1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
|
<html>
<head>
<meta http-equiv="Content-Language" content="en-us">
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<title>Content</title>
<base target="_self">
</head>
<body background="../../images/bg.gif" topmargin="0" leftmargin="0" marginheight="0" marginwidth="0" onLoad="window.focus();" link="#808080" vlink="#808080" alink="#808080">
<table border="0" cellpadding="0" cellspacing="0" width="100%">
<tr>
<td bgcolor="#336666" width="18" valign="top">
<img border="0" src="../../images/content_lines.gif" width="16" height="25">
<img border="0" src="../../images/transdot.gif" width="2" height="1"></td>
<td bgcolor="#336666"><font face="Arial" size="2" color="#FFFFFF"><b>1.2</font></b></td>
<td bgcolor="#336666"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
<td bgcolor="#336666" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">DHCP</font></b></td>
<td width="9" bgcolor="#336666"> </td>
</tr>
<tr>
<td bgcolor="#669999" height="25" width="18"> </td>
<td bgcolor="#669999" height="25"><b><font face="Arial" size="2" color="#FFFFFF">
1.2.8</font></b></td>
<td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
<td bgcolor="#669999" height="25" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">DHCP relay </font></b></td>
<td bgcolor="#669999" height="25" width="9"> </td>
</tr></table>
<table border="0" cellpadding="0" cellspacing="0" width="95%" bordercolor="#111111">
<tr>
<td width="15"></td>
<td>
<font FACE="Arial" SIZE="2">
DHCP clients use IP broadcasts to find the DHCP server on the segment.
What happens when the server and the client are not on the same
segment and are separated by a router? Routers do not forward these
broadcasts.</font><p>
<font FACE="Arial" SIZE="2">
DHCP is not the only critical service that uses broadcasts. Cisco
routers and other devices may use broadcasts to locate TFTP servers.
Some clients may need to broadcast to locate a TACACS server. A TACACS
server is a security server. Typically, in a complex hierarchical
network, clients reside on the same subnet as key servers. Such remote
clients will broadcast to locate these servers. However, routers, by
default, will not forward client broadcasts beyond their subnet.</font></p>
<p>
<font FACE="Arial" SIZE="2">
Because some clients are useless without services such as DHCP, one of
two choices must be implemented. The administrator will need to place
servers on all subnets or use the Cisco IOS helper address feature.
Running services such as DHCP or DNS on several computers creates
overhead and administrative difficulties making the first option
inefficient. When possible, administrators should use the </font>
<font FACE="Courier New" SIZE="2">
<b>ip helper-address</b></font><font FACE="Arial" SIZE="2">
command to relay broadcast requests for these key UDP services.</font></p>
<p>
<font FACE="Arial" SIZE="2">
By using the helper address feature, a router can be configured to
accept a broadcast request for a UDP service and then forward it as a
unicast to a specific IP address. By default, the </font>
<font FACE="Courier New" SIZE="2">
<b>ip helper-address</b></font><font FACE="Arial" SIZE="2">
command forwards the following eight UDP services:</font></p>
<ul>
<li>
<font FACE="Arial" SIZE="2">
Time</font></li>
<li>
<font FACE="Arial" SIZE="2">
TACACS</font></li>
<li>
<font FACE="Arial" SIZE="2">
DNS</font></li>
<li>
<font FACE="Arial" SIZE="2">
BOOTP/DHCP Server</font></li>
<li>
<font FACE="Arial" SIZE="2">
BOOTP/DHCP Client</font></li>
<li>
<font FACE="Arial" SIZE="2">
TFTP</font></li>
<li>
<font FACE="Arial" SIZE="2">
NetBIOS Name Service</font></li>
<li>
<font FACE="Arial" SIZE="2">
NetBIOS datagram Service</font></li>
</ul>
<p>
<font FACE="Arial" SIZE="2">
In the particular case of DHCP, a client broadcasts a DHCPDISCOVER
packet on its local segment.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> This
packet is picked up by the gateway. If a helper-address is configured,
the DHCP packet is forwarded to the specified address. Before
forwarding the packet, the router fills in the GIADDR field of the
packet with the IP address of the router for that segment. This
address will then be the gateway address for the DHCP client, when it
gets the IP address.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"></font></p>
<p>
<font FACE="Arial" SIZE="2">
The DHCP server receives the discover packet. The server uses the GIADDR field to index into the list of address pools, to find one
which has the gateway address set to the value in GIADDR. This pool is
then used to supply the client with its IP address.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12">
<img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12"></font></p>
<p>
<TABLE bgcolor="#B0AFAF" width="95%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top">
<TABLE bgcolor="#669999" width="100%" cellspacing="0" cellpadding="0" border="0">
<TR>
<TD width="5">
<img border="0" src="../../images/lab_toplft.gif" width="116" height="23"></TD>
<TD>
<IMG alt="" height="1" width="3" src="../../images/s.gif"></TD><TD align="right" valign="top">
<IMG alt="" src="../../images/corner_ur_7.gif" width="7" height="7"></TD>
</TR>
</TABLE>
<TABLE bgcolor="#B0AFAF" width="100%" cellspacing="0" cellpadding="0" border="0">
<TR>
<TD>
<TABLE width="100%" cellpadding="2" cellspacing="0" border="0" bordercolor="#111111">
<TR>
<TD bgcolor="#ffffff" width="15"> </TD>
<TD bgcolor="#ffffff"><font FACE="Arial" SIZE="2" COLOR="#000000">
<p><font color="#808080">
<a target="main" href="../lab1_2_8/index.html">
<img border="0" src="../../images/lab_icon.gif" width="25" height="25"> <b>
Lab Activity</b></a></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Lab Exercise: Configuring DHCP Relay</p>
<p>In this lab, a router will be configured for Dynamic Host
Configuration Protocol (DHCP).</p>
</font><font FACE="Arial" SIZE="2" COLOR="#000000">
<p></TD>
</TR>
</TABLE>
</TD>
</TR>
</TABLE>
</TD>
</TR>
</TABLE>
<TABLE bgcolor="#669999" width="95%" cellspacing="0" cellpadding="0" border="0">
<TR>
<TD width="5">
<img border="0" src="../../images/lab_toplft.gif" width="116" height="23"></TD>
<TD>
<IMG alt="" height="1" width="3" src="../../images/s.gif"></TD><TD align="right" valign="top">
<IMG alt="" src="../../images/corner_ur_7.gif" width="7" height="7"></TD>
</TR>
</TABLE>
<TABLE bgcolor="#B0AFAF" width="95%" cellspacing="0" cellpadding="0" border="0">
<TR>
<TD>
<TABLE width="100%" cellpadding="2" cellspacing="0" border="0" bordercolor="#111111">
<TR>
<TD bgcolor="#ffffff" width="15"> </TD>
<TD bgcolor="#ffffff"><font FACE="Arial" SIZE="2" COLOR="#000000">
<p><font color="#808080">
<a href="javascript:void(0)" onClick="window.open('../../elabs/mod01/elab_1_2_8/elab1_2_8.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');">
<img border="0" src="../../images/lab_icon.gif" width="25" height="25"><b></a>
<a href="javascript:void(0)" onClick="window.open('../../elabs/mod01/elab_1_2_8/elab1_2_8.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');">Lab Activity</a></b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>e-Lab Activity: Configuring DHCP Relay</p>
</font><font FACE="Arial" SIZE="2" COLOR="#000000">
<p><font color="#808080">In this lab, the student will configure a router for DHCP, add the ability for workstations to remotely obtain DHCP
addresses and dynamically assign addresses to the attached
hosts.</font></p>
<p></TD>
</TR>
</TABLE>
</TD>
</TR>
</TABLE>
<TABLE bgcolor="#B0AFAF" width="95%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top">
<TABLE bgcolor="#669999" width="100%" cellspacing="0" cellpadding="0" border="0">
<TR>
<TD width="5">
<img border="0" src="../../images/lab_toplft.gif" width="116" height="23"></TD>
<TD><IMG alt="" height="1" width="3" src="../../images/s.gif"></TD><TD align="right" valign="top">
<IMG alt="" src="../../images/corner_ur_7.gif" width="7" height="7"></TD>
</TR>
</TABLE>
<TABLE bgcolor="#B0AFAF" width="100%" cellspacing="0" cellpadding="0" border="0">
<TR>
<TD>
<TABLE width="100%" cellpadding="2" cellspacing="0" border="0" bordercolor="#111111">
<TR>
<TD bgcolor="#ffffff" width="15"> </TD>
<TD bgcolor="#ffffff"><font FACE="Arial" SIZE="2" COLOR="#000000">
<p><font color="#808080">
<img border="0" src="../../images/links_icon.gif" width="25" height="25">
<b>Web Links</b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>Configuring DHCP Relay, DHCP Server, and PAT<p>
<a target="_blank" href="http://www.cisco.com/univercd/cc/td/doc/product/access/acs_fix/750/cfgguide/700cgdhc.htm">
http://www.cisco.com/univercd/cc/td/
doc/product/ ccess/acs_fix/750/
cfgguide/700cgdhc.htm</a></font></p>
<p>
<IMG alt="" height="2" width="1" src="../../images/s.gif"></p>
</TD>
</TR>
</TABLE>
</TD>
</TR>
</TABLE>
</TD>
</TR>
</TABLE><p>
<font face="Arial" size="2"> </font>
</td>
</tr>
</table>
</body>
</html>
|
