I have a question ?

Dowima

Hi all,
i'm new to this world. and about the take ccna course! ..... but i have a question ! Why in big companies people use those big catalyst switchs to connect different areas? Why not using routers ?
isn't routers are for interconnect different broadcast domains ?

Netstudent

Routers separate collision and broadcast domains. switches and routers operate at different OSI layers and therefore provide different functions for moving data around.

Routers are used for routing datagrams or ip packets and switches are used to forward frames based on a destination MAC address. IP is layer 3 MAC is layer 2.

These big companies use switches to connect different segments of the network because they, for one, cause less latency. They also provide a full-duplex or collision free data transfer. Also switches are access devices where as routers are distribution devices.

This means that you use switches to enable users to access a network.

Routers are used to distribute data between different networks or subnets.

Also switches offer added scalability and flexibility (VLANS) to better optimize the data flow.

You may be referring to Layer3 switches, which offers all the functionality of both devices as one unit. When you say areas, do you mean physical areas, ospf areas, different networks? It just depends on the needs of that particular organization as far as layer 2 and layer 3 devices.

Paul Boz

Enterprise networks typically utilize vlans to segment large portions of networks. Vlans are managed by switches. Layer two switching is faster than layer 3 routing, so with less overhead you can push more packets in the same span of time. At the backbone level of a network you want as much data throughput with the least manipulation possible. At work we use several Catalyst 6513 switches and a 10,008 routers as our core backbone. Just about every data frame sees the switchs but since a lot of data never exits the segment (or layer 2 as a whole) there's no need to introduce routing.

Dowima

I really agree with all that!!!
But if i'm at the a different segment than the destination end system ( to reach it i must pass the core switch, this is the only route for example!) !!! How can the Core switch ( those 6500 i.e.) locate the destination without routing, and without dealing with the ip address ( MAC aren't addressable). how to know with path to select ? which area to switch the frame to the edge device there ????

EdTheLad

Dowima wrote:

I really agree with all that!!!
But if i'm at the a different segment than the destination end system ( to reach it i must pass the core switch, this is the only route for example!) !!! How can the Core switch ( those 6500 i.e.) locate the destination without routing, and without dealing with the ip address ( MAC aren't addressable). how to know with path to select ? which area to switch the frame to the edge device there ????

The switching core is a layer 2 network,the layer 2 network is broken up into different vlans to divide up the broadcast domain.Each vlan will contain a certain number of devices, these devices will all share a common ip subnet and have a default gateway pointing to the 6500.A 6500 running with an mfsc works as a layer 3 switch, this switch will have a trunk port connected to the L2 switching core and an ip address assigned to each vlan in the switching core.The 6500 will advertise the different vlan ip subnets to the rest of the layer 3 network and it will work as a default gateway for all the switching core vlans.So in the L2 core anything that is addressed to an off subnet ip address is sent to the 6500 default gateway.

Rearden

We use a 6513 as our core here and 3560s on the edge. The 6513 has RIPv2 enabled for routing between VLANS, and it has BGP enabled for I2 traffic. Anything else, goes to the I1 router.

mgeorge

Well to simplify every ones answer on this thread. The 6500 Series Catalyst can be a layer 2 or layer 3 switch. Most enterprise campuses are using Layer3 switching in the backbone for broadcast and segmentation control. While a Layer3 switch can preform the same functions of Layer2 switch. The L3 route processors on the 6500 series catalyst switches are way faster then most routers you can buy and can contain way more ports.

Recently within the past 3 years Cisco has added the 10Gig Xenpac ports to a card for the 6500 Series catalyst, and the only router I can think of that will support this type of speed is the CRS-1 Multiservice router. (which can be the size of a refridgerator)

Most Universities use 6500's with 10Gig backbones and utilizing L2 and L3 switching for vlan control, broadcast control, high speed transport and so much more.

Cisco has released a 3750 series switch that has a 1 port xenpak slot on it, in able to support 10GigLX for high speed backbone transport. Typically these routers are used at the distribution layer to enforce route policies, security, QoS, GoS, gateway ACL's etc...

With this switch being stackable, this switch has alot to offer in speed, preformance and flexibility.

So simplely put, the 6500 Series Cayalyst offers speed, flexibility, preformance and reliability that most enterprise networks require.