ccna question - cisco netacad correct or cbt nuggets videos

[C0mmUN1cAt0r]

I have come across a question which asked what is used to forward data packets from one interface router to another? - I answered mac address (incorrect according to cisco netacad) answer is destination network address.

Within the CBT nuggets videos I have watched there is a section which explains about 2 packets, one needing to get to a host within its network, another needing to get to a host outside its network. For the packet needing to get to the host outside the network it is explained that the mac address of the router is used as destination at each hop not the network address.

Either one or the other is incorrect or i'm missing something???

 

[leedsit]

Hi,

Think its in the wording. PACKETS normally refer to Layer 3 entities of data where FRAMES Layer 2. If the Question was FRAMES from one port to another, then your right, MAC Address..

Hope that makes sense.



LEEroy
CCNP,CCIP

 

[C0mmUN1cAt0r]

leedsit, think you have confused me further!

as the pdu's move there way down the osi table then at layer 3 they are packets, at layer 2 they are encapsulated into frames, then layer 1 they go out over the medium for instance as 1's and 0's, I cant see how a frame ever makes it onto the network and therefore ever has its contents examined by a router i.e. mac address!?

 

[unclerico]

Technically it will use both datalink and network layer information to send the payload towards its destination.

Think about how a typical end host forwards data to a remote network. It does a DNS lookup if required to find the destination IP address. The host does a binary calculation to determine if the destination IP address is in its broadcast domain (subnet). If it is then it will ARP for the destination hosts MAC address by sending a broadcast out asking "who is at 1.1.1.1?". The destination host will reply to the ARP request and the source host can then begin forwarding packets. If the destination is not in its broadcast domain it will ARP for the MAC address of its default gateway. The gateway will reply and the host can begin forwarding packets to the gateway.

When a router receives a packet it removes the frame header and examines the destination IP address in the packet header. It does binary calculation to determine if it has a route in its local routing table to reach the destination. The router will need to determine the next-hop layer 2 information (MAC address for Ethernet, DLCI for FR, etc.) and the exit interface to forward the packet. It will then recompute the network layer checksum and re-encapsulate the packet with a new L2 frame header using the MAC address (assuming Ethernet) of its exit interface as the new source address.

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[kishmbee]

I too answered mac address to a question regarding what is used to forward data packets from one interface router to another? It was a class test, don't know what the tutor will award me for this.