What is the purpose of no ip route cach on a 2950

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Mar 20th, 2008
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Most of the research I have down on cisco's site shows configurations of a vlan 1 with the no ip route cache command with no explanation.


What would be the purpose of this? Is this a default configuration?


As I understood.. route cache speeds up

packet forwarding



http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_19_ea1/configuration/guide/swipaddr.html#wp1027783

Correct Answer by Richard Burts about 9 years 4 months ago

Richard


Since the route cache is populated as a result of the layer 3 IP routing decision, I do not see how a layer 2 switch could use it.


HTH


Rick

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francisco_1 Thu, 03/20/2008 - 07:57
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you are right, it is used for packet forwarding. Use the ip route-cache interface configuration command to control the use of high-speed switching caches for IP routing

francisco_1 Thu, 03/20/2008 - 08:07
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See below for some more insight.


With fast switching, the first packet to a specific destination IP address is process switched With the first packet, the router adds an entry to the fast-switching cache, sometimes called the route cache. The cache has the destination IP address, the next-hop information, and the data link header information that needs to be added to

the packet before forwarding . Future packets to the same destination address match the cache entry, so it takes the router less time to process and forward the packet. Although it is much better than process switching, fast switching has a few drawbacks. The first

packet must be process switched. The cache entries are timed out relatively quickly, because otherwise the cache could get overly large as it has an entry per each destination address, not per destination subnet/prefix. Also, load balancing can only occur per destination with fast switching. CEF overcomes the main shortcoming of fast switching. CEF optimizes the route lookup process by using a construct called the Forwarding Information Base (FIB).

Richard Burts Thu, 03/20/2008 - 08:27
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Franco


The information that you posted is very good and helps explain fast switching and the route cache used in IP routing. But since the question that Richard asks is about a 2950 which is a layer 2 switch which does not route IP, I am not sure that it is an effective answer to his question.


Richard


I am not sure but I have a theory about the explanation for your question. Since the switch is based on IOS (not CatOS) it may be that the default behavior is to allocate some memory for the fast switching cache. If so it might optimize memory allocation if route cache was disabled. In terms of your comment about fast switching improving performance, that is true for layer 3 devices that are doing IP forwarding but I think not true for layer 2 devices like the 2950.


If anyone from Cisco has a more authoritative answer, please let us know.


HTH


Rick

nygenxny123 Sun, 03/23/2008 - 14:39
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So, althought route-cache is a command

that is configurable on a 2950, it is incapable of using it since it is a L2 device?

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Richard Burts Sun, 03/23/2008 - 15:01
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Richard


Since the route cache is populated as a result of the layer 3 IP routing decision, I do not see how a layer 2 switch could use it.


HTH


Rick

Richard Burts Tue, 03/25/2008 - 14:06
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Richard


I am glad that my answer was helpful. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that they will read a response which did resolve the question.


The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.


HTH


Rick

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