3 Replies Latest reply: Sep 19, 2018 6:07 PM by Charles RSS

    How is the processing of Frame done at Switch Port?

    Shaukat

      How is the processing of Frame done at Switch Port?

       

      Like what is processed first? FCS?

       

      And later VLAN TAG (if tagged) and then source MAC and then Destination MAC?

       

      or Destination MAC First then Source MAC and then VLAN TAG?

        • 1. Re: How is the processing of Frame done at Switch Port?
          Steven Davidson

          How a switch handles a frame, generically speaking, depends on the switch. Not all switches are created equally.  In general the bits of a frame are immediately inspected as soon as they are received.  The beginning of the frame (as opposed to the packet) the very first bit received is the bit which determines whether or not the frame is of type individual or group.  I imagine, at the very lowest levels, this information could be used by the switch to prepare itself for what it'll eventually need to do to subsequently egress the frame(s).

           

          There's two types of switching; store-and-forward and cut-through.  In cut-through switching the switch starts sending the frame before fully receiving the frame.  This is done to reduce latency.  If the ingress port is a trunk then the switch would be forced to wait for at least the 802.1Q tag portion of the frame because it needs that to figure out which port(s) are part of the flooding domain.  In the case of cut-through switching the switch cannot inspect the FCS of the frame, before forwarding, because it doesn't have that information yet.  In store-and-forward the switch would check the FCS to ensure that the frame is good before sending it out any other port(s).

          • 3. Re: How is the processing of Frame done at Switch Port?
            Charles

            This is from the CCENT/CCNA ICND1 100-105 Official Cert Guide, APPENDIX Q Topics from Previous Editions. Page 2 Table Q-1 Switch Internal Processing


            Switch Internal Processing

             

            Store-and-forward

             

            The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame.

             

            Cut-through

             

            The switch forwards the frame as soon as it can. This reduces latency but does not allow the switch to discard frames that fail the FCS check.

             

            Fragment-free

             

            The switch forwards the frame after receiving the first 64 bytes of the frame, thereby avoiding forwarding frames that were errored because of a collision.

             

            Also from Chapter 2: Fundamentals of Ethernet LANs 49, CCENT/CCNA ICND1 100-105 Official Cert Guide

             

            IEEE 802.3 Ethernet Header and Trailer Fields


            Preamble  - Synchronization.

            Start Frame Delimiter (SFD) - Signifies that the next byte begins the Destination MAC Address field.

            Destination MAC Address - Identifies the intended recipient of this frame.

            Source MAC Address - Identifies the sender of this frame.

            Type - Defines the type of protocol listed inside the frame; today, most likely identifies IP version 4 (IPv4) or IP version 6 (IPv6).

            Data and Pad* -Holds data from a higher layer, typically an L3PDU (usually an IPv4 or IPv6 packet). The sender adds padding to meet the

            minimum length requirement for this field (46 bytes).

            Frame Check Sequence (FCS) - Provides a method for the receiving NIC to determine whether

            the frame experienced transmission errors.

             

            The frame is processed from top to bottem.  Preamble first and FCS last.  The Vlan tag is placed between the Source and Type fields.

            See CCENT/CCNA ICND1 100-105 Official Cert Guide, Page 244. The 802.1Q and ISL VLAN Trunking Protocols,  Figure 11-6

             

            The tag contains the, type, priority, flag and vlan ID fields.