Solved: layer2, layer3 devices

sarahr202 · ‎09-20-2008

hi every body!

I am a bit confused about thses terms.

For example when we say layer 3 device, that means it can understand upto layer 3 information, anything beyond layer 3 is greek for layer 3 device.

But then we find layer 3 device , for example router using access list to filter, or identify the traffic based on ports.

My question being layer 3 device, router should not be able to identify or filter the traffic based on ports,(udp or tcp),as there are above layer 3 and thus be greek to router. But this is not the case in realty.Similarly, we find layer 2 switch filtering traffic based on ip addresses. Again being layer 2 , switch device should not be able to do that as ip address is in layer 3.

Does anyone care to elaborate on that?

thanks a lot and have a nice day!

scottmac · ‎09-21-2008

It's a general label used to describe the major functionality of the device.

As technology develops, features are added.

In the "good old days," a Layer two switch was truly just a fast, multi-port bridge. As the technology evolved, feature were added to address the needs of a larger network (enabled by the base technology, like switching).

For layer 2, some of the features added later were for things like multicast control (cgmp, igmp), flood/broadcast control, ACL capability, multi-link ...

Same thing for routers at layer 3. As new technologies emerged, and network diameters grew, features were developed and integrated beyond the base functionality (like PIM, HSRP/VRRP, tunneling ...)

So, some of the labeling is just legacy notation, some is describing a class of device, that by description gives a general indication of where it might fit within the larger network.

A layer 3 switch, by basic function is a fast multi-port router ... but the additional functionality usually added to the L3 switch and/or the broader classes of interfaces generally available to a router distinguish them as separate classes of device, and describe different roles within a typical network.

Good Luck

Scott

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lamav · ‎09-21-2008

Sarah:

Wow, what a coincidence. I just asked a similar question regrding L2 switches with QoS configs on them that use L3/L4 ACLs to match traffic.

Go to the this thread and read it; I think it will help you.

http://forums.cisco.com/eforum/servlet/NetProf;jsessionid=B883149F84C7543DCB0D8C5A4E68211A.SJ2B?page=netprof&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing&topicID=.ee71a04&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26l...

HTH

Victor

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Marwan ALshawi · ‎09-20-2008

ok interesting question

the thing is when we say L3 device that mean it is capable to work in L3 and same for L2

but at the same time tose device include L3 and L4 service intelegant for example

Cisco Catalyst 2960 Series

Layer 2 switching with intelligent

Layer 2 - 4 services

Cisco Catalyst 3750 Series

Layer 2-4 switching and intelligent services

with dynamic IP routing and IPv6

however:

Cisco Catalyst 2940 Series

Standalone fixed-configuration Layer 2

switches within this sriese dose not support L3-4 intelegant service

so it depends on the service intelgance on the device

while the actual role is diffrent if it is only L2 forwarding switch or multilayer switch

the same with router

by the way routers work even on L7 with the featuer if u heard about it NBAR

like with calss-map u can use matching based on L7

good luck

if helpful Rate

which mean this se

scottmac · ‎09-21-2008