IOS SLB in the Server Farm

Unanswered Question
Apr 21st, 2010

Hi, folks:


I'm looking at a data center of a company that has a huge enterprise network. The server farm uses a switched access layer and a collapsed core comprised of two 6509 switches. They have about 200 IOS SLB instances, with probes, configured in that core layer. These are old CatOS switch with dual SUP 2s.


From what I can recall, IOS SLB traffic is process switched, isnt it?  Now, given the caveat that I dont know much about this environment and its requirements, I cant, nonetheless, help but view this environment as being in desperate need of an upgrade in hardware, but also a redesign that is more scalable and appropriate for an enterprise-grade server farm.


My opinion of IOS SLB is that it can come in handy in the absence of a real load balancing appliance, or even a switched module in the chassis, but it is really a poor-man's verion of SLB that should be used more judiciously than 200 separate instances.


Any thoughts on this from anyone? Would love to hear different opinions...


Thanks


Victor

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Jon Marshall Thu, 04/22/2010 - 01:26

Victor


From what I can recall, IOS SLB traffic is process switched, isnt it? 


Not in dispatch mode. Dispatch mode means you must have L2 adjacency from the switch to the servers. It is process switched if you use directed mode though.


My opinion of IOS SLB is that it can come in handy in the absence of a real load balancing appliance, or even a switched module in the chassis, but it is really a poor-man's verion of SLB that should be used more judiciously than 200 separate instances.

Any thoughts on this from anyone? Would love to hear different opinions...


Call me crazy but i happen to agree with you on this. Obviously there are caveats in that if the switch can handle the additional load then why not but even then a dedicated load-balancer has more functionality/features built in. In a large data centre setup i would leave the 6500 to get on with it's primary purpose ie. switching packets and offload the load-balancing to a dedicated device. Note that this does include using a 6500 service module because that does offload the function off the main 6500 resources.


Having said that, if the features that the company needs can be met by IOS SLB and the switch is not showing any performance issues it could be a difficult sell in these economic circumstances to persuade them for a need to upgrade.


Jon

lamav Thu, 04/22/2010 - 02:25

Hi, Jon:


Thanks for your input. I agree that the economic situation these days may cause some wariness toward upgrades.


On a separate note, Im reviewing the SLB configs on this switch, and Im seeing something I havent seen before. I am in the process of researching the commands and understanding exactly what they do, but in the interim, can I ask you a few questions? (Please, no sarcasm!) lol


Have you ever seen this?



vlan 107 server
  ip address 10.176.6.251 255.255.255.0
  alias 10.176.6.254 255.255.255.0
!
vlan 104 client
  ip address 10.176.3.251 255.255.255.0
  gateway 10.176.3.254



The chassis has a CSM module in it, which makes me wonder why theyre using IOS SLB in the first place. Perhaps they just installed the module and just havent migrated traffic to it yet. It doesnt seem as though they have it configured either.


What does it mean when I see this on the CSM after doing a sess to enter its CLI? These are the only commands available.


CSM> ?
usage
      upgrade  slot0:|server-ip-addr filename
      tftp     core_dump tftp-ip-addr [filename]
      exit


CSM> exit


Laslty, when I execute the sh ip slb command, the only option I have is the mode keyword. I dont see any ability to view any other statistics or output.



de01-br11#sh ip slb ?
  mode  SLB system mode information

de01-br11#sh ip slb


And the mode is rp, which seems like a mode that is the default when using the CSM module -- but Im not executing the command on the CSM. Nor does it seem as though the CSM is even configured. But it may be a case in which the mere presence of the module forces even the IOS SLB mode into rp....?


[EDIT] Jon, as I said before, I am researching these answers to these questions -- just havent had a chance to do a deep dive yet. So, please dont waste your time researching for me. Just answer if you have seen this before and know right off the bat what the answers are. [EDIT]


Thanks


Victor

Giuseppe Larosa Thu, 04/22/2010 - 02:36

Hello Victor,

the CSM service module can be used in a way that you configure it from supervisor


in my case starts with:


module ContentSwitchingModule 6
ft group 1 vlan 999
  priority 100
  failover 3
  preempt
!
and then goes on with client and server vlan definitions


if it would be used you should see with commands like


sh module csm serverfarms

sh module csm vserver


all given in supervisor mode without connecting to CSM


the drawback of this is that there is no autosync between the two CSMs in the pair and we need to make changes on both.



http://www.cisco.com/en/US/docs/interfaces_modules/services_modules/csm/4.1.x/4.1.2/configuration/guide/getstart.html#wp1037341


RP Mode

You can use the ip slb mode rp command mode (the  default) to configure multiple CSMs in a chassis with Cisco IOS SLB. You  can only configure the CSM using this mode starting from release 2.1.

In this mode, the CSM is configured from this command submode:

mod csm X



Hope to help

Giuseppe

lamav Thu, 04/22/2010 - 02:44

"the CSM service module can be used in a way that you configure it from supervisor

in my case starts with:

module ContentSwitchingModule 6
ft group 1 vlan 999
  priority 100
  failover 3
  preempt
!
and then goes on with client and server vlan definitions

if it would be used you should see with commands like

sh module csm serverfarms

sh module csm vserver

all given in supervisor mode without connecting to CSM

the drawback of this is that there is no autosync between the two CSMs in the pair and we need to make changes on both.

http://www.cisco.com/en/US/docs/interfaces_modules/services_modules/csm/4.1.x/4.1.2/configuration/guide/getstart.html#wp1037341"



aaaaaaaaaaaaaaaaaaah, ok... got it! So, in this case the CSM is in the chassis and it IS being used, but it is configured from the supervisor? Who is actually doing the load balancing then -- the switch supervisor or the CSM module?


As I said, I still have to research (I just ran into these configs late last night), but I have never heard of this before. Why do it this way?


Thank you, Giuseppe!

Giuseppe Larosa Thu, 04/22/2010 - 02:54

Hello Victor,

aaaaaaaaaaaaaaaaaah, ok... got it! So, in this case the CSM is in the  chassis and it IS being used, but it is configured from  the supervisor?


use the show commands given above to see if it is used and it should be


Hope to help

Giuseppe

Jon Marshall Thu, 04/22/2010 - 04:36

Victor


aaaaaaaaaaaaaaaaaaah, ok... got it! So, in this case the CSM is in the chassis and it IS being used, but it is configured from the supervisor? Who is actually doing the load balancing then -- the switch supervisor or the CSM module?


The CSM would be doing the load-balancing. The CSM isn't quite like the FWSM or the ACE in that it does not support contexts and you don't session to the CSM to config it, unless it has an SSL card but we won't go there !!


Your example config if it is indeed being used is for a routed mode setup on the CSM.


Jon

lamav Thu, 04/22/2010 - 07:14

I've done some reading and a lot of clouds are starting to clear up.


I did run into this peculiar statement. perhaps one of you can tell me how you would interpret it.


"The RP mode is required to configure multiple CSM-S modules in one chassis as well as the Cisco IOS SLB in the same chassis with a CSM-S"


My understanding is that IOS SLB is used when a CSM module is not available.


So, why would one want to cofngiure it when it is? Moreover, what is the relationship between IOS SLB and SLB on a CSM?


Do you find this statement as confusing as i do?

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