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Fundamentals of Routing
Is the destination logical address in the routing table? If not, discard
the packet and send an ICMP (Internet Control Message Protocol)
message to the sender.
If the destination logical address is in the routing table, to which inter-
face will the packet be forwarded? Once this exit, or forwarding interface,
is chosen, the router must have an encapsulation in which to place the
packet. This is called framing and is required to forward the packet to
the next-hop logical device.
Once the packet is framed, it is forwarded from hop to hop until it reaches
the final destination device. Routing tables in each device are used to pass the
packet to the correct destination network.
Routing Tables
All the routing information needed for a router to forward packets to a next-
hop relay device can be found in the router's routing table. Again, if a des-
tination logical address is not found in the table, the router discards the
packets. A gateway of last resort can be set on the router to forward packets
not listed in the routing table. This is called setting the default route.
However, this is not a default gateway, nor does it act as a default gate-
way, so it is important to not think of setting the gateway of last resort as set-
ting a default gateway. Default gateways are used on hosts to direct packets
to a relay device if the destination logical device is not on the local segment.
Gateway-of-last-resort entries are used to send packets to a next-hop relay
device if the destination logical address is not found in the routing table. If
the destination logical address is in the routing table, then the gateway of last
resort will not be used.
A sample routing table is shown below:
2600B#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M -
mobile, B ­ BGP, D - EIGRP, EX - EIGRP external, O - OSPF,
IA - OSPF inter area. N1 - OSPF NSSA external type 1, N2 -
OSPF NSSA external type 2, E1 - OSPF external type 1, E2 -
OSPF external type 2, E ­ EGP, i - IS-IS, L1 - IS-IS
level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route, o - ODR
T - traffic engineered route
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