VLANs routed with C3560 and Proxy ARP

VLANs routed with C3560 and Proxy ARP
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Subject Author Date
VLANs routed with C3560 and Proxy ARP Morph 08-10-2008
Posted by Morph on August 10, 2008, 1:08 pm
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We have several offices and use a provider to route betwean the private
networks.
At one of the offices we have the network 192.168.2.0/24 that is routed
and accessible from the other offices.

Now we want to use VLANS in that office. We are planing to use
C2960G-48TC-L as access switch and have a C3560G-24TS-S to route between
the VLANs.

The address of the provider router is 192.168.2.254.

We are planing to create several VLANs:
192.168.2.1-62 /255.255.255.192
192.168.2.65-94 /255.255.255.224
etc...

The routed port of the catalyst 3560 connected to the router will have
the address 192.168.2.253 and the C3560 and C2960 will be connected
through a trunk.

All the VLANs will use their own default gateways set on the Catalyst
3560.

So the diagram will be:

ROUTER---C3560---C2960---VLANS

When the router will try to connect to any of the addresses in the VLANs
it will do so in a way that C3560 will answer through proxy ARP.

Will this work or are we missing something?

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Posted by Martin Bilgrav on August 12, 2008, 3:06 pm
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> We have several offices and use a provider to route betwean the private
> networks.
> At one of the offices we have the network 192.168.2.0/24 that is routed
> and accessible from the other offices.
>
> Now we want to use VLANS in that office. We are planing to use
> C2960G-48TC-L as access switch and have a C3560G-24TS-S to route between
> the VLANs.
>
> The address of the provider router is 192.168.2.254.
>
> We are planing to create several VLANs:
> 192.168.2.1-62 /255.255.255.192
> 192.168.2.65-94 /255.255.255.224
> etc...
>
> The routed port of the catalyst 3560 connected to the router will have
> the address 192.168.2.253 and the C3560 and C2960 will be connected
> through a trunk.
>
> All the VLANs will use their own default gateways set on the Catalyst
> 3560.
>
> So the diagram will be:
>
> ROUTER---C3560---C2960---VLANS
>
> When the router will try to connect to any of the addresses in the VLANs
> it will do so in a way that C3560 will answer through proxy ARP.
>
> Will this work or are we missing something?


you can do VLAN routing in two setups:
1. SVI
2. Routed interface with sub-interface.

to answer your Q:
> When the router will try to connect to any of the addresses in the VLANs
> it will do so in a way that C3560 will answer through proxy ARP.
The router do not want to connect, more likely it wants to forward some
packets to the VLANs.
when the router forwards packets, it looks up its own routetable and forward
accordingly.
so the router will most likely only have the C3560 in its ARP table, as it
will forward packets to the c3560, inorder to reach the VLANs.

Hope this answers your Q.
btw - you should disable Proxy ARP anyhow.

Regards
Martin



Posted by Morph on August 12, 2008, 5:47 pm
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Martin Bilgrav wrote:

| > We have several offices and use a provider to route betwean the private
| > networks.
| > At one of the offices we have the network 192.168.2.0/24 that is routed
| > and accessible from the other offices.
| >
| > Now we want to use VLANS in that office. We are planing to use
| > C2960G-48TC-L as access switch and have a C3560G-24TS-S to route between
| > the VLANs.
| >
| > The address of the provider router is 192.168.2.254.
| >
| > We are planing to create several VLANs:
| > 192.168.2.1-62 /255.255.255.192
| > 192.168.2.65-94 /255.255.255.224
| > etc...
| >
| > The routed port of the catalyst 3560 connected to the router will have
| > the address 192.168.2.253 and the C3560 and C2960 will be connected
| > through a trunk.
| >
| > All the VLANs will use their own default gateways set on the Catalyst
| > 3560.
| >
| > So the diagram will be:
| >
| > ROUTER---C3560---C2960---VLANS
| >
| > When the router will try to connect to any of the addresses in the VLANs
| > it will do so in a way that C3560 will answer through proxy ARP.
| >
| > Will this work or are we missing something?
|
|
| you can do VLAN routing in two setups:
| 1. SVI
| 2. Routed interface with sub-interface.

I used SVI.

| to answer your Q:
| > When the router will try to connect to any of the addresses in the VLANs
| > it will do so in a way that C3560 will answer through proxy ARP.
| The router do not want to connect, more likely it wants to forward some
| packets to the VLANs.

Yes :) I'm not native english speaker so the wording i used wasn't
appropriate.

| when the router forwards packets, it looks up its own routetable and forward
| accordingly.
| so the router will most likely only have the C3560 in its ARP table, as it
| will forward packets to the c3560, inorder to reach the VLANs.
|
| Hope this answers your Q.
| btw - you should disable Proxy ARP anyhow.

The router has an address 192.168.2.254/24 so it thinks that all the
hosts are in the same subnet as the router. I segmented 192.168.2.0/24
into several segments with a SVI as default gateway for every VLAN (like
192.168.2.0/26, 192.168.2.64/26, 192.168.2.128/27 etc.).
The port of the 3560 connected with the router is 192.168.2.253/30.

If I disable Proxy ARP then the router won't be able to send packets to
all the hosts since they are all in different subnets. With Proxy ARP
enabled the 3560 will forward the packets to the hosts.

Regards.

Posted by Merv on August 12, 2008, 8:45 pm
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Any particular reason the provider is not providing you with a dynamic
routing protocol so that you can address subnets at any of your sites
the way you want without having to use kludges like proxy-ARP?

What is the backbone transport technology used by your provider ?

Posted by Morph on August 13, 2008, 3:02 am
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In the message
wrote:

| Any particular reason the provider is not providing you with a dynamic
| routing protocol so that you can address subnets at any of your sites
| the way you want without having to use kludges like proxy-ARP?
| What is the backbone transport technology used by your provider ?

It's MPLS.

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