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subnetting

 
 
mmark751969
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      07-21-2008
we have a 400 node network running on multple 3750 stackable switches
at access layer and a 6509 at the core. We are talking about
subnetting the network. 8 different subnets have been proposed. I
agree that vlan subnetting has it's advantages(i.e reducing broadcast
domains). But i am wondering at what point it's too much. Or what is
the drawback of having too many subnets and would 8 be considered that
on a 400 node network. Thanks
 
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Trendkill
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      07-21-2008
On Jul 21, 2:25*pm, mmark751969 <(E-Mail Removed)> wrote:
> we have a 400 node network running on multple 3750 stackable switches
> at access layer and a 6509 at the core. *We are talking about
> subnetting the network. *8 different subnets have been proposed. *I
> agree that vlan subnetting has it's advantages(i.e reducing broadcast
> domains). *But i am wondering at what point it's too much. *Or what is
> the drawback of having too many subnets and would 8 be considered that
> on a 400 node network. *Thanks


It all depends on the nature of the traffic. If these are all user
stations, I have seen many times where a /22 is used (1000+
addresses), and 4-500 are active during regular usage. If its a mixed
use vlan with real-time traffic (such as video or voice, which would
not be good architecture anyway), then subnetting to smaller subnets
makes more sense. In short, 8 segments for 50 nodes each is
overkill. I would personally not do smaller than /25, and /24 in most
cases, unless of course the network is a retail or small site network
which will never get larger than that space, or there are special vlan
or traffic requirements. When you are talking modern gig speed
networks, broadcast traffic is minimal unless you have an issue (ie
bad worm or virus), or a network design problem.
 
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Trendkill
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      07-21-2008
On Jul 21, 5:28*pm, "Thrill5" <(E-Mail Removed)> wrote:
> There is no drawback to having too many subnets when routing with a 6500!!!!
> Layer 2 switching is not any faster than layer 3 switching and smaller
> subnets means a smaller broadcast domain. *If you are running windows, than
> smaller broadcast domains are good because windows workstations send out
> quite a bit of broadcast traffic.
>
> I never ever, ever use subnets bigger than a /24, and most subnets usually
> have an average of only about 60 workstations. *On my network, each and
> every wiring closet has its own subnet, plus a voice subnet if it is a VoIP
> is being used.
>
>
>
> >"Trendkill" <(E-Mail Removed)> wrote in message
> >news:(E-Mail Removed)....
> >On Jul 21, 2:25 pm, mmark751969 <(E-Mail Removed)> wrote:
> >> we have a 400 node network running on multple 3750 stackable switches
> >> at access layer and a 6509 at the core. We are talking about
> >> subnetting the network. 8 different subnets have been proposed. I
> >> agree that vlan subnetting has it's advantages(i.e reducing broadcast
> >> domains). But i am wondering at what point it's too much. Or what is
> >> the drawback of having too many subnets and would 8 be considered that
> >> on a 400 node network. Thanks

>
> >It all depends on the nature of the traffic. *If these are all user
> >stations, I have seen many times where a /22 is used (1000+
> >addresses), and 4-500 are active during regular usage. *If its a mixed
> >use vlan with real-time traffic (such as video or voice, which would
> >not be good architecture anyway), then subnetting to smaller subnets
> >makes more sense. *In short, 8 segments for 50 nodes each is
> >overkill. *I would personally not do smaller than /25, and /24 in most
> >cases, unless of course the network is a retail or small site network
> >which will never get larger than that space, or there are special vlan
> >or traffic requirements. *When you are talking modern gig speed
> >networks, broadcast traffic is minimal unless you have an issue (ie
> >bad worm or virus), or a network design problem.

>
> The speed of the interface has nothing to do with the amount of broadcast
> traffic, that is a function of the number of devices in the broadcast
> domain. *Each and every broadcast packet is sent to, and processed by, every
> device in the broadcast domain.


I agree, but in the days of 10/half interfaces, broadcast could easily
overwhelm a link or subnet. Today, it would take one hell of a
broadcast storm to overwhelm a modern backbone or LAN link. While I
don't disagree with your subnet solution (can't really go wrong here,
and have to design to your requirements), my experience is in two
global Fortune 50 companies, and when you have dozens of datacenters,
subnetting like that adds up quick on wasted addresses. That being
said, I would not go larger than a /22. And, always subnet real time
traffic if possible, particularly VoIP since the requirements/
segmentation work nicely.
 
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Trendkill
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Posts: n/a
 
      07-21-2008
On Jul 21, 5:28*pm, "Thrill5" <(E-Mail Removed)> wrote:
> There is no drawback to having too many subnets when routing with a 6500!!!!
> Layer 2 switching is not any faster than layer 3 switching and smaller
> subnets means a smaller broadcast domain. *If you are running windows, than
> smaller broadcast domains are good because windows workstations send out
> quite a bit of broadcast traffic.
>
> I never ever, ever use subnets bigger than a /24, and most subnets usually
> have an average of only about 60 workstations. *On my network, each and
> every wiring closet has its own subnet, plus a voice subnet if it is a VoIP
> is being used.
>
>
>
> >"Trendkill" <(E-Mail Removed)> wrote in message
> >news:(E-Mail Removed)....
> >On Jul 21, 2:25 pm, mmark751969 <(E-Mail Removed)> wrote:
> >> we have a 400 node network running on multple 3750 stackable switches
> >> at access layer and a 6509 at the core. We are talking about
> >> subnetting the network. 8 different subnets have been proposed. I
> >> agree that vlan subnetting has it's advantages(i.e reducing broadcast
> >> domains). But i am wondering at what point it's too much. Or what is
> >> the drawback of having too many subnets and would 8 be considered that
> >> on a 400 node network. Thanks

>
> >It all depends on the nature of the traffic. *If these are all user
> >stations, I have seen many times where a /22 is used (1000+
> >addresses), and 4-500 are active during regular usage. *If its a mixed
> >use vlan with real-time traffic (such as video or voice, which would
> >not be good architecture anyway), then subnetting to smaller subnets
> >makes more sense. *In short, 8 segments for 50 nodes each is
> >overkill. *I would personally not do smaller than /25, and /24 in most
> >cases, unless of course the network is a retail or small site network
> >which will never get larger than that space, or there are special vlan
> >or traffic requirements. *When you are talking modern gig speed
> >networks, broadcast traffic is minimal unless you have an issue (ie
> >bad worm or virus), or a network design problem.

>
> The speed of the interface has nothing to do with the amount of broadcast
> traffic, that is a function of the number of devices in the broadcast
> domain. *Each and every broadcast packet is sent to, and processed by, every
> device in the broadcast domain.


I should also qualify that this entire discussion assumes centralized
layer 2, which in itself is becoming a legacy design for very large
networks. Distributed layer 3 is not necessarily cheap, but is
feasible in most l3 switches today, and can help alleviate backbone
traffic, broadcast traffic, and inter vlan routing since subnets are
local to switch pairs only.
 
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