCDA List has been formed. For more information go to http://www.groupstudy.com/list/Associates.html _ UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html FAQ, list archives, and subscription info: http://www.groupstudy.com Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Cisco Express Forwarding
Are you talking about the one by Albritton? If so, then I second that! It is very good. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of Javier Contreras Sent: Tuesday, September 12, 2000 10:59 AM To: [EMAIL PROTECTED] Subject: Re: Cisco Express Forwarding Hi! I would really recommend you get the Inside Cisco IOS architecture book, it gives a lot of information on the different switching modes of cisco... In resume, CEF builds the "switching table" in configuration time (instead on the first packet of a flow, as fast-switching does). To hold the switching information it uses a "mtrie" instead of a mtree, which allows to reference external tables instead of pointing to the destinations in the mtree structure (as fs,distributed sw do) This external tables are updated every time new information is adquired (for example, a arp resolution, or cache time out) so the amount of information that needs to be changed overtime is a lot less than in other switching modes. In previous modes, it was necessary sometimes to invalidate the complete cache in some cases, now only specific entries are changed. It was designed to support very big caches (it was tested on backbone routers), so it should be quite ok for most applications. Just take care if you enable it in a working router, because probably you will need to restart it to get it working stable again (I did test on a 3640 + ipsec +12.1.3, and it needed a restart). Other difference is that the switching is done per source and destination and not per destination (as Fsw) so the loadbalancing of packets over parallel links is better. Regards. Bal Sandhu escribió: Guys, does anyone have any information on CEF ? How can I measure the improvements upon its implementation ? cheers, Bal Sandhu e-mail : mailto:[EMAIL PROTECTED] http://www.madge.com **NOTE: New CCNA/CCDA List has been formed. For more information go to http://www.groupstudy.com/list/Associates.html _ UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html FAQ, list archives, and subscription info: http://www.groupstudy.com Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] -- --- Javier Contreras Albesa Standard Trainer PRO IN Training S.L. PROfessional Information Networks World Trade Center, Moll de Barcelona S/N Edif Sur, Planta 4 Phone: (+34) 93-5088850 E-mail: [EMAIL PROTECTED] Fax: (+34) 93-5088860 Internet: http:// www.proin.com SHAPING THE FUTURE - BE PART OF IT! **NOTE: New CCNA/CCDA List has been formed. For more information go to http://www.groupstudy.com/list/Associates.html _ UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html FAQ, list archives, and subscription info: http://www.groupstudy.com Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] **NOTE: New CCNA/CCDA List has been formed. For more information go to http://www.groupstudy.com/list/Associates.html _ UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html FAQ, list archives, and subscription info: http://www.groupstudy.com Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
