提要：
讨论OSPF中的外部汇总命令summary-address的作用以及使用地点。
cisco文档关于summary-address的解释
summary-address {{ip-address mask} | {prefix mask}} [not-advertise] [tag tag]
Multiple groups of addresses can be summarized for a given level. Routes learned from other routing protocols can also be summarized. The metric used to advertise the summary is the smallest metric of all the more specific routes. This command helps reduce the size of the routing table.
Using this command for OSPF causes an OSPF Autonomous System Boundary Router (ASBR) to advertise one external route as an aggregate for all redistributed routes that are covered by the address. For OSPF, this command summarizes only routes from other routing protocols that are being redistributed into OSPF.
OSPF does not support the summary-address 0.0.0.0 0.0.0.0 command.
正文：
一般而言，summary-address的作用是用于在普通及骨干区域的ASBR上汇总重分布的外部路由，可是在NSSA区域，在ABR和ASBR上都可以汇总重分布的外部路由， 其中存在细微差别，下面以具体实验为例讲述：
TOPO如下：
基本配置如下：
l R1
l R2
l R3
R1
| hostname r1
!
interface Serial1/0
ip address 12.1.1.1 255.255.255.0
!
router ospf 10
router-id 1.1.1.1
network 12.1.1.0 0.0.0.255 area 0
!
End
|
R2
| hostname r2
!
interface Serial1/0
ip address 23.1.1.2 255.255.255.0
!
interface Serial2/0
ip address 12.1.1.2 255.255.255.0
!
router ospf 10
router-id 2.2.2.2
network 12.1.1.0 0.0.0.255 area 0
network 23.1.1.0 0.0.0.255 area 1
!
End
|
R3
| hostname r3
!
interface Loopback0
ip address 4.4.4.4 255.255.255.0
!
interface Loopback1
ip address 5.5.5.5 255.255.255.0
!
interface Serial2/0
ip address 23.1.1.3 255.255.255.0
!
router ospf 10
router-id 3.3.3.3
area 1 nssa
redistribute connected subnets
network 23.1.1.0 0.0.0.255 area 1
!
End
|
验证：
首先，我们在R3（ASBR）上对外部路由4.4.4.0/24做外部汇总为4.0.0.0/8
r3(config-router)#summary-address 4.0.0.0 255.0.0.0
然后，在R2
（ABR）上对外部路由5.5.5.0/24做外部汇总为5.0.0.0/8
r2(config-router)#summary-address 5.0.0.0 255.0.0.0
查看R2数据库中Forward Address信息
r2#sh ip ospf data external
OSPF Router with ID (2.2.2.2) (Process ID 10)
Type-5 AS External Link States
LS age: 780
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 4.0.0.0 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0xB8BF
Length: 36
Network Mask: /8
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 23.1.1.3
External Route Tag: 0
LS age: 76
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 5.0.0.0 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x6A29
Length: 36
Network Mask: /8
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0
可以看到，当summary-address在R2（ABR）上作NSSA外部路由汇总时，可以将Forward Address变为0.0.0.0，具体全零Forward Address的作用，大家可以参考one-tom论坛日历事务: OSPF在转换LSA 5时的转发地址抑制。
附注：
如果外部路由汇总时加上[not-advertise]参数时，它起到“prevent the type 7 LSAs from being translated outside the NSSA”的作用，即不将类型7 的LSA转换成类型5通告到骨干区域，实现了路由过滤，这里就不再赘述。
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