L3 switching config question.

[skk391]

Hi all,

I have a question relating to L3 switching. I am refering to an example on the Cisco website. URL :

http://www.cisco.com/image/gif/paws/41860/howto_L3_intervlanrouting.pdf

if anyone is interested.

Router (200.1.1.2 /24 ) -> Catalyst 3550 -> Layer 2 Switched Network -> 3 Vlans Configured
1) Vlan 2 - 10.1.2.2 /24
2) Vlan 3 - 10.1.3.2 /24
3) Vlan 10 - 10.1.10.2/24


If I needed the devices connected to the Layer 2 network to be able to communicate with each other I would need to configure the Vlans on the Catalyst 3550 to make it aware of the Vlans, ( or setup a VTP domain name so that the switch became automatically aware of the Vlan is this correct? )

My question is that I thought the routing was actually dont by the L3 switch i.e the Catalyst 3550 but according to the Config on the example used by Cisco they have the following command on the Catalyst

ip route 0.0.0.0 0.0.0.0 200.1.1.2

My understanding is that this statement is called the 'gateway of last resort ' and that if the Catalyst doesnt know where the route the packet it will send it to the Router (ip address 200.1.1.2 /24) so isn't it the router which is doing the routing and not the Catalyst switch?????

What happenes if there isnt a routing within the network? Only the L3 switch how would this then be configured?

Have I got this all confused in my head, but I thought that the L3 switch should be doing the nrouting shouldnt it???


Thanks

 

[stubnski]

Hi,
If you look at the network diagram in the PDF you will see Interface Vlan 2, Interface Vlan 3, and Interface Vlan 10 next to the L3 switch. Those are layer 3 interfaces that can route between Vlans.


==>My question is that I thought the routing was actually dont by the L3 switch i.e the Catalyst 3550 but according to the Config on the example used by Cisco they have the following command on the Catalyst
ip route 0.0.0.0 0.0.0.0 200.1.1.2

IP route 0.0.0.0 0.0.0.0 200.1.1.2 is the default route for all networks that the switch does not recognize (every network except for the three vlans)

Here is a Cisco doc on ip route 0.0.0.0 that will help you understand more about how it works Link

==> What happenes if there isnt a routing within the network? Only the L3 switch how would this then be configured?

L3 switches can also route. Enable routing and have the L3 vlan interfaces on the switch and voila, your switch is routing.


Hope this helps

 

[allworxguy]

In a router/L2 switch configuration with multiple vlans, all traffic destined for another vlan will be sent to and processed by the router even if the hosts are on the same switch.

In router/L3 switch configuration, traffic destined to another host on the same switch on a diffent vlan is processed by the switch, esentially turning the switch into a "local" router. This has the benefit of reducing traffic to the router to only addresses not on the switch and speeding up local switching. The line ip route 0.0.0.0 0.0.0.0 next hop address, takes care of addresses unknown to the L3 switch.

Marv ccna

http://www.telemarv.ca

 

[skk391]

Thanks guys, i thought that all the traffic was getting passed to the router. I dont see any commands on the switch to route between the VLAN's. something like ip route 10.1.2.0 255.255.255.0 etc, only the ip routing command. I take it that when this is run the switch then routes between all config VLAN's ?

 

[VinceWhirlwind]

I think your question is answered further down the document where it shows the Layer-3 switch's routing table:

Cat3550#show ip route
<snip>
Gateway of last resort is 200.1.1.2 to network 0.0.0.0
200.1.1.0/30 is subnetted, 1 subnets
C 200.1.1.0 is directly connected, FastEthernet0/48
10.0.0.0/24 is subnetted, 3 subnets
C 10.1.10.0 is directly connected, Vlan10
C 10.1.3.0 is directly connected, Vlan3
C 10.1.2.0 is directly connected, Vlan2
S* 0.0.0.0/0 [1/0] via 200.1.1.2

The 3550 has routes for the subnets it has interfaces in. It has a default route for anything else.