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Order of operation: Arp table vs bridge mac table

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  • 1.  Order of operation: Arp table vs bridge mac table

    Posted 12-26-2020 12:29

    Hi everyone.

    Please consider the following example:

    HOST1( 2.2.2.2)-----ge-0/0/0 -MX4--ge-0/0/1.100-------100.100.100.2 ( HSOT2, MAC:22:22:22:22:22:22)

    Above MX4 is  a router for host2, it uses irb.100 ( 100.100.100.100/24) to provide routing for host2.

    1)Host1 sends a ping to 100.100.100.2, which is received by MX4 on ge-0/0/0.

    2) MX4 finds this packet need to be routed out of irb.100, it therefore replace src mac with its mac associated with irb.100, and destination mac with 22:22:22:22:22:22 because MX4 finds the hosts2 mac in its arp table as shown below:
    MX4> show arp
    22:22:22:22:22:22 100.100.100.2 irb.100 [ge-0/0/1.100]

    3) MX4 also finds host2 mac in its bridge mac table for vlan 100 as shown below:
    root@MX4> show bridge mac-table

    Routing instance : default-switch
    Bridging domain : TEST, VLAN : 100
    MAC MAC Logical NH MAC active
    address flags interface Index property source
    22:22:22:22:22:22 D ge-0/0/1.100


    Question:
    Will MX4 use arp table to forwards the packet out of ge-0/0/1.100 to host2 or will it use bridge mac table and forwards the frame out of ge-0/0/1.100 to host2?

    Thanks and have a good weekend!!



  • 2.  RE: Order of operation: Arp table vs bridge mac table
    Best Answer

     
    Posted 12-26-2020 16:58

    Hi Sarah,

    ARP table only tells the router what MAC address to use as destination in the Ethernet header.  The bridge or L2 table tells it which interface to send the packet out of. 

    Regards, 



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    Original Message