I've inherited a moderate size network that I'm trying to bring some sanity to. Basically, its 8 public class Cs and a slew of private ranges all on one vlan (vlan1, of course). Most of the network is located throughout dark sites.

I need to start separating some of the network. I've changed the ports from the main cisco switch (3560) to the cisco router (3825) and the other remote switches to trunking with dot1q encapsulation. I'd like to start moving a few select subnets to different vlans.

To get some of the different services provided on our address space (and to separate customers) on to different vlans, do I need to create a subinterface on the router for each vlan and, if so, how do I get the switch port to work on a specific vlan? Keep in mind, these are dark sites and geting console access is difficult if not impossible at the moment. I was planning on creating a subinterface on the router for each vlan then setting the ports with services I want to move to a different vlan to allow only that vlan. Example of vlan3:


interface GigabitEthernet0/1.3
description Vlan-3
encapsulation dot1Q 3
ip address

the connection between the switch and router:

interface GigabitEthernet0/48
description Core-router
switchport trunk encapsulation dot1q
switchport mode trunk

show interfaces gi0/48 switchport

Name: Gi0/48
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk private VLANs: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL

Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none

So, if the boxen hanging off of gi0/18 on the 3560 are on an unmanaged layer2 switch and all within the range and are using as their gateway, what is left to do, especially to gi0/18, to get this working on vlan3? Are there any recommendations for a better setup without taking everything offline?


Pardon, in your cut and pasted configs, you appear to be describing Gi0/48 - your uplink to your router, but in your question refer specifically to hosts connected to Gi0/18. I'm going to assume you're describing two different ports here. Further, I'm assuming from details in your config statements and question, that vlan 3 is being used for the traffic. I'm going to assume that the vlan has already been declared on your 3560. (Check sh vlan)

First of all, your port Gi0/18 should be configured for access mode on vlan 3. Likely, something like this:

interface GigabitEthernet 0/18
switchport access vlan 3
switchport mode access

As far as for other recommendations. Will all/most of your traffic from your IP subnets be to and from the internet. Basically, If you have enough traffic between subnets, it may suit you to have the 3560 act as your internal router and then dedicate your 3825 to be your border router. The problem is that if your router is baring the entire load for all routing, then a packet from one subnet will arrive at your switch, then be forwarded via the dot1q to your trunk on some vlan X, the router then makes a routing decision and sends the same packet back along the dot1q trunk on some new vlan Y now destined for the destination machine. Btw, I'm simply describing the situation of internal traffic to your customers/organization that crosses your different subnets.

Instead, you can configure the 3560 at the, assuming normal conventions, first address of each vlan/subnet. E.g. and enable ip routing. The next step is you create a new subnet specifically for between the router and switch. For convenience, i'd use something completely different, for example, is reserved for documentation examples. Configure the router at and the switch at Have the switch use as the default route. Configure the router to reach via the switch at If your network is small enough, static routes should be sufficient. No need for OSPF or anything.

Of course, this would be a rather dramatic change; but it has potential for being a large improvement. It all depends on the nature of your traffic.

For reference, cisco lists the Cisco Catalyst 3560G-48TS and Catalyst 3560G-48PS having a 38.7 Mpps forwarding rate and the Cisco 3825 as having 0.35Mpps forwarding rate. Mpps, just in case you don't know, is millions of packets per second.

It's not bandwidth, but it's how many 64 byte packet routing decisions the device can make a second. The length of the packet doesn't affect how long it takes to making a routing decision. So the peak performance in bits/bytes will be somewhere in a range. In terms of bandwidth, it means that 350kpps is 180Mbps w/ 64byte packets and 4.2Gbps w/ 1500 byte packets. Mind you, that's in bits per second, so think of it as 18 Megabytes or 420 Megabytes per second in regular file-size terms.

In theory, this means that your 3560G can route somewhere between 19.8Gbps and 464Gbps or rougly 2GBps and 45GBps.

Actually, looking at those numbers, you most definitely should consider the plan I described above. Dedicate your 3825 to handling, presumably, NAT'd external traffic and let your 3560 handle the rest.

I'm sorry this is so long; I'm bored at work waiting for tapes to finish. Cheers.

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