Splitting/Shrinking Broadcast area

[TechPaulJohn]

Good morning,

I *think* my problem is pretty basic, but I want to make sure I do this right.

The system I work with has 7 sites. 4 of the sites are truly 'remote' from the other three geographically, and so their traffic is isolated from the main site by routers.

Of the 3 remaining sites, 2 out of the 3 are currently on the same subnet, and so broadcast traffic carries across from site to site. This probably isn't a huge problem, but we'd like to 'break off' the other site, and shrink the broadcast domain a bit.

The connection from Site 1 to Site 2 is via fiber. From a 6509 at site 1 to a 3560 at site 2.

My thought was to create a vlan, put both endpoints of that link into the vlan, then update IPs at site 2 into their new subnet (10.NN.HHH.HHH). Make sure Site 2's equipment looks to the 3560 as their gateway...then...well, that's where I'm not sure if I'm missing something.

Will I have to manually configure routing to the new subnet, or will the first couple of PCs that come online 'teach' the location of the new subnet to the rest of the network? Based on I see now, data destined for that range of IPs is already heading for the 6509.

Or, could I do it even simpler by simply assigning the port on the 6509 an address in the new subnet and use that as the gateway IP for the Site 2?

Advice, links, or other information is appreciated.

Thanks,

Paul

 

[ADB100]

You are fundamentally correct - your network should be 'broken down'............

BUT based on the comments from the 3rd paragraph onwards I suggest you get someone in with more knowledge and experience to throw together a small design or you are likely to cause yourself some grief.

Apologies if this doesn't help but what you are asking has implications that are likely not to get covered on a forum and will end up with you being the 'cause' of the problems.

Andy

 

[TechPaulJohn]

Well, things being what they are, I don't expect we'll be paying anyone to help with this. Fortunately, I've got some time to do more research.

Then, I'll just have to hope that with the SmartNet contract for the 3560 I can get a little tech support from Cisco to help me smooth out any rough edges. That plus calling in a favor or two.

Thanks,

Paul

 

[NetworksMD]

If you are going to break up your network into subnets or VLAN, and communication is needed between devices in the different subnets or vlan's, you will need to router to route the packets between different subnets or vlans. If you do use VLANs you will also have to set up VLAN trunking between the switches and router. You will have to manually set up routing for these subnets in the router. PC's only teach themselves about other devices, and they do so by doing a broadcast ARP of an IP to populate it's ARP table with a mac address. Layer 2 doesn't understand anything about IP addressing or routing. If there is no response to an ARP request, then the PC uses its default gateway. Look up interVlan routing for more info and help.

 

[TechPaulJohn]

I may have mis-spoken when I said 'teach the rest of the network'. What I meant was that when my 6509 switch, which can handle Layer 3, sees traffic coming from the new subnet, it would learn that the new subnet was located through that particular port/vlan.

And I had forgotten that if I set up the vlan with those two end points that yes, I would have to make that link a trunk as well.

We did something simlar last summer with Site #3, but in that case, we actually put the entire site as its own vlan. In between we had a strong Cisco person looking at that portion of the network, and he said that because the 6509 can handle layer 3, we could have made the break without setting that whole site up in the seperate vlan.

Funny though, I just got an email from his company, and it turns out I may have a few available tech support hours, so maybe I *can* get some high dollar help to assist with the change-over.

While I don't expect anyone to tell me every change that I need to make, comments, and issues to try and avoid are appreciated.

Thanks,

Paul

 

[vipergg]

For what you are saying sure I would put the layer 3 SVI 's definitions on the 6509 seeing that you have a fiber link over to the 3560 . I'll go on the assumption that your 6509 is running native IOS , if it's hybrid then thats a different story and config altogther. Once you configure the layer 3 SVI definition on the 6500 it will know how to get to that subnet. If you are running any kind of routing protocol then you would have to make sure your network statements cover any new vlans you add. Your clients default gateway will be pointed to the address on the 6509 not the 3750 because he is doing the routing .

6509
conf t
vlan 5
exit


interface vlan 5
ip address 192.168.5.254 255.255.255.0


interface g1/0 (interface to 3560)
switchport
switchport access vlan 5

--------------------------------------------------------

3560 config

conf t
no ip routing
enter
type vlan 5 then hit enter
exit

interface vlan 5 (This is for managing the switch only,no routing involved on this switch)
ip address 192.168.5.250 255.255.255.0
no shut

conf t
ip default-gateway 192.168.5.254 (this is the gateway for all client pcs also,the address on your 6509)

-----------------------------------------------------------
All ports on the 3560 would have to be put into vlan 5 .

interface g1/0/1
description link to 6509
switchport
switchport access vlan 5