Understanding the network.pdf - Back to Home

3. Router B—Router B receives the packet on interface e0. It extracts the IP
datagram and performs route lookup. Table 2.11 shows two gateways that
advertise routes to the destination network.
NOTE
Table 2.11. Routing Table for Router B
Network Address Netmask Gateway Interface
172.168.1.0 /24 172.168.0.1 direct connect e0
172.168.3.0 /30 172.168.3.1 direct connect s0
172.168.3.8 /30 172.168.3.9 direct connect s1
172.168.0.0 /24 172.168.1.254 e0
172.168.3.4 /30 172.168.3.2 via s0
172.168.3.10 via s1
192.168.20.0 /24 172.168.3.2 via s0
172.168.3.10 via s1
192.168.21.0 /24 172.168.3.2 via s0
172.168.3.10 via s1
0.0.0.0 /0 172.168.3.2 s0
4. Router D—Router D is the transition point from the remote delivery model to
the local delivery model. The IP datagram is now extracted from the Frame
Relay frame, the route lookup is made, and the destination is local to the
router. Now, the router is interested in the host address. Routers are only
interested in a datagram's host address when the destination is on the
router's own network. The router must perform an ARP lookup to find out the
destination host's Layer 2 address. After the ARP lookup is complete, the
datagram is placed in an Ethernet frame (source: d61; destination: 0x1) and
forwarded directly to the destination host, where the Layer 4 data is
extracted from the Layer 2 and Layer 3 packets.
All things being equal, the router uses both interfaces to deliver traffic to a network.
Determining just how equal these two paths are is left to the routing protocol. In this
case, the route via the 172.168.3.2 address adds an additional hop, so the route via
the 172.168.3.10 address will be used. The IP datagram is placed inside a Frame
Relay packet and forwarded to the next hop. This part of the journey provides an
example of a router being used as a Layer 2 media gateway.