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house of cards puzzeler

My clients router has the Fe0/0 configured with an ip address and 3 secondary addresses. The Fe0/0 is pluggeded into a switch that doesn't support VLANS. (Older non-cisco switch)

The 4 networks (flat on the same wire in the switches) are then moved out with static routes. Until the client gets funded and is able to purchase more modern hardware I would like to move these networks to sub-interfaces and add a routing protocol because they are having experiencing issues. (Surprise?) When I try to configure the sub-interfaces it tells me that the trunk port needs to be configured. I have the ability to migrate them to a 3560 Switch as well. Thanks in advance.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Gold

Re: house of cards puzzeler

jason

If you have 4 logical subnets in 1 flat network then secondary addressing is the only alternative that works for you if the switch does not support VLANs.

Trying to do subinterfaces on the router interface requires trunking to be enabled. And Jon is quite right that if the existing switch does not support VLANs then it would not support trunking.

If you are able to migrate to a 3560 switch (as you indicated in the original post) it would provide a solution for this situation. The 3560 does support VLANs and trunking. So you could configure the 4 VLANs on the 3560 and do inter-vlan routing on the 3560. If you wanted you could use the old switch as an access switch in a single access VLAN and connect it to an access port on the 3560. You would have a choice of how to connect the 3560 to the router. You could make the connection between the 3560 and the router a trunk and extend each VLAN to the router. Or you could make the link between the 3560 and the router a routed link with its own subnet. Of these options I would prefer the routed subnet option (I see little reason to extend the VLANs to the router if the 3560 is doing inter-vlan routing). But it would be a decision that you make as seems best to you.

HTH

Rick

11 REPLIES
Hall of Fame Super Blue

Re: house of cards puzzeler

Hi

The port on the switch that the router is connected to must be configured as a trunk port running 802.1q tagging.

If the switch doesn't support vlans, then it won't support trunks which mean you can't run subinterfaces on your fe0/0 interface.

Jon

New Member

Re: house of cards puzzeler

If I use the 3560 switch would it work if I used SVI's and assigned the ip address of the default gateway of the differient subnets configured on the "multi networked" switch? Configure IP routing on the 3560 to route the differient networks in the backplane?

Hall of Fame Super Blue

Re: house of cards puzzeler

Hi

Yes, if you have a 3560 then you can use that to the inter-vlan routing.

I didn't realise you had a L3 switch.

** Edit. Just seen Narayan's and Rick's postings. I agree that you should use the L3 switch rather than subinterfaces on the router as using subinterfaces each vlan will not get the full bandwidth of the router interface whereas the limitation on the switch is the backplane **

Jon

Re: house of cards puzzeler

Yes,

The best option would be to use the 3560 switch and create 3 vlans each with their respective SVI's.

you can then enable ip routing and use static or dynamic protocols to route to different networks

HTH

Narayan

Hall of Fame Super Gold

Re: house of cards puzzeler

jason

If you have 4 logical subnets in 1 flat network then secondary addressing is the only alternative that works for you if the switch does not support VLANs.

Trying to do subinterfaces on the router interface requires trunking to be enabled. And Jon is quite right that if the existing switch does not support VLANs then it would not support trunking.

If you are able to migrate to a 3560 switch (as you indicated in the original post) it would provide a solution for this situation. The 3560 does support VLANs and trunking. So you could configure the 4 VLANs on the 3560 and do inter-vlan routing on the 3560. If you wanted you could use the old switch as an access switch in a single access VLAN and connect it to an access port on the 3560. You would have a choice of how to connect the 3560 to the router. You could make the connection between the 3560 and the router a trunk and extend each VLAN to the router. Or you could make the link between the 3560 and the router a routed link with its own subnet. Of these options I would prefer the routed subnet option (I see little reason to extend the VLANs to the router if the 3560 is doing inter-vlan routing). But it would be a decision that you make as seems best to you.

HTH

Rick

Hall of Fame Super Gold

Re: house of cards puzzeler

Jason

I am glad that my ansers were helpful. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that they will read a response that did resolve the question.

I encourage you to continue your participation in the forum.

HTH

Rick

New Member

Re: house of cards puzzeler

I have one final question. To configure the SVI on the 3560 switch I create vlans with the IP address of the routers secondary address (remove the address from the router first of courst) enable ip routing on the switch. Here's the question.... I plug one cable into the legacy switch and all of the devices arp to go to the new Vlan interface of the switch. They are routed in the backplain of the switch. Do I need assign ports to the Vlans or does the routing accour within the backplain. I appoligize but I don't have a test environment. Thank you in advance.

Hall of Fame Super Blue

Re: house of cards puzzeler

Jason

The devices connected to the legacy switch should all be in the same vlan and the port on the 3560 that the legacy switch connects to should be allocated to that vlan as well.

Any devices connected directly into the 3560 should have their ports allocated into the relevant vlans.

Does this answer your question ?

Jon

Hall of Fame Super Gold

Re: house of cards puzzeler

Jason

I do not understand clearly your question. I will explain what I think it is asking about and if I do not answer what you need then perhaps you can clarify the question.

On the 3560 you will enable ip routing. Then you will configure SVIs. If you have 4 VLANs then you need 4 SVIs (interface vlan x). Then you assign IP addresses to the vlan interface on the switch (and it would allow the end stations to continue using the default gateway that they are currently configured to use if you move the secondary addresses from the router and use them on the vlan interfaces). Then you need to assign ports on the 3560 to the appropriate VLAN. You can use the original switch if you want to. All of its ports will be in a single VLAN. You would connect the original switch to some port on the 3560 which would be configured as an access port (not a trunk port) and you assign the port on the 3560 to whichever VLAN you want the original switch to belong to.

I hope that covers your question. If not please clarify.

HTH

Rick

New Member

Re: house of cards puzzeler

Sorry for the confusion. The design and functionality of this doesn't follow anything I have seen since the early 90's. The legacy switch (access level) doesn't have any configuration on it besides a management IP address. So all of the devices are in one domain with four IP networks running on it. They (the workstains plugged into the legacy switch) find the IP network configured on the router interface/secondary interface. They all connect via the same cable from f0/0 to the switch. My question would be do I configure an access port for each network on the 3560 switch and need to plug a individual cable to the legacy switch for each network or can I get all of the networks on the legacy switch to come into the new 3560 switch and route into the backplane? Thanks once again.

Hall of Fame Super Gold

Re: house of cards puzzeler

Jason

It feels like there has been a significant failure to communicate on some basic concepts. So let me try again. In your present environment there is a single VLAN on the switch which connects to the router. The single VLAN is a single broadcast domain in which there are 4 subnets. Every device in any subnet will hear all the broadcasts from every device no matter what subnet they are in. And any device could communicate directly with any other device in the VLAN no matter what subnet they belong to.

We tend to think of subnets as providing separation for devices within the subnet and tend to think that for a device in one subnet to communicate with a device in a different subnet that it must go through a router to route between subnets. This is true if the assumption is true that there is a one to one relationship between subnet and VLAN. If each VLAN has a single subnet and if each subnet is in only a single VLAN then communication between subnets must go through a router. But your implementation puts 4 subnets into a single VLAN and they are not separated from each other.

Part of our discussion has been about using a 3560 switch and implementing VLANs on that switch. Using the switch and configuring VLANs would restore the assumption about a one to one relationship between VLAN and subnet. And the 3560 could do the inter VLAN routing. To do this and for it to work, then all devices which remain connected to the legacy switch must be in a single subnet. If you need to keep devices on the legacy switch in multiple subnets then there is no point in trying to do VLANs anywhere. So a fundamental question that needs to be answered is whether you need to keep the legacy switch operating the same way or can it change? If it changes then it will have a single VLAN and have a single subnet, and will need only a single connection from the legacy switch to the 3560.

HTH

Rick

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