1、简介
目前，在IP网络中实现实时语音、视频通信和应用已经成为网络应用的一个主流技术和发展方向，本文详细介绍IP协议族中用于实时语音、视频数据传输的标准协议RTP（ Real-time Transport Protocol）和RTCP（RTP Control Ptotocol）的主要功能

IP地址和路由协议：R1
Ethernet0/0 192.168.12.1/24 运行RIP
Loopback0 1.1.1.1 /32 运行RIP
IP地址和路由协议：R2
Ethernet0/1 192.168.12.2/24 运行RIP
Ethernet0/0 192.168.23.2/24 运行OSPF
Loopback0 2.2.2.2 /32 运行OSPF
在R2路由器配置了rip和ospf的双向路由重分发。
IP地址和路由协议：R3
Ethernet0/0 192.168.23.3/24 运行OSPF
Loopback0 3.3.3.3 /32 运行OSPF
2）实现以上配置后，观察R3路由器的类型5 lsa
R3#sh ip ospf database external
OSPF Router with ID (3.3.3.3) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 848
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 1.1.1.1 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x7D17
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0

Routing Bit Set on this LSA
LS age: 848
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 192.168.12.0 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x75AD
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0
现在的Forward Address是正常值: 0.0.0.0
R3#sh ip route
O E2 192.168.12.0/24 [110/20] via 192.168.23.2, 00:18:59, Ethernet0/1
1.0.0.0/32 is subnetted, 1 subnets
O E2 1.1.1.1 [110/20] via 192.168.23.2, 00:18:59, Ethernet0/1
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 192.168.23.2, 00:18:59, Ethernet0/1
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
C 192.168.23.0/24 is directly connected, Ethernet0/1
3）根据前面的描述的条件，把R2路由器的外部接口（运行RIP）e0/1也配置到ospf中，可以观察到Forward Address的变化:
R3#sh ip ospf database external
OSPF Router with ID (3.3.3.3) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 22
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 1.1.1.1 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000002
Checksum: 0x56C6
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 192.168.12.1
External Route Tag: 0
注意，此时R3路由器的类型5的lsa中，Forward Address为 192.168.12.1，该地址是一个外部路由域的路由器R1的接口地址。但是观察下面的路由表，O E2路由的下一跳并没有发生任何变化。

R3#sh ip route
O 192.168.12.0/24 [110/20] via 192.168.23.2, 00:00:32, Ethernet0/1
1.0.0.0/32 is subnetted, 1 subnets
O E2 1.1.1.1 [110/20] via 192.168.23.2, 00:00:32, Ethernet0/1
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/11] via 192.168.23.2, 00:00:32, Ethernet0/1
3.0.0.0/32 is subnetted, 1 subnets
C 3.3.3.3 is directly connected, Loopback0
C 192.168.23.0/24 is directly connected, Ethernet0/1
5、结束语
上述实验结果表明，在ASBR路由器配置ospf时，如果外部接口参与到ospf中，并且接口不是point－to－point或者point－to－multipoint，类型5的LSA中的forwarding address就会改变为一个外部路由器的地址。
一般情况下，不建议将ASBR路由器的外部接口配置到ospf中，这样forwarding address就是0.0.0.0，流量发送给ASBR即可。
如果ASBR的外部接口是广播网段或者NBMA，即ASBR可能有多个外部直连邻居，在这种情况下，建议将ASBR路由器的外部接口配置到ospf中，使ASBR有更准确的外部转发地址，避免次优路径。
北京华尔思网络实验室 周军 [email protected]