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544 Chapter 8: WAN Protocols and Design
Figure 8-14
Basic Point-to-Point Network
The core routing logic must be considered to fully appreciate mapping. Router A receives an
Ethernet frame from some host and strips the Ethernet header (and trailer). The destination IP
address of the packet is compared to the IP routing table, and an entry is matched. The matched
routing table entry tells the router to route the packet out serial 0 to 10.1.2.2 next (Router B's
S1 IP address). Router A builds the HDLC header/trailer and sends the frame.
The fact that Router B's IP address on the common serial link is 10.1.2.2 has nothing to do with
the contents of the HDLC header and trailer; Router B's IP address is immaterial in this case.
If Router A can get the frame across the link, there is only one possible recipient--Router B.
So, no mapping is needed between the Layer 3 address and the HDLC address on a point-to-
point link.
Now consider a diagram with Ethernet between the routers (see Figure 8-15) and the routing
table that follows (see Table 8-18).
Table 8-17
Partial Routing Table on Router A for Figure 8-14
Subnet
Outgoing Interface
Next Router
10.1.3.0
serial 0
10.1.2.2
HDLC L3 Packet
HDLC
Eth.
L3 Packet
Eth.
Eth.
L3 Packet
Eth.
A
B
10.1.1.0/24
10.1.3.0/24
10.1.2.0/24
10.1.2.1
10.1.2.2
S0
S1
ch08.fm Page 544 Monday, March 20, 2000 5:17 PM