Layer 3 switching vs Routing

I'm happy with Layer 2 switching using the MAC address to determine where the frame should go. I'm also happy with Layer 3 routing examining the destination IP address of the packet and the routing table to determine how to forward (or drop) the packet, but what about Layer 3 switching? I've read that it uses the destination IP address to determine what to do and, whilst it's similar to routing, it's faster and tends to be done with hardware (presumably rather than software in a router?). I've also read that some modern switches have some routing capability and some modern routers have some switching capability.

Despite understanding this so far, I still don't *really* know why I'd want to use Layer 3 switching rather than routing. What are the circumstances in a real live network when one or the other would be preferred? What are the *actual* differences between Layer 3 switching and routing? It strikes me that they are very similar but I suspect that won't suffice when I come to take the exam!

You would want to use Layer 3 switching when dealing with interVLAN's. When you configure VLANs you are trying to control the size of the broadcast domain and keep all the traffic local. But when VLANs need to communicate with other VLANs, interVLAN communication is required. This type of communication is supported by interVLAN routing. A layer 3 type switch has L3 routing protocols to be used for interVLAN routing.

Now depending on the L3 switch you use, there will be some routing limitations. Here are two examples:

(Catalyst 3750 or 3560 switches) The switch does not support tunnel interfaces for unicast routed traffic. Only Distance Vector Multicast Routing Protocol (DVMRP) tunnel interfaces are supported for multicast routing.

(Catalyst 3750 or 3560 switches) A route map that has an ACL with a Differentiated Services Code Point (DSCP) clause cannot be applied to a Layer 3 interface. The switch rejects this configuration and displays a message that the route map is unsupported. There is no workaround.
***These are release notes for a Catalyst 3750 and 3560.


Cheers,

ZiPPy

Layer three switches forward packets using hardware approx6.6 million pps . But routers forward packets based on hardware approx 60,000 packets per sec .

Second layer three switches can route between ehthernet interfaces only but routers support multifarious interfaces and are capable of routing between them .

Other than that they both are the same ...

Thanks
Patiot

Thanks for the feedback.

@ZiPPy - I see that VLANs are approaching so haven't covered them yet. I was under the impression that they required a router to route between the different VLANs, but apparantly not. A lot of your response went over my head but it should all fall into place over the next few weeks. I really hate it when Cisco throw something new into the equation because I'm the sort of guy who likes to understand everything. If they said something like "Layer 3 switching is similar to routing and we'll cover this topic in further detail when we cover VLANs in chapter X", I'd be content to wait!

@Patiot - wow, I didn't realise there was such a difference in the forwarding rates. I realise that Layer 3 switches use hardware and are faster than routers, but a hundred-fold difference is amazing!

I think thats an excellent discussion guys.

I was under the impression that they required a router to route between the different VLANs, but apparantly not.

Your impression was NOT wrong SteveP. You can route between VLANs using either a router or a layer 3 switch. Thats for sure:

www.cisco.com/en/US/docs/switches/lan/ca...routing.html#wp13354
www.cisco.com/en/US/tech/tk389/tk815/tec...186a008019e74e.shtml

One thing here -to avoid confusion- that I would like to point out regarding this page in the firewall.cx guides: www.firewall.cx/vlans-routing.php

Although the page obviously confirms that routing between VLANs is possible in both cases, with a router and L3 switch. The term "InterVLAN" routing is described as to be specific for L3 switching (Solution No.4 in the page). I'm not sure why is that so. The above cisco links clearly state the term "InterVLAN" for both cases. Hope that Chris or some one can clarify this. Or may some one correct me if I'm missing some thing here.

1. If you look at the architecture of the switch we create logical vlan SVI to create a bridge between the switch processor and the routing engine so that you can perform routing . Which is much like a physical interface on the router that is directly connected to the routing engine .
2. Other than that the routing concept is the same (ie ) the router puts the packet in the physical interface but a Layer 3 switch puts the packet in the respective SVI interfaces that are connected to the switch processor and then the switch port .

Hope this helps

Thanks
Patiot