05-12-2009 12:31 PM - edited 03-06-2019 05:41 AM
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
Solved! Go to Solution.
05-12-2009 12:35 PM
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
05-12-2009 12:35 PM
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
05-12-2009 12:38 PM
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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