MTU的问题

1，mtu的范围只和硬件设备有关，有的mtu是400多，有的更少，也有更大的。。

2，路由有路由的mtu，你自己的网卡有自己的mtu，不互相影响，但是最好别调，用默认

MTU是可通过此接口最大包的大小（单位字节）。。。127.0.0.1是环回接口，就是你自己自身的IP，先这么理解吧。。

思科路由器设置VPDN的方法

CCNA 重新认证：CCNA认证证书的时效性是三年。要重新更新证书，必须通过CCNA的'考试(如果通过的下一阶段CCNP的考试，CCNA认证证书会被自动更新)下面是我整理的关于思科路由器设置VPDN的方法，欢迎大家参考!

1 启用aaa new-model

aaa new-model

2、建立认证方式

aaa authentication ppp mytest local (这里为本地认证)

3、启用VPDN

vpdn enable

4、建立VPDN组

vpdn-group mytest

! Default L2TP VPDN group

accept-dialin

protocol l2tp

virtual-template 1

local name mytest-name

lcp renegotiation always //注意非常重要，否则LCP不会自协商，那么路由器的MTU值一定要和对方LAC发过来的要匹配，否则LCP失败

l2tp tunnel password 7 00090A1201481F

source-ip x.x.x.x

5、建立IP 本地POOL

ip local pool mytest-01 172.23.1.2 172.23.1.254

6、配置虚模板

interface Virtual-Template1

ip address 172.23.1.1 255.255.255.0

peer default ip address pool mytest-01

ppp authentication pap mytest

5200g

domain qh.org

authentication-scheme vpdn_none

accounting-scheme vpdn_none

l2tp-group qh.org

l2tp-group qh.org

tunnel password simple mytest

tunnel name mytest

start l2tp ip x.x.x.x

tunnel source LoopBack0

附：L2TP协议结构

1、IP包头 20字节

2、UDP报头 8字节

3、L2TP报头 12字节：包括：

1)the version and flag fields (2 bytes)

2)the tunnel id and session id fields (2 bytes each)

3)2 bytes of padding offset

4)4 bytes of Point-to-Point Protocol (PPP) encapsulation

//以下是没有启用LCP自协商的DEBUG 连接失败

Jul 25 00:03:48.084: Vi1 VPDN: Virtual interface created for [email protected]

Jul 25 00:03:48.084: Vi1 PPP: Phase is DOWN, Setup

Jul 25 00:03:48.084: Vi1 VPDN: Clone from Vtemplate 1 filterPPP=0 blocking

Jul 25 00:03:48.204: %LINK-3-UPDOWN: Interface Virtual-Access1, changed state to up

Jul 25 00:03:48.204: Vi1 PPP: Using set call direction

Jul 25 00:03:48.204: Vi1 PPP: Treating connection as a callin

Jul 25 00:03:48.204: Vi1 PPP: Phase is ESTABLISHING, Passive Open

Jul 25 00:03:48.204: Vi1 LCP: State is Listen

Jul 25 00:03:48.204: Vi1 VPDN: Bind interface direction=2

Jul 25 00:03:48.204: Vi1 LCP: I FORCED CONFREQ len 14

Jul 25 00:03:48.204: Vi1 LCP: MRU 1492 (0x010405D4)

Jul 25 00:03:48.204: Vi1 LCP: AuthProto PAP (0x0304C023)

Jul 25 00:03:48.204: Vi1 LCP: MagicNumber 0x01022143 (0x050601022143)

Jul 25 00:03:48.204: Vi1 VPDN: PPP LCP accepted rcv CONFACK

Jul 25 00:03:48.204: Vi1 LCP: I FORCED CONFACK len 10

Jul 25 00:03:48.204: Vi1 LCP: MRU 1480 (0x010405C8)

Jul 25 00:03:48.204: Vi1 LCP: MagicNumber 0x082D5DCE (0x0506082D5DCE)

Jul 25 00:03:48.204: Vi1 VPDN: PPP LCP not accepting sent CONFACK

Jul 25 00:03:48.204: Vi1 VPDN: Unbind interface

Jul 25 00:03:48.296: Vi1 PPP: No remote authentication for call-in

Jul 25 00:03:48.560: Vi1 PPP: Missed link down notification

Jul 25 00:03:48.560: Vi1 LCP: State is Closed

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