Help with switches & routers in a per-packet load balanced network setup!!

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Jul 14th, 2007
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Hi to all,


I wanted some help regarding the type of switches and routers to use in a network setup of mine (attached "setup1.gif" image), which makes use of per-packet load balancing. I wanted some help regarding 2 network traffic scenarios:


1) When traffic from Host 1 is to be load balanced in a per-packet manner onto PCs A, B, C and D!! What kind of switch should I use for Switch 1? Switches that incorporate some routing capabilities I presume but I'm not sure which will be the best (MLS switches??).


2) When Traffic from Host 2 is to be load balanced in a per-packet manner onto PCs A, B, C and D!! Here again, what kind of configuration should I use? I know that if the router were directly connected to switch 2, then 4 static routes on the router, with A, B, C and D as nexthop and ip load-sharing per-packet would do the trick. But does it still hold good when an additional switch is added here (in my case Switch 1)?


I'm unable to find convincing answers to my queries.. Any help will be great.


Warm regards,

Visham



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Edison Ortiz Sat, 07/14/2007 - 16:03
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On the image provided, I only see single links between switches, therefore there won't be any load-balancing.


Load-balancing only takes place when you have multiple links from the same source and destination and you want to send the same amount of traffic via those links.


If you were to have multiple links between these switches, you can achieve this with any kind of Cisco switch and you don't need a Layer 3 switch, you can do this with Layer 2 by implementing etherchanneling between switches.


You can form an etherchannel between switch 1 and switch 2 and all packets sourcing from PCs (A-D) will be balanced over the bundled links.


However, you can't do etherchanneling between the router and switch 1 - a single link will do on this diagram out to the internet.


You can do the same between switch 1 and switch 3.


I recommend going with the Cisco 2960 switch, if you are on a budget -or- a 3560 switch if you want layer 3 features.


I suggest reading etherchanneling configuration at:


http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12225see/scg/swethchl.htm

vishamr2000 Sun, 07/15/2007 - 10:04
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Hi EdisonOrtiz,


Many thx for the reply..


I actually didnt want to use etherchannel...I wanted to know if there's a switch that can use some command to load balance traffic from Host 1 & 2 onto Pcs(A-D). The reason for not using etherchannel is that I don't have many free ports on switch 1. Only two switches have been shown in the picture but there are in fact more switches connected to switch 1. Is there a switch that uses the "ip load-sharing per-packet" command for doing per-packet load balancing onto PCs directly connected to it (a bit like the router with static routes on its interface)?


Warm regards,

Visham

Edison Ortiz Sun, 07/15/2007 - 10:14
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Visham,


As I stated before, a sharing/balancing mechanism is only implemented if you have multiple links from src/dst device. Keep in mind, src/dst device must be the same on each end. The per-packet load balancing you mentioned, it's implemented when you have multiple links and you don't want to saturate just one link. The devices are smart enough to create an algorithm to share the load on the per-packet basis. This implementation does not work for PCs. With PCs, you can have multiple NICs on PC 'A' and create an etherchannel between the switch and PC 'A' in order to perform load balancing and fault tolerance. However, you can't have multiple PCs being part of the same etherchannel. Each PC will have its own etherchannel.


If I understand you correctly, you want to share resources that PCs (A-D) are supplying to the rest of the network. If that's the case, you have to look at the application those devices are providing and see if there is some kind of load sharing/balancing feature.


As for the switches, there isn't a way to determine that PC 'A' is overloaded and to send the packet to PC 'B,C or D', for instance. That would be an application level mechanism.


If I'm off mark with my assumption, please clarify.


Thanks


vishamr2000 Sun, 07/15/2007 - 21:57
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Dear EdisonOrtiz,


>The per-packet load balancing you mentioned, it's implemented when you have multiple links..

You are right because the "ip load-sharing per-packet" & static routes method works only if the switch, to which PCs(A-D) are attached, is directly connected to the router (i.e Switch 2 is diretly connected to the router and not via Switch 1 as in the picture). The router will then load balance between the static routes in a round-robin fashion, with the appropriate dst MAC address set [for each packet the MAC address of one of the PCs(A-D) is set]. The problem is when an additional switch (Switch 1) is added between the router and the switch connecting the PCs(A-D) (here switch 2). Packet traversal from switch to switch changes the src/dst MAC addresses such that the load balancing effect of the command on the router is lost.


You are right to say that etherchannel will work in this case.. but if I dont use multi-link load balancing, i believe the only way to make this topology work is to bring the routing element closer to the PCs. That is why I was thinking of a switch/router, on which we can place the static routes towards the PCs. The switch/router will take the place of Switch 1 in the picture. I dont know if this is possible to do or if there's such a switch/router that accepts static routes pointing towards the same subnet and sends packets in a round-robin fashion along those static routes.


Please note that these PCs(A_D) are on the same subnet (they have IPs 192.168.10.11-14). When Switch 2 was directly connected to the router, the static routes point towards the subnet 192.168.10.0/24, with PCs(A_D) as nexthop. These PCs are basically forwarding firewalls that inspect the packets and then forward valid ones towards a cluster of end-user hosts.


If anything is still not clear, pls let me know...my apologies if I'm being unknowingly stubborn about things you've mentioned previously. It may be due to my lack of experience with such setups.


Warm regards,

Visham

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