2014年思科认证考试Cisco路由技术：Cisco路由器PPPoE配置命令
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配置PPPoE时，使用的ip tcp adjust-mss命令
ip tcp adjust-mss
To adjust the maximum segment size (MSS) value of TCP SYN packets going through a router, use the ip tcp adjust-mss command in interface configuration mode. To return the MSS value to the default setting, use the no form of this command.
ip tcp adjust-mss max-segment-size
no ip tcp adjust-mss max-segment-size
Syntax Description
max-segment-size
Maximum segment size, in bytes. The range is from 500 to 1460.
Defaults
If the ip tcp adjust-mss command is not configured, the MSS is determined by the originating host.
Command Modes
Interface configuration
Command History
Release Modification
12.2(4)T
This command was introduced.
12.2(8)T
This command was changed from ip adjust-mss to ip tcp adjust-mss.
Usage Guidelines
When a host (usually a PC) initiates a TCP session with a server, it negotiates the IP segment size by using the MSS option field in the TCP SYN packet. The value of the MSS field is determined by the maximum transmission unit (MTU) configuration on the host. The default MSS value for a PC is 1500 bytes.
The PPP over Ethernet (PPPoE) standard supports a MTU of only 1492 bytes . The disparity between the host and PPPoE MTU size can cause the router in between the host and the server to drop 1500-byte packets and terminate TCP sessions over the PPPoE network. Even if the path MTU (which detects the correct MTU across the path) is enabled on the host, sessions may be dropped because system administrators sometimes disable the ICMP error messages that must be relayed from the host in order for path MTU to work.
The ip tcp adjust-mss command helps prevent TCP sessions from being dropped by adjusting the MSS value of the TCP SYN packets.
The ip tcp adjust-mss command is effective only for TCP connections passing through the router.
In most cases, the optimum value for the max-segment-size argument is 1452 bytes. This value plus the 20-byte IP header, the 20-byte TCP header, and the 8-byte PPPoE header add up to a 1500-byte packet that matches the MTU size for the Ethernet link.
If you are configuring the ip mtu command on the same interface as the ip tcp adjust-mss command, it is recommended that you use the following commands and values:
ip tcp adjust-mss 1452
ip mtu 1492
Instance:
interface Dialer1
ip address negotiated
ip mtu 1492 (将原来的mtu1500变为1492，因pppoe 的包头是8个字节)
ip tcp adjust-mss 1452
ip nat outside
encapsulation ppp
dialer pool 1 (通过拨号池，将物理端口和虚拟拨号端口绑定)
ppp pap sent-username ****** password 0 ******(ppp pap的单向验证)
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