菱形之GRE篇
OSPF为IGP协议，保证PE1和PE4互通，建立PE1到PE4 GRE隧道。
interface Tunnel0/0/14
ip address 10.1.4.1 255.255.255.0 （统一用10段的，可以借用）
tunnel-protocol gre
keepalive period 20 （保证隧道的连通性，定时发送探测数据包，避免对端不可达造成数据丢失）
source 100.1.2.1 （也可以用loopback地址）
destination 100.2.4.2
源地址和目的地址分别是物理接口的IP地址和隧道本身的IP地址不同，隧道协议是GRE，决定了隧道性质。
在PE1的隧道接口pingPE4的隧道接口
[PE1]ping -a 10.1.4.1 10.1.4.2
PING 10.1.4.2: 56 data bytes, press CTRL_C to break
Reply from 10.1.4.2: bytes=56 Sequence=1 ttl=255 time=12 ms
Reply from 10.1.4.2: bytes=56 Sequence=2 ttl=255 time=16 ms
可以看到keepalive报文
[PE1-Tunnel0/0/14]display keepalive packets count
Send 2 keepalive packets to peers, Receive 2 keepalive response packets from pee
rs
Receive 3 keepalive packets from peers, Send 3 keepalive response packets to pee
rs.
查看tunnel接口信息
[PE1]dis interface Tunnel 0/0/14
Tunnel0/0/14 current state : UP
Line protocol current state : UP
Last line protocol up time : 2015-12-07 16:07
Description:HUAWEI, Tunnel0/0/14 Interface
Route Port,The Maximum Transmit Unit(L3) is 1500 bytes
Internet Address is 10.1.4.1/24
Encapsulation is TUNNEL, loopback not set
Tunnel source 100.1.2.1 (GigabitEthernet0/2/5), destination 100.2.4.2
Tunnel protocol/transport GRE/IP, key disabled
keepalive enable period 20 retry-times 3
Checksumming of packets disabled
Current system time: 2015-12-07 16:07
20 seconds input rate 48 bits/sec, 0 packets/sec
20 seconds output rate 16 bits/sec, 0 packets/sec
0 seconds input rate 0 bits/sec, 0 packets/sec
0 seconds output rate 0 bits/sec, 0 packets/sec
63 packets input, 4616 bytes
0 input error
37 packets output, 2076 bytes
0 output error
Input:
Unicast: 58 packets, Multicast: 0 packets
Output:
Unicast: 5 packets, Multicast: 0 packets
Input bandwidth utilization : 0%
Output bandwidth utilization : 0%
[PE1]dis ip routing-table
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
Destinations : 23 Routes : 23
Destination/Mask Proto Pre Cost Flags NextHop Interface
10.1.4.0/24 Direct 0 0 D 10.1.4.1 Tunnel0/0/14
10.1.4.1/32 Direct 0 0 D 127.0.0.1 Tunnel0/0/14
10.1.4.255/32 Direct 0 0 D 127.0.0.1 Tunnel0/0/14