OSPF特殊区域：
R2，R3各有一个环回口通告进area0，所有的外部路由全部重分布进OSPF进程。在R1上应该能看到4条E2路由，2条IA路由。
看下R1上EIGRP进来的两条路由的下一跳。
2.0.0.0/32 is subnetted, 1 subnets
O E2 2.2.2.2 [110/20] via 12.0.0.2, 00:02:56, Serial1/0
22.0.0.0/32 is subnetted, 1 subnets
O E2 22.22.22.22 [110/20] via 12.0.0.2, 00:02:51, Serial1/0
看下这两条路由的forward metric
R1(config-router)#do show ip ro 2.2.2.2
Routing entry for 2.2.2.2/32
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 64 因为没有FA，将包发到始发的ASBR上。forward metric就是去往ASBR的metric，如果有forward address就是到达forward address的metric。
Last update from 12.0.0.2 on Serial1/0, 00:01:47 ago
Routing Descriptor Blocks:
* 12.0.0.2, from 2.2.2.2, 00:01:47 ago, via Serial1/0
Route metric is 20, traffic share count is 1
将area 1设为stub区域，再看下路由
R1，R2，R4
area 1 stub
再看下R1的路由表：
2.0.0.0/32 is subnetted, 1 subnets
O E2 2.2.2.2 [110/20] via 13.0.0.3, 00:00:05, Serial1/1
22.0.0.0/32 is subnetted, 1 subnets
O E2 22.22.22.22 [110/20] via 13.0.0.3, 00:00:08, Serial1/1
100.0.0.0/32 is subnetted, 1 subnets
O IA 100.0.0.1 [110/65] via 12.0.0.2, 00:00:05, Serial1/0
看下同一条路由的forward metric
R1(config-router)#do show ip ro 2.2.2.2
Routing entry for 2.2.2.2/32
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 65 经由R3到达R2的路由。 收到E2路由，
Last update from 13.0.0.3 on Serial1/1, 00:01:27 ago
Routing Descriptor Blocks:
* 13.0.0.3, from 2.2.2.2, 00:01:27 ago, via Serial1/1
Route metric is 20, traffic share count is 1
从多个方向收到一条E2路由的时候，AD相同的情况下，先比较 metric然后再比较forward metric(其中有FA的影响)。选路不做重点讨论，在下节课会着重去讲。
看下R4的路由表：
O*IA 0.0.0.0/0 [110/129] via 14.0.0.1, 00:03:22, Serial1/2
LSDB：
OSPF Router with ID (4.4.4.4) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 248 0x80000012 0x001E82 4
2.2.2.2 2.2.2.2 254 0x8000000C 0x00600C 2
4.4.4.4 4.4.4.4 247 0x80000004 0x002D32 2
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 259 0x80000001 0x0075C0
13.0.0.0 2.2.2.2 259 0x80000006 0x00449F
100.0.0.1 2.2.2.2 259 0x80000006 0x004883
123.0.0.0 2.2.2.2 259 0x80000006 0x00268F
200.0.0.1 2.2.2.2 259 0x80000006 0x00392D
结论：STUB区域隔离4类LSA和5类LSA。会由ABR强制(任何两个非骨干区域，都不能直接通信，必须要经过骨干区域)
totally stub：
由于3类LSA，和默认都是ABR下放的，totally只需要在ABR上做即可
R2(config-router)#area 1 stub no-summary
R4上路由表：
12.0.0.0/24 is subnetted, 1 subnets
O 12.0.0.0 [110/128] via 14.0.0.1, 00:13:40, Serial1/2
14.0.0.0/24 is subnetted, 1 subnets
C 14.0.0.0 is directly connected, Serial1/2
O*IA 0.0.0.0/0 [110/129] via 14.0.0.1, 00:01:50, Serial1/2
R4上LSDB：
OSPF Router with ID (4.4.4.4) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 159 0x80000013 0x001C83 4
2.2.2.2 2.2.2.2 100 0x8000000D 0x005E0D 2
4.4.4.4 4.4.4.4 821 0x80000006 0x002934 2
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
0.0.0.0 2.2.2.2 100 0x80000003 0x0071C2
可以看出，totally stub 是隔离3,4,5类LSA，并且下放一条3类的默认路由。
NSSA：
R1，R2，R4：
area 1 nssa
看R4的路由表：
1.0.0.0/32 is subnetted, 1 subnets
O N2 1.1.1.1 [110/20] via 14.0.0.1, 00:05:06, Serial1/2
2.0.0.0/32 is subnetted, 1 subnets
O N2 2.2.2.2 [110/20] via 14.0.0.1, 00:05:06, Serial1/2
100.0.0.0/32 is subnetted, 1 subnets
O IA 100.0.0.1 [110/129] via 14.0.0.1, 00:05:06, Serial1/2
200.0.0.0/32 is subnetted, 1 subnets
O IA 200.0.0.1 [110/130] via 14.0.0.1, 00:05:06, Serial1/2
22.0.0.0/32 is subnetted, 1 subnets
O N2 22.22.22.22 [110/20] via 14.0.0.1, 00:05:06, Serial1/2
11.0.0.0/32 is subnetted, 1 subnets
O N2 11.11.11.11 [110/20] via 14.0.0.1, 00:05:06, Serial1/2
123.0.0.0/24 is subnetted, 1 subnets
O IA 123.0.0.0 [110/129] via 14.0.0.1, 00:05:07, Serial1/2
12.0.0.0/24 is subnetted, 1 subnets
O 12.0.0.0 [110/128] via 14.0.0.1, 00:05:07, Serial1/2
13.0.0.0/24 is subnetted, 1 subnets
O IA 13.0.0.0 [110/193] via 14.0.0.1, 00:05:07, Serial1/2
14.0.0.0/24 is subnetted, 1 subnets
C 14.0.0.0 is directly connected, Serial1/2
NSSA区域会将外部路由引入，用7类LSA代替5类LSA在本NSSA区域泛洪。NSSA不会下放默认路由，在R2上下放默认路由。
R2(config-router)#area 1 nssa default-information-originate
R4上看到一条N2的路由：
O*N2 0.0.0.0/0 [110/1] via 14.0.0.1, 00:00:05, Serial1/2
对于R4来说，R1是ASBR，R2是ASBR的同时也是ABR，但是R2下放了一条默认路由，访问这个区域以外的路由，除了R1进来的路由，其他的都可以通过默认路由走。R2进来的7类LSA，完全没有必要。
在R2上：
R2(config-router)#area 1 nssa no-redistribution
在R4上看，没有了R2原来的两条7类路由。
由此得出结论，NSSA区域可以隔离4,5类LSA，不会下放默认路由。
totally NSSA(CISCO私有)
R2上：
R2(config-router)#area 1 nssa no-summary
看下R4的路由表
1.0.0.0/32 is subnetted, 1 subnets
O N2 1.1.1.1 [110/20] via 14.0.0.1, 00:01:30, Serial1/2
11.0.0.0/32 is subnetted, 1 subnets
O N2 11.11.11.11 [110/20] via 14.0.0.1, 00:01:30, Serial1/2
12.0.0.0/24 is subnetted, 1 subnets
O 12.0.0.0 [110/128] via 14.0.0.1, 00:01:30, Serial1/2
14.0.0.0/24 is subnetted, 1 subnets
C 14.0.0.0 is directly connected, Serial1/2
O*IA 0.0.0.0/0 [110/129] via 14.0.0.1, 00:00:04, Serial1/2
totally NSSA会干掉3,4,5类LSA，并且会下放一条3类的默认路由。
注：area 1 nssa no-redistribution default-information-originate no-summary 3条命令可以连用，后两条多此一举，3类默认路由会优于7类的默认路由。虽然LSDB里面7类还有。
