Router and L3 switch

[mkoniuszko]

Hello,

I'm pretty new in Cisco (still preparing for CCNA), so be understanding if question is obvious.

I just don't understand what is a difference betweet router with integrated L2 switch (for example 877) and Layer 3 switch ?

I cannot find it anywhere, and in video courses I've seen they just mention that L3 switches exist, that's it.

Regards,

Mariusz

[Jon Marshall · Accepted solution]

mkoniuszko wrote:
Hello,
I'm pretty new in Cisco (still preparing for CCNA), so be understanding if question is obvious.
I just don't understand what is a difference betweet router with integrated L2 switch (for example 877) and Layer 3 switch ?
I cannot find it anywhere, and in video courses I've seen they just mention that L3 switches exist, that's it.
Regards,
Mariusz

Mariusz

A router does it's routing in software.

A L3 switch does most of it's routing in hardware.

This means that L3 switches outperform routers in forwarding of L3 packets.

So why does anybody buy a router ? Well routers are much more flexible devices than L3 switches eg.

1) Apart from the 6500 switch no other switch supports NAT

2) Routers support a much more rich set of QOS features so if you want WAN QOS routers are often the way to go

3) Routers support a far greater range of interface types

4) Routers support IPSEC/VPNs etc.

Note that the 6500 switch can do a lot of the above than other switches but you do pay for that.

Jon

[mkoniuszko]

Thank you Jon for this quick clarification.

Mariusz

[sourabh1000_2]

hello,

now i come to know only 1 difference between Router and L3 switch is that Router is doing routing part in Cisco IOS software and L3 switch is doing their routing by using hardware like ASICs but what does this means and how they are doing? are there any diferences available between Router and L3 switch exept this?

Thanks and Regards,

sourabh

[Jon Marshall]

sourabh1000 wrote:
hello,
now i come to know only 1 difference between Router and L3 switch is that Router is doing routing part in Cisco IOS software and L3 switch is doing their routing by using hardware like ASICs but what does this means and how they are doing? are there any diferences available between Router and L3 switch exept this?
Thanks and Regards,
sourabh

Sourabh

There is more than 1 difference that i wrote about in this thread - see the list under why you would buy a router rather than a switch.

The difference between hardware and software routing

a router has a CPU & memory and the L3 routing is done using some of these resources. The CPU is the general CPU used by the router to carry out all of it's functions not just L3 routing

a switch on the other hand has dedicated ASICs which are chips which carry out a number of limited operations. The circuitry on the chip is designed specifically for this limited set of operations. And because it only deals with a limited number of operations it has been optimized to do these operations very quickly.

Jon

[stevejennings]

Jon,

When you say . . .

"This means that L3 switches outperform routers in forwarding of L3 packets."

. . . are you comparing the L3 switch speed to a router doing "process switching"? I thought that CEF copied routing tables to an ASIC on a router. Or are you implying that routers don't have an ASIC? Can you clarify?

Thanks

[Jon Marshall]

stevejennings wrote:
Jon,
When you say . . .
"This means that L3 switches outperform routers in forwarding of L3 packets."
. . . are you comparing the L3 switch speed to a router doing "process switching"? I thought that CEF copied routing tables to an ASIC on a router. Or are you implying that routers don't have an ASIC? Can you clarify?
Thanks

Steve

The lines between a traditional router and layer 3 switch are somewhat blurred these days eg. the 7600 router which is very similiar to the 6500 switch.

But generally speaking routers do not have ASICs and CEF on routers is merely a cache that allows forwarding of packets without having to go the main processor for every packet but as far as i know this forwarding is still not implemented in hardware as it is on L3 switches.On routers this CEF cache may be download to VIP cards etc. but still routers are not actually forwarding in hardware.

So a L3 switch compared to a comparable router both using CEF will still outperform the router.

In fact it is the ASICs on a L3 switch that are it's strong point and it's achilles heel. ASICs allow them to forward packets in hardware but it also limits what features you can implement on a switch because you are constrained by the hardware. Routers are far more flexible because most of the features are performed in software. It also means routers can be far more standard across models than switches. QOS is a great example of this. QOS across router platforms is pretty standard and full featured. On catalyst switches it is anything but standard and not so feature rich. To make it as feature rich would mean doing more in software and suddenly you have lost the wire speed selling point of L3 switches.

So there is a tradeoff between speed and functionality.

Anyway i seem to have digressed somewhat from your original question, so i'll shut up now

Jon

[stevejennings]

Thanks Jon. That's exactly what I was looking for.

Steve

[amrelec]

Thanks Jon. i also want to add something here for the differences between a router & L3 switch...

Router (Layer 3 Routing )

1- Packets are forwarded between networks based on Layer 3 addresses.

2- An optimal path is determined for a packet to take through a network to the next router.

3- Routers communicate with each other using routing protocols.

4- L3 routing is generally performed by microprocessor-based engines (L3 Engine), which require CPU cycles to examine each packet's network layer header.

L3 Switch (Layer 3 Switching)

1- Packets are forwarded at layer 3, just like as a router do.

2- Packets are switched using specialized hardware, ASIC (Supervisors) , for high speed & low latency.

NOTE:

Layer 3 switches are designed to examine and forward packets in high-speed LAN environment. Whereas a router might impose a bottleneck to forwarding throughput, a layer 3 switch can be placed anywhere in the network, with little or no performance penalty.

Amro

[Jon Marshall]

amrelec wrote:
Thanks Jon. i also want to add something here for the differences between a router & L3 switch...
Router (Layer 3 Routing )
1- Packets are forwarded between networks based on Layer 3 addresses.
2- An optimal path is determined for a packet to take through a network to the next router.
3- Routers communicate with each other using routing protocols.
4- L3 routing is generally performed by microprocessor-based engines (L3 Engine), which require CPU cycles to examine each packet's network layer header.
L3 Switch (Layer 3 Switching)
1- Packets are forwarded at layer 3, just like as a router do.
2- Packets are switched using specialized hardware, ASIC (Supervisors) , for high speed & low latency.
NOTE:
Layer 3 switches are designed to examine and forward packets in high-speed LAN environment. Whereas a router might impose a bottleneck to forwarding throughput, a layer 3 switch can be placed anywhere in the network, with little or no performance penalty.
Amro

Amro

Router (Layer 3 Routing) - 2 & 3 - as far as i can see L3 switches also do this ie. communicate using routing protocols and therefore also 2).

Jon

[amrelec]

yes you are right. L3 switches make use of routing protocols. but this is more specific to routers.that's why it's listed only w/ L3 routing.

In other words, if ASICs are deployed with CEF technology or (Netflow) and henc, packets are routed throgh the hardware (FIB), therefore, in this methodology.. the device is called L3 switches (combining the SE and RP).

hope that helps.

thanks

[guru_ganesha]

From Jon.Marshall: 3) Routers support a far greater range of interface types

This is a big one.