Limitation of layer 3 switch compared to router

[a.pijpaert]

Hi there,

I've been reading about layer 3 switching and understand that it is basically routing, done by hardware i.s.o. software like in the more traditional routers.

I've been reading about all the router functionality that a layer 3 switch can handle but I hardly can't find any information about what layer 3 switches CANNOT do if compared to routers.

The only thing I found is that routers are still needed to connect to different types of networks.

Anyone know more about this? or can tell me where I can find more information on these differences between routers and layer 3 switches?

Will the router be replaced by layer 3 switches in the future?

Thanx.

[mark-obrien]

Here's my two cents. You mostly have it right, that routers have more variety in the types of interfaces than layer three switches do. At this time, it looks as though routers have more features than layer three switches, i.e., I don't see where you can implement NAT on certain switches. I expect that these features will be implemented in layer 3 switches in the future.

Will the router be replaced by layer 3 switches? I think that as the technology progresses there will be more and more instances in which incoming packets to a device, whether you call that device a router or switch, will be processed out of another interface without needing to be processed by the routing engine. This is the idea behind CEF. I wouldn't call this replacing the router so much as blurring the lines between the swich and the router.

FWIW.

Mark

[konigl]

Cisco's stackable Layer 3 switches are generally limited to routing IP, though some can route IPX as well. (Foundry's stackables can actually do IP, IPX, and AppleTalk; I'm not sure, but I think HP resells these too. Most other manufacturers limit themselves to IP only.) Routers, on the other hand, can handle all sorts of protocols.

Cisco's low-end L3-capable switches generally can't do access-list filtering, but routers can.

Cisco's L3-capable switches generally cannot terminate a telco-provided WAN circuit like a T1/E1 or T3/E4, whereas routers can.

Having said all that, I should also mention that some switches cut across these generalizaions. For example, in certain configurations Cisco's Catalyst 6500 series can do everything above that I said the routers were better at, and can do it better and faster than the routers. In fact, in these cases a comparably-equipped and -capable router would probably cost more than the 6500.

Ultimately which device(s) you use, L3 switch or router, depends on what the particular requirements of your network are. Sometimes the L3 switch is more appropriate, sometimes the router is; and sometimes, you need both.

Hope this helps.

[a.manosca]

Just to add some info using examples:

1.) Catalyst 3550 switches running EMI software, supports BGP, but it still has some 'Unsupported' commands:

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12113ea1/3550scg/swuncli.htm#1013377

2.) Catalyst 6000 series switches has many features and interface types but still has some 'Unsupported' hardware:

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/122sx/ol_4164.htm#314139