当一台L1/L2路由器发送L1 LSP到一个区域时,它会通过在LSP中设置ATT位来通知其他L1路由器能够达到另一个域。当需要转发数据包到区域外时，L1路由器会将数据包转发到最近的L2路由器(到设置了ATT位的路由器具有最小代价的路径)
考虑到此图，在区域1中,R2和R3都为L1/L2 路由器,它们都是在L1的更新包中设置了ATT位,并发送到路由器R5,并且R5为L1路由器,下面是R5的IS数据库
r5#sh isis database
IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
r2.00-00              0x0000000F   0x7474        789                   1/0/0
r3.00-00              0x0000000A   0x09DB        465               1/0/0
r5.00-00            * 0x0000000B   0x6374        1133              0/0/0
r5.01-00            * 0x00000005   0xE631        367               0/0/0
查看路由表:
r5#sh ip route
Gateway of last resort is 192.168.35.3 to network 0.0.0.0
i L1 192.168.12.0/24 [115/20] via 192.168.25.2, FastEthernet0/0
C    192.168.25.0/24 is directly connected, FastEthernet0/0
     5.0.0.0/24 is subnetted, 1 subnets
C       5.5.5.0 is directly connected, Loopback0
i L1 192.168.23.0/24 [115/20] via 192.168.35.3, Serial1/1
                     [115/20] via 192.168.25.2, FastEthernet0/0
C    192.168.35.0/24 is directly connected, Serial1/1
i*L1 0.0.0.0/0 [115/10] via 192.168.35.3, Serial1/1
               [115/10] via 192.168.25.2, FastEthernet0/0
可以看到一条通往外部的缺省路由将在R2和R3之间实现负载
因为R2和R3都是离R5最近的L1/L2路由器
测试:

r5#ping  
Protocol [ip]:
Target IP address: 1.1.1.1
Repeat count [5]:
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface:
Type of service [0]:
Set DF bit in IP header? [no]:
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]: r
Number of hops [ 9 ]:
Loose, Strict, Record, Timestamp, Verbose[RV]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
Packet has IP options:  Total option bytes= 39, padded length=40
 Record route: <*>
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
Reply to request 0 (500 ms).  Received packet has options
 Total option bytes= 40, padded length=40
  Record route:
   (192.168.25.5)
   (192.168.12.2)
   (1.1.1.1)
   (192.168.12.1)
   (192.168.25.2)
   (192.168.25.5) <*>
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
 End of list
Reply to request 1 (412 ms).  Received packet has options
 Total option bytes= 40, padded length=40
  Record route:
   (192.168.35.5)
   (192.168.23.3)
   (192.168.12.2)
   (1.1.1.1)
   (192.168.12.1)
   (192.168.25.2)
   (192.168.35.5) <*>
   (0.0.0.0)
   (0.0.0.0)
可以清楚的看到以往1.1.1.1的数据包在R2和R3之间负载
 
很显然，对于R5来说去往外面最近的路径是经过R2，而不是经R3再经R2出去
通过将1.1.1.0通知到区域1的L1路由器，可以控制从R5到外部业务流量，这个把L2路由通告能L1路由器的做法就叫做路由泄漏（route leaking）
实现方法：
在R2上：
access-list 100 permit ip host 1.1.1.0 any
router isis
 net 49.0001.1111.1111.1112.00
 redistribute isis ip level-2 into level-1 distribute-list 100
就这么简单，查看路由表：
r5#sh ip route
Gateway of last resort is 192.168.35.3 to network 0.0.0.0
i L1 192.168.12.0/24 [115/20] via 192.168.25.2, FastEthernet0/0
     1.0.0.0/24 is subnetted, 1 subnets
i ia    1.1.1.0 [115/158] via 192.168.25.2, FastEthernet0/0  //外部路由进入了L1 ，并标识为ia
C    192.168.25.0/24 is directly connected, FastEthernet0/0
     5.0.0.0/24 is subnetted, 1 subnets
C       5.5.5.0 is directly connected, Loopback0
i L1 192.168.23.0/24 [115/20] via 192.168.35.3, Serial1/1
                     [115/20] via 192.168.25.2, FastEthernet0/0
C    192.168.35.0/24 is directly connected, Serial1/1
i*L1 0.0.0.0/0 [115/10] via 192.168.35.3, Serial1/1
               [115/10] via 192.168.25.2, FastEthernet0/0
在R5上测试：
r5#ping      
Protocol [ip]:
Target IP address: 1.1.1.1
Repeat count [5]:
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface:
Type of service [0]:
Set DF bit in IP header? [no]:
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]: r
Number of hops [ 9 ]:
Loose, Strict, Record, Timestamp, Verbose[RV]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
Packet has IP options:  Total option bytes= 39, padded length=40
 Record route: <*>
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
Reply to request 0 (220 ms).  Received packet has options
 Total option bytes= 40, padded length=40
 Record route:
   (192.168.25.5)
   (192.168.12.2)
   (1.1.1.1)
   (192.168.12.1)
   (192.168.25.2)
   (192.168.25.5) <*>
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
 End of list
Reply to request 1 (268 ms).  Received packet has options
 Total option bytes= 40, padded length=40
 Record route:
   ( 192.168.25.5)
   (192.168.12.2)
   (1.1.1.1)
   (192.168.12.1)
   (192.168.25.2)
   (192.168.25.5) <*>
   (0.0.0.0)
   (0.0.0.0)
   (0.0.0.0)
 End of list
 
可以看到，数据包仅通过R2发送，实验结束！
 
 
全部配置如下：
 
R1
nterface Loopback0
 ip address 1.1.1.1 255.255.255.0
 ip router isis
!
interface FastEthernet0/0
 no ip address
 shutdown
 duplex half
!
interface Serial1/0
 ip address 192.168.12.1 255.255.255.0
 ip router isis
 serial restart-delay 0
!
interface Serial1/1
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial1/2
 no ip address
 shutdown
 serial restart-delay 0
!        
interface Serial1/3
 no ip address
 shutdown
 serial restart-delay 0
!
router isis
 net 49.0002.1111.1111.1111.00

R2
interface Loopback0
 ip address 2.2.2.2 255.255.255.0
!
interface FastEthernet0/0
 ip address 192.168.25.2 255.255.255.0
 ip router isis
 duplex half
!
interface Serial1/0
 ip address 192.168.23.2 255.255.255.0
 ip router isis
 serial restart-delay 0
!
interface Serial1/1
 ip address 192.168.12.2 255.255.255.0
 ip router isis
 serial restart-delay 0
!
interface Serial1/2
 ip address 192.168.24.2 255.255.255.0
 serial restart-delay 0
!
router isis
 net 49.0001.1111.1111.1112.00
 redistribute isis ip level-2 into level-1 distribute-list 100
!
access-list 100 permit ip host 1.1.1.0 any
!
R3
interface Loopback0
 ip address 3.3.3.3 255.255.255.0
!
interface FastEthernet0/0
 no ip address
 shutdown
 duplex half
!
interface Serial1/0
 ip address 192.168.35.3 255.255.255.0
 ip router isis
 serial restart-delay 0
!
interface Serial1/1
 ip address 192.168.23.3 255.255.255.0
 ip router isis
 serial restart-delay 0
!
interface Serial1/2
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial1/3
 no ip address
 shutdown
 serial restart-delay 0
!
router isis
 net 49.0001.1111.1111.1113.00
!
R5
interface Loopback0
 ip address 5.5.5.5 255.255.255.0
!
interface FastEthernet0/0
 ip address 192.168.25.5 255.255.255.0
 ip router isis
 duplex half
!
interface Serial1/0
 ip address 192.168.57.5 255.255.255.0
 serial restart-delay 0
!
interface Serial1/1
 ip address 192.168.35.5 255.255.255.0
 ip router isis
 serial restart-delay 0
!
interface Serial1/2
 ip address 192.168.45.5 255.255.255.0
 serial restart-delay 0
!
interface Serial1/3
 no ip address
 shutdown
 serial restart-delay 0
!
router isis
 net 49.0001.1111.1111.1115.00
 is-type level-1