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Hardware switching and Process switching on EX switches

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  • 1.  Hardware switching and Process switching on EX switches

    Posted 03-03-2019 13:59

    Hi everyone.

     

    Please consider the following scenarios:

     

    1) Let say Ex 4200 Switch operating as layer 2, receives unknown Unicast destination MAC,  will  EX switch use harware switching to forward it ?

    2) Let say Ex 4200 Switch operating as layer 2, receives multicast frame  as a transit traffic i.e frame is not deemed " Exceptional traffic" ,   will Switch use harware switching to forward it or frame will be sent to CPU for forwarding?

     

    Thanks, and have a nice weekend!!

     



  • 2.  RE: Hardware switching and Process switching on EX switches

     
    Posted 03-03-2019 18:03
    Hi Sarah,

    The port on the switch reside the hardware ASICs aka PFE (packet forwarding engine) and a group of ports belong to one PFE. Each switch will have multiple ASICs/PFEs.

    Ingress PFE performs destination MAC address lookup after checking source mac lookup (for MAC learning). If destination is unknown unicast, the ingress PFE replicates the packet multiple times so that there is 1 copy for each of the local ports that are in the same VLAN as the ingress port, and 1 copy for each of the VCP ports. The same lookup and replication processes take place on all the other PFEs if the EX4200 is part of a virtual chassis as these PFEs receive the packet one after the other on the VC backplane.

    The same process as mentioned above applies for unregistered multicast i..e if destination is unregistered multicast, the ingress PFE replicates the
    packet multiple times so that there is 1 copy for each of the local ports that are in the same VLAN as the ingress port, and 1 copy for each of the VCP ports.


    For L2 registered multicast packets (i.e. through IGMP snooping) with either unknown or known source MAC address, the forwarding process and packet flow is almost
    identical to those of L2 unregistered multicast packets, except:
    * Instead of flooding to all ports, registered multicast packets are only replicated to those ports belonging to same registered IP multicast group and are in the same VLAN as the source port.
    * One copy of the registered multicast packet will still be transmitted out of each of the VCP ports of each PFE so that it reaches all of the PFEs in the VC.

    Note that in case of unknown source-MAC, the MAC learning is done by the CPU though.

    In simple words, to answer your question the unknown unicast or multicast packets are flooded to all ports in the same VLAN at the hardware level.

    Hope this helps.

    Regards,
    -r.

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  • 3.  RE: Hardware switching and Process switching on EX switches
    Best Answer

     
    Posted 03-03-2019 18:46

    1) Let say Ex 4200 Switch operating as layer 2, receives unknown Unicast destination MAC, will EX switch use harware switching to forward it ?

    Ans.  Yes EX switch will use hardware switching as explained above.


    2) Let say Ex 4200 Switch operating as layer 2, receives multicast frame as a transit traffic i.e frame is not deemed " Exceptional traffic" , will Switch use harware switching to forward it or frame will be sent to CPU for forwarding?

    Ans.  EX will use hardware switching to forward the frame as explained above.

     

    Hope this helps.

     

    Regards,
    -r.

    --------------------------------------------------

    If this solves your problem, please mark this post as "Accepted Solution."
    Kudos are always appreciated :).