Frames for the switch

[dosel99]

I recently started improving my understanding of protocols such as BGDP and CDP. I realized something about switch behavior that I can't find a clear answer for.

My original understanding was to talk to the switch; it required an SVI. The SVI was given an IP address and a mac address from the baked mac on the switch. If I wanted to ssh or ping the switch just had to give the ip address of the switch (assuming no vlans at play).

I have started reading about CDP and BGDP and noticed that those protocols use multicast addresses, and headers have info about those protocols using 803.2 frame format. I noticed that the switch that gets the frame accepts the frame and processes it.

This leads to a question, do switches require a virtual interface (SVI) to have frames processed that are distended to the switch? Going off what I am seeing I would say no it seems the switch's can detect certain types of traffic like BGDP and CDP and know to grab the frame and process it for those tasks.

Is that correct or am I completely missing something here?

[MHM Cisco World]

any L2 protocol not need IP and hence not need SVI 
the L3 protocol need IP and need SVI in SW to run it.

[Richard Burts]

I would say that the original poster is missing something (and that is common for people who are just learning about networking so this is not a negative comment). I would start my response by emphasizing the distinction between a layer 2 switch and a layer 3 switch. A layer 2 switch is the earlier version of a switch and the more simple version. A layer 2 switch forwards frames by looking that the mac address (particularly the destination mac address). So a layer 2 switch looks at every frame that arrives on an interface and decides what it should do with the frame. So a layer 2 switch can receive, process, and forward a frame that is an IP packet as long as the switch does not need to process the IP information. So there is no need for an SVI on a layer 2 switch. A layer 3 switch is a more advanced type of switch. It does all the same things that a layer 2 switch does, and it has the capability to process IP information. To process IP information the switch needs an SVI.

If you want to access the switch using SSH or telnet, or if you want the switch to send SNMP to a server, or want the switch to send syslog to a server, or to learn time from an NTP server, or to do other things that are based on IP then the switch needs a single SVI. The SVI allows the switch to process IP on that interface and that vlan (and still be able to do layer 2 forwarding of IP packets on the other vlans). If you want the switch to forward frames based on IP then each vlan on the switch needs it own SVI (and the switch needs ip routing to be enabled).

[Joseph W. Doherty]

"My original understanding was to talk to the switch; it required an SVI."

Depends on the switch.

L2 switches pre-VLANs, would not have a SVI, but could have a management address.

Many L3 switches, support "routed" interfaces, which can be used to communicate with that switch, i.e. w/o an SVI.

When it comes to L2 frames, they too have unicast MACs, multicast MACs and broadcast MACs.

A switch, will accept for "processing" any frame it receives, but if the destination MAC is of no interest to the switch, itself, it will just "process" the frame as a transit frame (the primary purpose of a switch [BTW, is also only what a "dumb" L2 switch does]).

If the destination MAC is of possible interest to the switch itself (as a network host), besides still possibly treating the frame as a transit (e.g. multicast and broadcast), it will analyze the frame and take whatever action it believes it should (which could also include - "don't care - and if unicast, discard"), based on the contents of the frame.

[MHM Cisco World]

friend the Control Plane divide the protocol to L2 and L3, L2 dont need IP, L3 need IP and in SW you can use the SVI IP for L3 protocol.