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Community Member

Routing Protocols in Layer 3 Switches

Hello there.

When I read data sheets of switches such as 3750x and 6500 models, it is usually indicated that they are capable of routing protocols such as OSPF, EIGRP and BGP.

Can someone please enlighten me when and why are these protocols used in a layer 3 switch?

Thanks in advance.

2 ACCEPTED SOLUTIONS

Accepted Solutions
Cisco Employee

Routing Protocols in Layer 3 Switches

Kevin,

Julio explained it very well.

I would like to add from my end:

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)

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

Regards

Inayath

*****Plz rate all usefull posts***************

Cisco Employee

Re: Routing Protocols in Layer 3 Switches

Kevin,

Yes your understanding is correct.

The major difference between L3 switch and router is regarding performance. Routing on router is software base which is cpu intensive and routing done on layer 3 switches can be done on hardware which is less cpu intensive and gives you better performance.

Yes ofcource if you use L3 switch for EIGRP & ospf routing protocol also it can be detected as DR and BDR depending upron your ospf configuration.

Also DR and BDR depends on whether you have broadcast media or not if you have let it be a router or layer 3 switch it does not matter and it will elect DR and BDR.

Problem comes when you want to terminate you wan lines like frame relay and lease lines. Not all layer 3 switches support that. Big daddy like cat6k still supports wan modules on which you can even terminate your wan lines.

So the major difference lies on performance.

You can find information about the functions of a L3 switch in comparison to a L2 switch and a router by clicking on the following link:

http://www.cisco.com/en/US/about/ac123/ac147/archived_issues/ipj_1-2/switch_evolution.html

HTH

Regards

Inayath

****Plz rate all usefull posts*******

8 REPLIES
Community Member

Routing Protocols in Layer 3 Switches

If you do not configure a routing protocol on your Layer 3 switch then the only routing it will be able to do is static - therefore you would have to manually configure all routes for the network.

These protocols are for dynamic routing. So they will be able to learn different routes and route your traffic without you having to manually configure static routes for everything.

Here is a quick comparison of dynamic routing protocols available:

http://www.cisco.com/en/US/docs/net_mgmt/active_network_abstraction/3.7/reference/guide/routpro.html

Routing Protocols in Layer 3 Switches

Hello Kevin,

Cause if you have a L3 device you definetly want to be able to route packets on such a way that is :

  • Scalable
  • Dynamic
  • Fast

And that's why L3 routing protocols exist.

A L3 switch that does not support that it just a waste of the name

For more information about Core and Security Networking follow my website at http://laguiadelnetworking.

Any question contact me at jcarvaja@laguiadelnetworking.com

Cheers,

Julio Carvajal Segura

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Cisco Employee

Routing Protocols in Layer 3 Switches

Kevin,

Julio explained it very well.

I would like to add from my end:

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)

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

Regards

Inayath

*****Plz rate all usefull posts***************

Community Member

Re: Routing Protocols in Layer 3 Switches

Hello Inayath.

Thanks for the response. Just to clarify things out,

1.) If  I am running EIGRP in my branches, and each has a L3 switch (theoretically),

- Does it mean that the L3 switch will also endorse the different networks / subnetworks connected to it?

- Will it be the one to perform a summary route instead of the router?

2.) Another case is, if OSPF is running on my network,

- Can the L3 switch also do route summarization or can it also be an ABR?

- Is it also capable of being a DR or a BDR?

3.) Generally, the main purpose of enabling routing protocols in a layer 3 switch is to be able to form a neighbor with the router and endorse

the subnetworks in the organization. Am I correct?

Regards,

Kev

Cisco Employee

Re: Routing Protocols in Layer 3 Switches

Kevin,

Yes your understanding is correct.

The major difference between L3 switch and router is regarding performance. Routing on router is software base which is cpu intensive and routing done on layer 3 switches can be done on hardware which is less cpu intensive and gives you better performance.

Yes ofcource if you use L3 switch for EIGRP & ospf routing protocol also it can be detected as DR and BDR depending upron your ospf configuration.

Also DR and BDR depends on whether you have broadcast media or not if you have let it be a router or layer 3 switch it does not matter and it will elect DR and BDR.

Problem comes when you want to terminate you wan lines like frame relay and lease lines. Not all layer 3 switches support that. Big daddy like cat6k still supports wan modules on which you can even terminate your wan lines.

So the major difference lies on performance.

You can find information about the functions of a L3 switch in comparison to a L2 switch and a router by clicking on the following link:

http://www.cisco.com/en/US/about/ac123/ac147/archived_issues/ipj_1-2/switch_evolution.html

HTH

Regards

Inayath

****Plz rate all usefull posts*******

Community Member

Routing Protocols in Layer 3 Switches

It seems clear now.

Thank you.

Super Bronze

Re: Routing Protocols in Layer 3 Switches

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Posting

Hi, just wanted to mention what Inayath described, L3 switches providing better performance, is a generalization (although usually true at the low end).  Do understand that sometimes the performance aspect is the same or even better for a "router". Perhaps an even more important distinction is feature support.  "Routers" usually have more features than L3 switches.

A wonderful example of such a difference, Cisco markets the 6500 series as L3 switches and the 7600 series as routers, yet basically they both use the same architecture, can use some of the same supervisors and some of the same line cards (and they even used to run the same IOS).  However, their feature orientation is different.

For "raw" performance, Cisco's CRS series, I believe, are the most "powerful" Cisco offers yet they are considered routers.

Generally L3 switches trade off features for better performance, especially to keep their price down.

When working with, or using, L3 switches, you'll usually find they are "faster" than a router of the same price yet often will not have features such a router might offer.

Re: Routing Protocols in Layer 3 Switches

I would also like to add to Joseph's comments that routers are required in environments where this a handoff to a different interface or layer 2 encapsulation type (e.g., Ethernet vs serial/HDLC).

Sent from Cisco Technical Support iPad App

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