CSM Traffic Flows

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Feb 28th, 2007

Need to understand who the traffic flow/session goes thru the CSM module. My my concern is to undestand who a catch-all vserver works, Is the CSM making a entry to forward back the traffic to the origin host?



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Gilles Dufour Fri, 03/02/2007 - 00:20

the csm always try to first match a packet with a flow entry [you can do a 'sho mod csm X conn' to see the list of flows].

If there is a match, the CSM will use the flow info to perform nat and forward the packet.

If no entry exist, it will try to find a matching rule and perform the required action and create a new flow.


ruggirello Fri, 03/02/2007 - 05:12

Hi Gilles,

Thank you for your answer. But what happens with the reply from the balanced server? How this packet is going thru the CSM back to the client?



slavb Mon, 03/12/2007 - 11:50


It would go to the gateway set on CSM client vlan.


Gilles Dufour Tue, 03/13/2007 - 06:40


this is exactly what I explained in my previous post.

Let's try it differently.

when a client opens a TCP connection with the CSM vserver, the CSM will make a loadbalancing decision and select a server.

It will then create 2 flow entries.

1 for client -> vserver and 1 for server-> client.

This flow entries contain client ip and src mac address.

So, when the response from the server comes back, the CSM will match to the flow server -> client, extract the src mac-address and client vlan and forward the server response using that info.


slavb Tue, 03/13/2007 - 08:24

From Catalyst 6500 Series Content Switching

Module Configuration Note

"There are three situations in which the CSM must make a routing decision:

? Upon receiving a new connection.

At this time, the CSM needs to decide where to send the return traffic for that connection. Unlike

other devices, the CSM will not perform a route lookup, but memorizes the source MAC address

from where the first packet of the connection was received. Return traffic for that connection is sent

back to the source MAC address. This behavior also works with redundancy protocols between

upstream routers, such as HSRP.

? The CSM is configured in router mode.

The servers are pointing to the CSM as their default gateway and the servers are originating


? A server farm is configured with the predictor forward option (see Chapter 5, ?Configuring Real

Servers and Server Farms?). This predictor instructs the CSM to route the connection instead of load

balancing it."

This pretty much goes along with what you've said Gilles, with the exception of the client mac address, as CSM more then likely will see mac address of its client vlan L3 interface.

slavb Mon, 03/12/2007 - 11:36


I am not sure if I understand your question. Do you want to know how does vserver works that allows direct aaccess to behind the CSM?



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