Some questions on Routing and Switching

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b@nD

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Hope some of the certificated dudes can help out


I have some questions -- and -- confusion in respect of addressing , switching , trunking and routing.

I would really appreciate some direction or a config of the correct way of doing this


I have an ADSL router that I would like to attach to a Layer three switch ( the router does not have any switched ethernet add in cards so NO VLAN's on the router )

The drawback of Layer three switch is that it is not really a WAN device. It cannot do NAT / PAT or firewall ( CBAC / Zones ) and does not have WAN interfaces ( but it does have wire-speed ASICS )

I am familiar with the concept of "Router-on-a-Stick" and sub-interfaces

The Scenario

I have two subnets in private address space that need to be put into VLAN's but also need to be natted ( PAT ) and firewalled ( yes I know I need an ASA )

I would like to set one of the eth int of the router to a specific subnet ( it's IP address )
and
The other int to the other subnet ( subnet IP address )

( In essence two interface router on a stick but without trunking. I am not using the two eth interfaces for anything else )

Is there any other way of connecting a router to a layer three switch ? )

I can and have set up trunking and got that working but I am confused in respect of management VLAN's ( not needed on a Layer three if ip routing is enabled ) and the VLAN's that are going to be used for traffic.

eg

Say on the router
int fa 0/1 is configured as 192.168.10.1 ( Vlan 10 )
int fa 0/2 is configured as 192.168.20.1 ( Vlan 20 )

So the cabling of the router physical int goes to the 3550 on two separate physical ports ( fa 0/10 and fa 0/20 )

IF there are NO sub-interfaces on the eth on the router ie. trunking is not set
then
Should the ports on the 3550 be set as VLAN switched ports
or
Directly assigned routed IP addresses ( same subnet as router )

VLANs will have to be set on the 3550 so that hosts can be assigned to their respective VLAN sub-nets
ie. VLAN 10 and VLAN 20 -- these SVI's will also have to have IP addresses. ( acts as host gateway )

The problem is you cannot have an SVI ( VLAN ) in the same IP address space as a routed port

ie.
VLAN 10 is say 192.168.20.6
and
port fa 0/20 is assigned 192.168.20.3 ( to put it into the same subnet as the router )

then an error message is generated !


I surmise that if I just assign the physical ports that the connections from the router are attached to into their respective VLANS then this will all work ?
ie.
The 3550 ports are assigned as ACCESS ports in their respective VLANs ( SVI's ) ?

Inter-VLAN-Routing will be set up to take place in the 3550

----------------

The other question

Does a management interface on a layer two switch need to be in the "routed" Vlan or should it be in a seperate VLAN of its own ?

I assume that if it is in a separate / separately addressed VLAN then the ip default-gateway command will allow it to be reached from other hosts ?

Apart from confusion I am keen to know what the correct "industry-standard" way is for setting this up.

OK -- fire away and tell me I am DOF and missing what is obvious .......... ( I learn slowly ) :)
 
Im not 100% sure what you want here.

But, if you just assign the ports to each vlan respectively.
Then obviously the router will be doing your routing between vlans.

If you create an SVI, you can assign that to the vlan, and have layer 3 between the router and the switch (and from the switch on multiple ports on the same vlan).
The switch will obviously then be able to route between different SVI's.

For your management question,
on a strictly layer 2 switch, then yes, your ip default-gateway command will use the next hop IP to break out of its current vlan/network.

I am not a R&S guy, but i am pretty sure the above is correct
 
If i understand it correctly... what you need to do is the following.

Use routing...

Create a ptp link between your rtr and l3 switch. example 10.0.0.1 255.255.255.252( router) 10.0.0.2 255.255.255.252 ( switch ). On the switch use the no switchport command on the port connected to the Router.

Then make sure not just to create the vlans but create the interface vlans as this will make them routed vlans :)

Then setup your default route to point to 10.0.0.1 and do the natting on the router ( layer 3 switches add a default route*).

thus

(router) Fa0/1 10.0.0.1 255.255.255.252
| network 10.0.0.0 255.255.255.252
[SWITCH] Gi 0/1 10.0.0.2 255.255.255.252
make the one port a routed port with the no switchport command. conf t , int vlan10 ( ip address 92.168.10.1 255.255.255.0 ) + int vlan20 ( ip address 92.168.20.1 255.255.255.0 ) + default route
(ip route 0.0.0.0 255.255.255.255 10.0.0.1)

Correct?


ooohhhh just read 3550 ... burn it and get a layer2... As i hate that switch... the devs at Cisco was smoking blunt when they created that things vlan management interface.
 
Last edited:
Overlapping Range

syntax said:
Im not 100% sure what you want here.
Click to expand...
I am -- kind of :)
I need to write in order to clarify my thoughts
syntax said:
But, if you just assign the ports to each vlan respectively.
Then obviously the router will be doing your routing between vlans.
Click to expand...
I suspect that because the two eth ports on the router are directly connected they will be able to talk to each other ?
It is only two VLAN's so not really a sweat if the inter vlan routing happens in the router -- but then why have a Layer Three switch ? ( to torture yourself on of course )
syntax said:
If you create an SVI, you can assign that to the vlan, and have layer 3 between the router and the switch (and from the switch on multiple ports on the same vlan).
The switch will obviously then be able to route between different SVI's.
Click to expand...
Correct -- Layer Three Inter-VLAN-Routing -- I forget that a VLAN ( SVI ) is layer three
syntax said:
For your management question,
on a strictly layer 2 switch, then yes, your ip default-gateway command will use the next hop IP to break out of its current vlan/network.
I am not a R&S guy, but i am pretty sure the above is correct
Click to expand...
The default gateway must be in the same subnet as the management VLAN ?

Yes but unfortunately it does not answer the snafu

Say I have an interface VLAN 4 with an address of 192.168.20.4 /24 configured on the 3550
( Other ports can then be assigned to VLAN 4 -- switchport access vlan 4 )

but

I configure one of the 3550 fa ports as a routed port ( to attach to the router )

fa0/4 no switchport
ip address 192.168.20.10

I then get an error message saying that the address space overlaps and cannot be configured ?

I tried different masks but still the same error message ?

THIS is what has got me confused ?
 
UUMMM what's so confusing about it? Can't you see it's overlapping?

If you want to connect a router to fa0/4 you don't need to assign a IP to that port on the switch. You already have that range routable on VLAN4 on the switch, so you can't assign another IP in that range to another port on the switch. Make the native VLAN of port 4 VLAN4...


Then again all depends on the VLAN's on the router side, you might need to draw a diagram so that we can help...
 
Last edited:
You can still use the 3550 for routing but you will have to leave the nat up to the ADSL router.


ADSL router <-----> 3550 routed port

Whatever ip space you use on the vlans on the 3550, you will have to create static routes on the ADSL router to point those to the 3550, and then I hope you ADSL router can be set to NAT those too outbound over ADSL.

And yes, ip subnets can only be assigned to either a single routed port, or a single vlan. The Cisco IOS is actually helping you by preventing you from making silly mistakes that can't work.
 
Fanks ........

Tinuva said:
Whatever ip space you use on the vlans on the 3550, you will have to create static routes on the ADSL router to point those to the 3550,
Click to expand...
Thanks will try this -- need to fully understand the concept of "directly-attached" when and how this works as opposed to having to set up routes ( and what kind of routes )

I am probably going to be using "route-maps" .......... ( I have a complete working setup on an 877 [which has four switchable ports and is actually an incredibly powerful little machine ] that I am now going to move over to an 1841 [which does NOT ] )

I would really like an 2811 -- lots of places to put lots of "goodies" :)

Tinuva said:
And yes, ip subnets can only be assigned to either a single routed port, or a single vlan. The Cisco IOS is actually helping you by preventing you from making silly mistakes that can't work.
Click to expand...
Yes thanks -- kinda what I thought -- but now confirmed.

I DID say I learn slowly :D
 
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