integrating new 3550 with routing into existing routing structure?

Greetings all,

I've got six 3550-24 switches with three vlans vlan_a vlan_b and vlan_c
set up across all six 3550s. They are interconnected with gigabit fiber.

Vlan_a 10.77.11.0/24
Vlan_b 192.168.11.0/24
Vlan_c 192.168.0.0/24

Up until recently we had a router (cisco, not sure what model, but old)
that was connected between vlan_a and vlan_b (vlan_c is separate, and
does not factor into this question) and handled routing between the two,
as well as to the rest of the network.

Eth0 on the router is 10.77.11.1
Eth2 on the router is 192.168.11.1

Both of these ip’s are the gateways for the hosts on their respective
subnets.

So a host on vlan_a needs to talk to a host on vlan_b, it sends it’s
packets out of the box, to the router’s eth0, out the router’s eth2
back to the 3550 to vlan_b, and vice versa.

If a host on vlan_a needs to talk to somewhere else, the packet goes out
of the 3550, up to the router’s eth0, and out one of the router’s
other ports to wherever.

I recently got a 3550 with the routing capability. So for various
reasons, I want to use the 3550 routing to route between vlan_a and
vlan_b.

I understand how to do this is to make the vlan_a the subnet’s gateway
by assigning it the ip of the subnet’s gateway 10.77.11.1.

Ditto for vlan_b 192.168.11.1

A few more route and network statements, and presto chango, 3550 is
routing between 10.77.11.0 and 192.168.11.0. I’ve got that working.

But now, I’m kinda stuck.

1. How do I integrate this into the rest of the network, so that packets
not destined for 10.77.11.0 or 192.168.11.0 go where they are supposed to
go?

2. What do I do with the old router’s ip addresses on the eth0 and
eth2, the previous gateway addresses? I’m assuming I can still leave
them plugged into their respective cat 5 ports (router eth0 to some 3550
port on vlan_a, router port eth2 to some 3550 port on vlan_b) to provide
physical connectivity, but what do I do with the router’s ip’s. Now
there are two 10.77.11.1 and 192.168.11.1, which I’m pretty sure is a
bad thing...8)

I think I’ve probably described this badly, and I apologize. If I can
clarify, just ask. And I’m sure this is a common problem with an easy
solution. But I’m stumped. Any pointers? Thanks in advance!

 

Hi,

I would configure the network you describe like this:
(Pre-condition; all switches already know vlan-a,b and c in their
vlan-databases.
commands to do this:
- vlan database
vlan 1 name vlan-a
vlan 2 name vlan-b
vlan 3 name vlan-c

- Switch A:
ip routing
interface vlan1
description vlan-a, gateway interface for hosts on vlan-a
ip address 10.77.11.1 255.255.255.0
no shutdown
interface vlan2
description vlan-b, gateway interface for hosts on vlan-b
ip address 192.168.11.1 255.255.255.0
no shutdown
interface FastEthernet0/1
switchport mode access
switchport access vlan 1
interface FastEthernet0/2
description ToRouter
switchport mode access
switchport access vlan 1
...
interface FastEthernet0/23
switchport mode access
switchport access vlan 2
interface FastEthernet0/24
switchport mode access
switchport access vlan 2
interface GigabitEthernet0/1
switchport mode trunk
ip route 0.0.0.0 0.0.0.0 10.77.11.2 ! Direct everything else to the
router (which is using 10.77.11.2)

Switch B / C / D / E / F:
Like switch A, but without the virtual Vlan interfaces. You can
leave one of them there if you want, for management purposes, but use
different addresses!. Also don't enable "ip routing"".

Router:
interface ethernet0
description ToSwitchA
ip address 10.77.11.2 255.255.255.0
interface ethernet1
description To Vlan-C
ip address 192.168.0.1 255.255.255.0 ! Don't specify IP adres and
shutdown interface if you want to keep vlan-c isolated.
interface ethernet2
shutdown ! You don't have to use this port anymore, routing to
vlan-b is done by switch.
no ip address
ip route 192.168.11.0 255.255.255.0 10.77.11.1 ! Direct traffic for
192.168.11.x to switchA

Leave the router's default gateway like it is if you have more networks
that are already connected to this router.

Hope this is sufficient!?

Erik

 

Thanks!

That's kinda what I suspected. This has to be a common problem, but I
could not decide whether to :

1. do it like you described, or

2. no ip the router ethernet0

interface ethernet0
description ToSwitchA
no ip address

and then

switch A:
ip route 0.0.0.0 0.0.0.0 Gi 0/1.

That is, send the default route out a port, rather than a destination ip.

or

3. Something incredibly easy that I was completely missing.

And as I said, this has got to be a common problem, so I was wanting to
know what best practices are.

Thanks again.
--------------------------------------

<snip>

 

Only use routing via an interface when it is a point to point link (as
in, there is only one person on the other end). Otherwise always route
via an IP.

Follow this rule and it will hold you in good stead.