L2 vs L3 Switching: benefits

[fabio.marino]

Hi to all, during my CCNP 642-813, my instructor issued a challenge: benefits of L3 switching over L2 switching.

I was no able to understand the question. In my opinion, L2 and L3 switching have two different target so you cannot compare each others: L2 switching is involving with L2 information, basically MAC address, whereas L3 switching is in the context of topic suach IP addressing, fast routing and more functions that you can not perform using L2 switching. But sure i'm wrong something: I'm studying L2 topic and i have no great work experience!!!!

In brief, can someone discuss about this question?

Thanks, Fabio.

[Amit Singh]

Hi Fabio,

You are correct about your thoughts.Layer 3 switching is builtin on the new platforms from all the vendors and ideally your switch is doing both the fuanctionality a traditional L2 switching and Layer3 routing/switching.  Traditionaly routing capabilities was built into the routers only and   in a LAN environment with lost of Vlans a routers was used to provide  the routing among the vlans. As the technology had advanced Layer 3  switches have taken the functionality of both being a layer 2 switch and  routing device.

The Key difference between the Layer 3 routing on a L3 switch vs the traditional router is the use of special ASICS. L3 switches your specific hardware asics like ASICS for L2 switch, ASICS for Security and QOS ACL's, ASICS for IPv4 and IPv6 routing and many more. This improves the overall switching thgoughput and capabilties of the L3 switches as you have dedicated engines to perform the look-up's hardware thus cutting/improving the switching latency across the fabric and also put less load on the CPU for packet processing.

The overall benefits of the L3 switching vs L2 switch comes withe use of special ASICS to perform the specialized functions to perform both L2/L3 funcationality. Cisco's L2 switch also use some ASICS for the effective L2 switching and key features but there are many vendors in the industry who still use software based switching or merchant ASICS on their  switches.

HTH,

-amit singh

[Amit Singh]

Fabio :

The benefits of layer 3 switching include the following

• Hardware-based packet forwarding
• High-performance packet switching
• High-speed scalability
• Low latency
• Lower per-port cost
• Flow accounting
• Security
• Quality of service (QoS)

Cheers,

-amit singh

[George Stefanick]

We are routed access in our enviroment. The big up sell for L3 vs L2 swicthing was the convergence time and the lack of spanning tree issues...

[fabio.marino]

gstefanick ha scritto:
We are routed access in our enviroment. The big up sell for L3 vs L2 swicthing was the convergence time and the lack of spanning tree issues...

Hi Stefanick, thanks for you answer, but i was not able to understand the term "convergence time". Convergence time of which protocol? I mean in L2 environment the only protocol related to"convergence time" is STP, whreas in L3 environment protocols related to "convergence time"  is some routing protocol, so i can not understand how can you compare the two convergence times.

[fabio.marino]

Ok thanks to all, each opinion i think is valid and open my knowledge.

Now i'm going to say what my instructor email to me one minute ago, in order to reveal it's challenge: he wrote to me that behind the clear benefits (how you report in your post) there is one very important. L3 switching is able to provide first-hop redundancy to the clients, by means of HSRP, VRRP, GBLP.  I think we can discuss for hours about this answer. I mean, HSRP or VRRP are not useful term to ompare L2 with L3 switching.

What do you think? Thanks, but really i want to discuss about this qith my instructor.

Thanks very much for your help,

cheers Fabio.

[Amit Singh · Accepted solution]

Fabio,

If I have understood your questions correctly, HSRP/VRRP and GLBP only cannot be the real benefit of L3 switching. I would kindaa agree to that. I mean we can achieve the same by using these portocols on a router which is trunked to a L2 switch. This will go to the original router on a stick model. The real benefit of having a L3 switch is a simplied and scalable topology and able to run multiple services within hardware. Obviously L3 switch simplifies the above design and you can have an L3 gateway on the same switch where you have host connected to it. In a typically Cisco Hierarchical network you typically run these first hop gateway protocols like VRRP/HSRP at the aggreation layer. Without an L3 switch, you would end up strecthing your L3 inter-vlan routing to core and eventually you would extend your STP domain which will create a lot of STP issues over the network. Hence it is typically suggested to run an L3 routing protocol between your L3 capable aggregation layer switch and the core layer for faster L3 convergence in the backbone.

-As

[fabio.marino]

Yap, in some way i agree with you and you answer satisfy my doubt and support the answer of my instructor. May be we can talk about design/architectural  rule or best practice in a future post. Thanks.