156 Chapter 4: Bridges/Switches and LAN Design
Figure 4-13
Example Protocol Flows--Using a Router
The ARP broadcasts are not forwarded by the router. In fact, the logic in Step 1 begins with an
ARP looking for the MAC address of the client's default router--namely, the router's E0 MAC
address. This broadcast was not forwarded by the router, a fact that causes a router to be called
a broadcast firewall. Comparing this to a transparent bridge or a LAN switch, this difference in
broadcast treatment is the biggest advantage of routers.
0200.3333.3333
0200.4444.4444
0200.5555.5555
Web
Client
E0
E1
DNS
DMAC = FFFF.FFFF.FFFF
SMAC = 0200.1111.1111
ARP (Router-E0)
0200.1111.1111
1
DMAC = 0200.1111.1111
SMAC = 0200.4444.4444
ARP Reply
2
DMAC = 0200.4444.4444
SMAC = 0200.1111.1111
DNS Request
3
DMAC = 0200.1111.1111
SMAC = 0200.4444.4444
DNS Reply
4
DMAC = FFFF.FFFF.FFFF
SMAC = 0200.1111.1111
ARP (Web)
5
DMAC = 0200.1111.1111
SMAC = 0200.3333.3333
ARP
6
DMAC = 0200.3333.3333
SMAC = 0200.1111.1111
Connect to Web
7
0200.2222.2222
(Note: Addresses of frames on left-side Ethernet)
ch04.fm Page 156 Monday, March 20, 2000 5:02 PM