SW forwarding arp request

szar · October 2015

hello i have a question. i have a switch with 2 pc connected in fa 0/5 ,0/6 and a router with DG in fa 0/1. first the mac table of switch is empty.(using reload command and erase nvram to be sure )but when i ping from pc1 to pc2 i see entry for the router dg also.is it normal ?shouldnt router atached ignore these packets as they arp frames destined for specific hosts in these two ports?in cisco book it says that router igore broadcast packets that dont have router mac
-the ip scheme is and i am using packet tracer
172.16.1.1 DG
172.16.1.5 PC1
172.16.1.6 PC2

EdTheLad · October 2015

It's normal, you reload the switch, the router port goes down. Once the router port comes up it sends a "gratuitous arp reply" this is sent every time a router interface comes up. This "gratuitous arp reply" should populate the switch mac table.
Ignoring the gratuitous arp reply, the arp from the PCs is sent to the broadcast mac address ffff.ffff.ffff which means all devices in the layer 2 domain will process that frame. The router will populate its arp cache with the received arp requests, after the ping is successful check the router arp cache "show ip arp".

szar · October 2015

oc i understand this about gratit arp.the second question is why he router will populate its arp cache with the received arp requests as i send arp for only one host .with this logic the other hosts(eg pc7 with mac 7777)shoul also respond as it is broadcast arp?

EdTheLad · October 2015

You ping from R1 to R2, arp request is sent from R1 to L2 multicast ffff.ffff.ffff, this arp is received by R2 and the router, both will update their arp cache with the SRC IP and SRC MAC. R2 will process the arp request and reply to R1. R1 can now ping R2, so the router will only have R1 arp entry.

SW forwarding arp request

Comments