Discovery 19: Configure a Provider-Assigned IPv4 Address探索活動 19：設定服務提供者指派的 IPv4 位址

R1 is currently configured with a static IPv4 address on Ethernet0/3 interface. Verify this fact.

R1 目前在 Ethernet0/3 介面上設定了靜態 IPv4 位址，請確認這一點。

Access the console of R1 and examine the current IPv4 configuration on Ethernet0/3.

進入 R1 的主控台，檢查 Ethernet0/3 上目前的 IPv4 設定。

Routing has not been configured on R1. Verify that only local and connected routes are present in the R1 routing table.

R1 尚未設定路由。請確認 R1 路由表中僅存在本地路由與連接路由。

On R1, enter the following command:

在 R1 上輸入以下指令：

Only L and C entries are present in the routing table.

路由表中只存在 L 與 C 項目。

Reconfigure the Ethernet0/3 interface on the R1 router to obtain its IPv4 address and default gateway via DHCP.

重新設定 R1 路由器上的 Ethernet0/3 介面，使其透過 DHCP 取得 IPv4 位址與預設閘道。

On R1, enter the following commands:

在 R1 上輸入以下指令：

Wait up to 30 seconds. Verify that R1 displays a syslog message indicating that the Ethernet0/3 interface has been assigned an IPv4 address via DHCP.

等待最多 30 秒，確認 R1 顯示一則 syslog 訊息，指出 Ethernet0/3 介面已透過 DHCP 取得 IPv4 位址。

Verify that the IPv4 address is assigned to the Ethernet0/3 interface.

確認 IPv4 位址已指派給 Ethernet0/3 介面。

On R1, enter the following command:

在 R1 上輸入以下指令：

The address assigned is 198.51.100.101. Note the address now, because it will be useful later in the lab exercise, when you will examine DHCP server configuration on R2 router.

取得的位址是 198.51.100.101。請記下這個位址，因為在稍後的實驗練習中，當你要檢查 R2 路由器上的 DHCP 伺服器設定時會用到。

Verify that there is now a default route on R1, which uses the R2 198.51.100.1 address as the next hop.

確認 R1 上現在有一條預設路由，其以 R2 的 198.51.100.1 位址作為下一跳。

On R1, enter the following command:

在 R1 上輸入以下指令：

Verify the connectivity from R1 to the public IPv4 address side of the topology by pinging SRV2 (203.0.113.30). The ping should be successful.

透過 ping SRV2（203.0.113.30）來確認 R1 到拓樸中公用 IPv4 位址一側的連線。ping 應該要成功。

On R1, enter the following command:

在 R1 上輸入以下指令：

Now that R1 connects to the service provider via a DHCP assigned IPv4 address, configure PC1 to also acquire an address and a default gateway via DHCP. The R2 router is configured as a DHCP server also for the 10.10.1.0/24 subnet. First, remove the current default gateway configuration, and then configure the Ethernet0/0 interface to acquire an IPv4 address via DHCP and go to the privileged EXEC mode.

既然 R1 已透過 DHCP 指派的 IPv4 位址連接到服務提供者，接下來設定 PC1 也透過 DHCP 取得位址與預設閘道。R2 路由器同時也被設定為 10.10.1.0/24 子網路的 DHCP 伺服器。首先移除目前的預設閘道設定，然後設定 Ethernet0/0 介面透過 DHCP 取得 IPv4 位址，並進入特權 EXEC 模式。

On PC1, enter the following commands:

在 PC1 上輸入以下指令：

Verify whether PC1 has the IPv4 address and default gateway assigned.

確認 PC1 是否已被指派 IPv4 位址與預設閘道。

On PC1, enter the following commands:

在 PC1 上輸入以下指令：

PC1 is not assigned an IPv4 address, although it is configured to receive information via DHCP. This situation is expected.

PC1 並未被指派 IPv4 位址，儘管它已設定為透過 DHCP 接收資訊。這是預期中的情況。

DHCP clients need to use UDP broadcasts to send their initial DHCPDISCOVER messages, because they don’t have information about the network to which they are attached to. If the client is on a network segment that does not include a server, UDP broadcasts are normally not forwarded, because most routers are configured to not forward broadcast traffic. So broadcast messages that are sent by PC1 arrive only to the Ethernet0/0 interface of R1 and do not cross the R1 router. Because the DHCP server is located on the R2 router, PC1 DHCP requests never reach the server.

DHCP 用戶端需要使用 UDP 廣播來傳送其初始的 DHCPDISCOVER 訊息，因為它們並不知道自己所連接網路的資訊。如果用戶端所在的網路區段中沒有伺服器，UDP 廣播通常不會被轉送，因為大多數路由器都設定為不轉送廣播流量。因此 PC1 傳送的廣播訊息只會到達 R1 的 Ethernet0/0 介面，不會穿越 R1 路由器。由於 DHCP 伺服器位於 R2 路由器上，PC1 的 DHCP 請求永遠無法到達伺服器。

You can remedy this situation by configuring the interface of the router that is receiving the broadcasts to forward certain classes of broadcast traffic to a specific IPv4 address, called helper address. You can use more than one helper address per interface. When configured with a helper address, the router is considered to be the relay agent.

你可以透過設定接收廣播的路由器介面，將特定類別的廣播流量轉送到一個稱為輔助位址（helper address）的特定 IPv4 位址，來解決此問題。每個介面可以使用一個以上的輔助位址。當設定了輔助位址後，該路由器就被視為中繼代理（relay agent）。

To configure a helper address, use the ip helper-address IPv4-address interface configuration mode command.

若要設定輔助位址，請使用 ip helper-address IPv4-address 這個介面設定模式指令。

When a helper address is configured for DHCP, the router "picks up" a broadcast packet from the interface it receives it on, and it generates new DHCP messages to send out to the indicated helper address. In the new DHCP message, the router (relay agent) inserts the IPv4 address of its interface, where broadcast message was received and where the ip helper-address command is configured, into the gateway IPv4 address (giaddr) field of the DHCP packet. This giaddr IPv4 address enables the DHCP server to determine which subnet should receive the offer and to identify the appropriate IPv4 address pool from which to select the address. The DHCP relay agent forwards the received broadcast as a unicast packet, to the address of the DHCP server specified by the ip helper-address command.

當 DHCP 設定了輔助位址後，路由器會從收到廣播封包的介面「取出」該封包，並產生新的 DHCP 訊息傳送到指定的輔助位址。在這則新的 DHCP 訊息中，路由器（中繼代理）會把收到廣播訊息、也就是設定了 ip helper-address 指令的那個介面的 IPv4 位址，填入 DHCP 封包的閘道 IPv4 位址（giaddr）欄位。這個 giaddr IPv4 位址讓 DHCP 伺服器能判斷應由哪個子網路接收此提供，並識別應從哪個適當的 IPv4 位址池中挑選位址。DHCP 中繼代理會將收到的廣播以單播封包的形式，轉送到 ip helper-address 指令所指定的 DHCP 伺服器位址。

Configure the R1 router to relay DHCP broadcasts.

設定 R1 路由器以中繼 DHCP 廣播。

On R1, enter the following command:

在 R1 上輸入以下指令：

The 198.51.100.1 address, specified as a helper address, is the address of the R2 router. R2 is the DHCP server for the 10.10.1.0/24 subnet.

設定為輔助位址的 198.51.100.1 位址，就是 R2 路由器的位址。R2 是 10.10.1.0/24 子網路的 DHCP 伺服器。

All broadcasts that R1 receives on its Ethernet0/0 interface will be forwarded to the 198.51.100.1 address as unicast messages. The router will add the 10.10.1.1 IPv4 address information to each of the unicast messages. It will add the address into the giaddr field of the DHCP message.

R1 在其 Ethernet0/0 介面上收到的所有廣播，都會以單播訊息的形式轉送到 198.51.100.1 位址。路由器會將 10.10.1.1 這個 IPv4 位址資訊加入每一則單播訊息中，並將該位址填入 DHCP 訊息的 giaddr 欄位。

Go to the console of PC1, and wait for about 30 seconds for the syslog message, informing about the DHCP assignment to be displayed in the console. Then, verify the IPv4 address information on PC1.

進入 PC1 的主控台，等待約 30 秒，直到主控台顯示告知 DHCP 指派情況的 syslog 訊息。接著確認 PC1 上的 IPv4 位址資訊。

On PC1, enter the following commands:

在 PC1 上輸入以下指令：

PC1 is assigned the IPv4 address of 10.10.1.101 and the default gateway 10.10.1.1.

PC1 被指派了 IPv4 位址 10.10.1.101 與預設閘道 10.10.1.1。

Using the show dhcp lease and show dhcp server commands, verify DHCP-specific information on PC1.

使用 show dhcp lease 與 show dhcp server 指令，確認 PC1 上與 DHCP 相關的資訊。

On PC1, enter the following command:

在 PC1 上輸入以下指令：

The show dhcp commands verify the basic DHCP IPv4 address and default gateway information, along with DHCP-specific information. In the output of the show dhcp lease command, you can see the assigned IPv4 address, the local interface to which the DHCP address was assigned, the DHCP server that provided the information, default gateway, and lease duration expressed in seconds. The value of 432000 seconds corresponds to 5 days. The output of the show dhcp server command indicates which DHCP server is chosen by the device. For PC1, it is any available server.

show dhcp 指令可用來確認基本的 DHCP IPv4 位址與預設閘道資訊，以及 DHCP 相關的資訊。在 show dhcp lease 指令的輸出中，你可以看到已指派的 IPv4 位址、被指派 DHCP 位址的本地介面、提供資訊的 DHCP 伺服器、預設閘道，以及以秒為單位表示的租約期間。432000 秒的值相當於 5 天。show dhcp server 指令的輸出則指出該裝置所選用的 DHCP 伺服器。對 PC1 而言，是任何可用的伺服器。

R2 is configured as the DHCP server for both the 198.51.100.0/24 and 10.10.1.0/24 segments. Although the DHCP server configuration is beyond the scope of this course, you can view the configuration, to understand what information is required to be configured on the DHCP server. DHCP configuration lines are present in the running configuration and startup configuration file of R2. To filter out only DHCP-related information from the startup configuration file, use the show startup-config | section dhcp command.

R2 同時被設定為 198.51.100.0/24 與 10.10.1.0/24 兩個網段的 DHCP 伺服器。雖然 DHCP 伺服器設定超出本課程範圍，你仍可以查看其設定，以了解 DHCP 伺服器需要哪些設定資訊。DHCP 設定命令列存在於 R2 的執行中設定與啟動設定檔案中。若要從啟動設定檔案中僅篩選出與 DHCP 相關的資訊，可使用 show startup-config | section dhcp 指令。

On R2, enter the following command:

在 R2 上輸入以下指令：

IPv4 parameters meant to be assigned on the 198.51.100.0/24 segment are specified in the DHCP pool ClientRouter. The pool specifies that addresses from the range 198.51.10.0/24 should be offered to DHCP clients. The default gateway for the segment is 198.51.100.1, and the lease period is 7 days. The configuration also specifies, in the ip dhcp excluded-addresses command, that addresses from 198.51.100.1 to 198.51.100.100 should not be offered. Therefore, the R1 router has received the address 198.51.100.101, the first available address from the ClientRouter pool. Address 198.51.100.1 is excluded, because it is already statically assigned to the R2 router Ethernet 0/3 interface, and it should not be assigned to any other host.

要在 198.51.100.0/24 網段上指派的 IPv4 參數，是在 DHCP 位址池 ClientRouter 中指定的。該位址池指定應向 DHCP 用戶端提供 198.51.10.0/24 範圍內的位址。此網段的預設閘道為 198.51.100.1，租約期間為 7 天。設定中也在 ip dhcp excluded-addresses 指令中指定，198.51.100.1 到 198.51.100.100 之間的位址不應被提供。因此，R1 路由器取得了 198.51.100.101 這個位址，也就是 ClientRouter 位址池中第一個可用的位址。198.51.100.1 位址被排除在外，因為它已靜態指派給 R2 路由器的 Ethernet 0/3 介面，不應再指派給任何其他主機。

IPv4 parameters meant to be assigned on the 10.10.1.0/24 segment are specified in the DHCP pool Client_10_10_1_0. The pool specifies that addresses from the range 10.10.1.0/24 should be offered to DHCP clients. The default gateway for the segment is 10.10.1.1, and the lease period is 5 days. The configuration also specifies, in the ip dhcp excluded-addresses command, that addresses from 10.10.1.0 to 10.10.1.100 should not be offered. Therefore, PC1 has received the 10.10.1.101 address, the first available address from the Client_10_10_1_0 pool. The 10.10.1.1 address is excluded, because it is already statically assigned to the R1 router Ethernet 0/0 interface and it should not be assigned to any other host in the 10.10.1.0/24 segment.

要在 10.10.1.0/24 網段上指派的 IPv4 參數，是在 DHCP 位址池 Client_10_10_1_0 中指定的。該位址池指定應向 DHCP 用戶端提供 10.10.1.0/24 範圍內的位址。此網段的預設閘道為 10.10.1.1，租約期間為 5 天。設定中也在 ip dhcp excluded-addresses 指令中指定，10.10.1.0 到 10.10.1.100 之間的位址不應被提供。因此，PC1 取得了 10.10.1.101 位址，也就是 Client_10_10_1_0 位址池中第一個可用的位址。10.10.1.1 位址被排除在外，因為它已靜態指派給 R1 路由器的 Ethernet 0/0 介面，不應再指派給 10.10.1.0/24 網段中的任何其他主機。