A. Router TestKingD will strip off the source MAC address and replace it with the MAC
address on the forwarding Fast Ethernet interface
B. Router TestKingD will strip off the source IP address and replace it with the IP address
on the forwarding Fast Ethernet interface
C. Router TestKingD will strip off the destination MAC address and replace it with the
MAC address of Host TestKing2
D. Router TestKingD will strip off the destination IP address and replace it with the IP
address of Host B
E. Router TestKingD will forward the data frame out interface Fast Ethernet0/1
F. Router TestKingD will forward the data frame out interface FastEthernet0/2
Answer: A, C, F
Whereas switches can only examine and forward packets based on the contents of the
MAC header, routers can look further into the packet to discover the network for which a
packet is destined. Routers make forwarding decisions based on the packet's
network-layer header (such as an IPX header or IP header). These network-layer headers
contain source and destination network addresses.
Local devices address packets to the router's MAC address in the MAC header. After
receiving the packets, the router must perform the following steps:
1. Check the incoming packet for corruption, and remove the MAC header . The
router checks the packet for MAC-layer errors. The router then strips off the MAC header
and examines the network-layer header to determine what to do with the packet.
2. Examine the age of the packet. The router must ensure that the packet has not come
too far to be forwarded. For example, IPX headers contain a hop count. By default, 15
hops is the maximum number of hops (or routers) that a packet can cross. If a packet has
a hop count of 15, the router discards the packet.
IP headers contain a Time to Live (TTL) value. Unlike the IPX hop count, which
increments as the packet is forwarded through each router, the IP TTL value decrements
as the IP packet is forwarded through each router. If an IP packet has a TTL value of 1,
the router discards the packet. A router cannot decrement the TTL value to 1 and then
forward the packet.
3. Determine the route to the destination. Routers maintain a routing table that lists
available networks, the direction to the desired network (the outgoing interface number),
and the distance to those networks. After determining which direction to forward the
packet, the router must build a new header. (If you want to read the IP routing tables on a
Windows 95/98 workstation, type ROUTE PRINT in the DOS box.)
4. Build the new MAC header and forward the packet.Finally, the router builds a new MAC header for the packet. The MAC header includes
the router's MAC address and the final destination's MAC address or the MAC address of
the next router in the path.
Figure 5 shows the contents of a packet before and after it has been forwarded by a router.
Figure 5 also shows the contents of the router's routing tables.
Figure 5: Routers forward packets based on the network address.