Cisco subnetting help!!

[sigipa]

Hello All,

I am a newbie. I am in need of some advice on how to subnet a class C that was provided to me by our ISP. The ISP also provides a Cisco 3740 that routes the entire class C network. I have purchased a 3640 with 4 NM-4E cards. I want to create /28 subnets for the entire class C, but I'm not sure about how to configure the 3640. Given the equipment that I have, what would be the right way to do this? I would like to find a way to do this without touching the ISP's router if possible. They are very difficult to work with.

Thanks for your time,
-S

 

[plshlpme]

is your gateway address for your isp part of that class c or have they given you a /30 as well ?

 

[sigipa]

Hi,

Thanks for your reply. The ISP provides a router with the class C on the ethernet interface. We do not have access to the router so I don't know what address range is provided on the T1 interface. We use the class C address space by pointing to the ISP router's ethernet address as a gateway.

Thanks,
-S

 

[sigipa]

Hello Again,

Sorry, I forgot to answer the actual question. Yes. The gateway address is part of the class C.

Thanks,
-S

 

[plshlpme]

ok so you will have 16 ethernet ports that you basically wish to address and use a single port per subnet

you will basically only have 15 /28s because the one you use as a gatway to the isp will be taken.

so you just need to go into each ethernet interface and assign it the first usable ip in each subnet...
if you want more detailed help youll have to post a sho run so we can fill in some blanks...

usually ide have the isp assign a /30 between us and just have them route the class c to us.

as your configured now does the isp have a /24 address assigned to the connection between your two routers?

 

[sigipa]

Hi,

Thanks for your help. It is much appreciated. I've managed to get the config you mentioned working. However, I'm not sure what to do about the route between the ISP and the class C. The ISP has assigned one of the class C addresses with a netmask of /24 to the ethernet interface of their router. In order for the ISP router to route traffic to the Internet, I would need to send traffic to the ethernet interface. It seems like the /24 netmask is going to be a problem. I can assign an address from the appropriate ip range (outside of the other subnets) with a netmask of /24 to one of my router's ethernet interfaces by using ip classless, but will this really work?

Thanks,
-S

 

[plshlpme]

well your router should have a

ip route 0.0.0.0 0.0.0.0 WAN INT

that way your router will send all traffic that it doesn't know to your isp router. the isp router should only forward your class c to you.. so it should work.

 

[sigipa]

Hi,

I have setup the config you recommended. It works, but performance is really bad. If I give myself a /24 address all is okay. Have I missed something? Here's my config:

Building configuration...

Current configuration : 948 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname xxx
!
boot-start-marker
boot-end-marker
!
enable secret 5 xxx.
enable password xxx
!
no aaa new-model
!
resource policy
!
no ip subnet-zero
!
no ip cef
!
interface Ethernet0/0
ip address x.x.x.x 255.255.255.240
full-duplex
!
interface Ethernet0/1
ip address x.x.x.x 255.255.255.240
full-duplex
!
interface Ethernet0/2
ip address x.x.x.x 255.255.255.240
full-duplex
!
interface Ethernet0/3
ip address x.x.x.x 255.255.255.240
full-duplex
!
ip http server
no ip http secure-server
ip route 0.0.0.0 0.0.0.0 x.x.x.x
ip route 0.0.0.0 0.0.0.0 Ethernet0/3
!
snmp-server community xxx RO
!
control-plane
!
line con 0
line aux 0
line vty 0 4
password xxx
login
!
end

Thanks,
-S

 

[plshlpme]

is e/3 the interface facing your isp?
what is the speed of the link?

 

[sigipa]

Hi,

Yes. e3 is the link to the ISP router. The speed should be 100mbs.

Thanks,
-S

 

[plshlpme]

these are just 10mb ethernet ports though.. not fast e's??

 

[sigipa]

Hi,

Yeah, I meant that my router plugs into a 10/100 switch. I believe the ports on the ISP router are 100 though. At any rate the internet connection is 4.5M which the 10mb should handle. Any idea what the performance problem could be? Is it the fact that I've got a subnetted class C sending traffic to a router (ISP) that has an address in the same class C with a different netmask?



Thanks,
-S

 

[plshlpme]

that part shouldn't matter because your router is not sending data to that routers address.. it should just be forwarded through to the isp regardless.
have you tried doing some trace routes to see at which point it is being held up

 

[sigipa]

Hi,

Well.. it looks like it's just a basic capacity issue. If I pull all of the wics but one, all is okay. Performance degrades as I add each subnet. I created an identical config on a linux server and it does not have any of the performance issues.

I sincerely appreciate the time and interest of plshlpme. I hope I can return the favor someday.

-S

 

[plshlpme]

you might be better off using a fast e connection and trunking to a switch and creating a vlan for each /28. that would be alot less work for the router...
you would have a bunch of subinterfaces on your faste interface going to a 24port switch which you could assign vlans to and have the same overall affect as what you are trying to do now.