routing a packet through network

carl_townshend · ‎06-12-2007

Hi all, when a packet goes from a source to a destination, at each hop towards the destination, does the source ip address or mac address change to the previous hops router?

Harold Ritter · ‎06-12-2007

Carl,

Generally speaking, only the destination MAC address would change from hop to hop. The source and destination IP adddresses would remain the same from source to destination (given that NAT is not used).

Hope this helps,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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mohammedmahmoud · ‎06-12-2007

Hi,

When a packet is routed via a router or Layer 3 switched from a source in one subnet to a destination in another subnet, the router/L3 Switch performs a packet rewrite at the egress port:

Packet rewrite alters five fields:

Layer 2 (MAC) destination address

Layer 2 (MAC) source address

Layer 3 IP Time to Live (TTL)

Layer 3 checksum

Layer 2 (MAC) checksum (also called the frame checksum or FCS)

Note: Packets are rewritten with the encapsulation appropriate for the next-hop subnet.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.