I'm sure this HAS to be somewhere on Cisco's web site, but a brief general explanation is this: Cisco, and most other vendor's hardware now-adays, has ASIC chips that inspect ingress traffic coming into the switch. It also has a shared memory buffer that it stores cached route-switch information. This information generally contains all of the information necessary, in the proper format, that the ASIC needs to re-write the packet on the outbound interface (which is usually part of the cached information). If a new flow is being established, there obviously won't be any information in the cache on how to re-write that packet in hardware (ASIC). So, the "switch" has to send the packet to the "routing engine" to have it layer-3 routed. The router makes the usual routing decisions, and stores the information necessary for the ASIC to handle future packets between this source-destination pair in the shared memory cache. Any future packets are handled "in hardware" by the ASIC, and don't need to go back to the route engine.
The specific architecture obviously depends on what specific hardware you are talking about. Fred Reimer - CCNA Eclipsys Corporation, 200 Ashford Center North, Atlanta, GA 30338 Phone: 404-847-5177 Cell: 770-490-3071 Pager: 888-260-2050 NOTICE; This email contains confidential or proprietary information which may be legally privileged. It is intended only for the named recipient(s). If an addressing or transmission error has misdirected the email, please notify the author by replying to this message. If you are not the named recipient, you are not authorized to use, disclose, distribute, copy, print or rely on this email, and should immediately delete it from your computer. -----Original Message----- From: Steven Aiello [mailto:[EMAIL PROTECTED] Sent: Thursday, September 04, 2003 10:05 AM To: [EMAIL PROTECTED] Subject: ??? Layer 2 routing ??? [7:74788] Ok all I have a question on this subject. I know routing takes place at the network layer, and switching takes place at the data link layer because it works based on physical addresses. So how do we get route switching? I've just started my CCNP and we were learning about different cache methods to speed up performance, is this how route switching is done, is the routing calculation be performed on a per packet basis? I was reading that by default, Cisco routers only perform a routing calculation on the first packet for a destination network and then on less the no route-cache option is set all the rest of the packets are really only "switched" to the correct interface. Am I missing something? I would invision that a router would by default perform a lookup for each connection sequence. does layer 3 routing not do a look up for each sequence of packet? Does is look at an address and use an "old" pre say route that was cached in memory? If some one can give a good explanation I would greatly appreciate it. 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=7&i=74797&t=74788 -------------------------------------------------- **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
