Source and Destination MAC L3 switches

Answered Question
May 12th, 2009

Hi,

got a very very basic question !!

When we are using L3 swicth with MLS-CEF feature enabled ( for speedy packet processing ). If a PC from VLAN_A sends "Ping" packet to PC in VLAN_B and gets a ping responce back showing the proper connectivity.

what are the MAC address involved in the process. In normal case PC A will send the packet to default gateway in this case VLAN_A ip address and corresponding VLAN MAC address. Then swicth will forward this packet to destination PC in the VLAN_B by ARP request and over-writing the farme destination by this MAC address. ( MAC of PC B ). But what is the source address of this frame which PC b will receive from switch.

In a normal router case fa0/0 will receive the frame from PC a, ( souce MAC of PC a and dest MAC of fa0/0 of router ) it will route this packet to fa0/1 and then to PC b. the destination frame will have source MAC of fa0/1 and PC B MAC address. How is this related to L3 switch source/dest MAC addreswise?

please share teh experience.

Thanks in advance.

subodh

Correct Answer by Jon Marshall about 7 years 9 months ago

Subodh


"But what is the source address of this frame which PC b will receive from switch."


It works same as on a physical interface ie. the source mac-address will the L3 vlan interface for VLAN_B.


Jon

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Correct Answer
Jon Marshall Tue, 05/12/2009 - 12:35

Subodh


"But what is the source address of this frame which PC b will receive from switch."


It works same as on a physical interface ie. the source mac-address will the L3 vlan interface for VLAN_B.


Jon

Giuseppe Larosa Tue, 05/12/2009 - 12:38

Hello Subodh,

a multilayer switch behaves as a router.


PCA sends a frame to its default gateway:

SA = PCA.NIC DA = SVI Vlan_A MAC address


the multilayer performs packet rewrite

(some slight changes are done to ip header: TTL is decremented by 1 and header CRC is recomputed)

and sends out in Vlan-B broadcast domain


SA= SVI VlanB MAC address DA= PCB.NIC


incidentally without explicit config

SVI VLanA MAC = SVI VlanB MAC in most platforms but this is not a problem


You can guess what happens for return path


Hope to help

Giuseppe



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