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Irfan Salam
Beginner

MAC and ARP aging on MSFC

I need your help on very basic concept….

Could please help me to understand paragraph form “http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094afd.shtml#t8

                         Consequences of Asymmetric Routing

Consider the case of the continuous ping of host B by host A. Remember that host A sends the echo packet to MSFC1, and host B sends the echo reply to MSFC2, which is in an asymmetric routing state. The only time that Switch 1 learns the source MAC of host B is when host B replies to an ARP request from MSFC1. This is because host B uses MSFC2 as its default gateway and does not send packets to MSFC1 and, consequently, Switch 1. Since the ARP timeout is four hours by default, Switch 1 ages the MAC address of host B after five minutes by default. Switch 2 ages host A after five minutes. As a result, Switch 1 must treat any packet with a destination MAC of host B as an unknown unicast. The switch floods the packet that comes from host A and is destined for host B out all ports. In addition, because there is no MAC address entry host B in Switch 1, there is no MLS entry as well.

How could MAC Address can timeout if continuous ping of host B by host A is there?

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