一、实验目的

理解ISIS协议中Overload-bit的作用以及扩展应用

二、实验拓扑

http://s14/mw690/002j7VyEzy6RNFaNxaRdd&690



三、基本配置

R1配置
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip router isis
!
interface Ethernet0/0
 ip address 10.1.123.1 255.255.255.0
 ip router isis
!
router isis
 net 49.0001.1111.1111.1111.00
 is-type level-1

R2配置
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
 ip router isis
!
interface Ethernet0/0
 ip address 10.1.123.2 255.255.255.0
 ip router isis
!
interface Serial2/0
 ip address 10.1.24.2 255.255.255.0
 ip router isis
 serial restart-delay 0
!
router isis
 net 49.0001.2222.2222.2222.00
 set-overload-bit

R3配置
interface Loopback0
 ip address 3.3.3.3 255.255.255.255
 ip router isis
!
interface Ethernet0/0
 ip address 10.1.123.3 255.255.255.0
 ip router isis
!
interface Serial2/1
 ip address 10.1.34.3 255.255.255.0
 ip router isis
 serial restart-delay 0
!
router isis
 net 49.0001.3333.3333.3333.00
!

R4配置
interface Loopback0
 ip address 4.4.4.4 255.255.255.255
 ip router isis
!
interface Serial2/0
 ip address 10.1.24.4 255.255.255.0
 ip router isis
 serial restart-delay 0
!
interface Serial2/1
 ip address 10.1.34.4 255.255.255.0
 ip router isis
 serial restart-delay 0
!
interface Serial2/2
 ip address 10.1.45.4 255.255.255.0
 serial restart-delay 0
!
router isis
 net 49.0002.4444.4444.4444.00
 is-type level-2-only

四、实验结果

在R2上面Overload-bit置位之前

R1#show isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x00000005   0xADA0        1035              0/0/0
R2.00-00              0x00000005   0xE237        1158              1/0/0    //OL为0
R3.00-00              0x00000004   0x7E23        1163              1/0/0   
R3.01-00              0x00000004   0xFD2F        1034              0/0/0

R1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is 10.1.123.3 to network 0.0.0.0

i*L1  0.0.0.0/0 [115/10] via 10.1.123.3, 00:01:15, Ethernet0/0
                [115/10] via 10.1.123.2, 00:01:15, Ethernet0/0  //R1将R2和R3作为出口
      1.0.0.0/32 is subnetted, 1 subnets
C        1.1.1.1 is directly connected, Loopback0
      2.0.0.0/32 is subnetted, 1 subnets
i L1     2.2.2.2 [115/20] via 10.1.123.2, 00:03:19, Ethernet0/0
      3.0.0.0/32 is subnetted, 1 subnets
i L1     3.3.3.3 [115/20] via 10.1.123.3, 00:03:19, Ethernet0/0
      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
i L1     10.1.24.0/24 [115/20] via 10.1.123.2, 00:01:23, Ethernet0/0
i L1     10.1.34.0/24 [115/20] via 10.1.123.3, 00:01:15, Ethernet0/0
C        10.1.123.0/24 is directly connected, Ethernet0/0
L        10.1.123.1/32 is directly connected, Ethernet0/0
R1#

小结：在没有将overload-bit置位时，ATT会置位，路由器会生成ISIS默认路由

扩展应用一： set-overload-bit

在R2上面修改配置

router isis
 net 49.0001.2222.2222.2222.00
 set-overload-bit

查看Overload-bit置位后的结果

R1#show isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x00000005   0xADA0        959               0/0/0
R2.00-00              0x00000006   0xDC40        1183              0/0/1  //OL位变为1
R3.00-00              0x00000004   0x7E23        1088              1/0/0
R3.01-00              0x00000004   0xFD2F        959               0/0/0
R1#

R1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override

Gateway of last resort is 10.1.123.3 to network 0.0.0.0

i*L1  0.0.0.0/0 [115/10] via 10.1.123.3, 00:00:17, Ethernet0/0  //R1只把R3作为出口
      1.0.0.0/32 is subnetted, 1 subnets
C        1.1.1.1 is directly connected, Loopback0
      2.0.0.0/32 is subnetted, 1 subnets
i L1     2.2.2.2 [115/20] via 10.1.123.2, 00:04:02, Ethernet0/0
      3.0.0.0/32 is subnetted, 1 subnets
i L1     3.3.3.3 [115/20] via 10.1.123.3, 00:04:02, Ethernet0/0
      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
i L1     10.1.24.0/24 [115/20] via 10.1.123.2, 00:02:06, Ethernet0/0
i L1     10.1.34.0/24 [115/20] via 10.1.123.3, 00:01:58, Ethernet0/0
C        10.1.123.0/24 is directly connected, Ethernet0/0
L        10.1.123.1/32 is directly connected, Ethernet0/0
R1#

小结：配置set-overload-bit后，level-1-2路由器泛洪的LSP会将overload-bit置位，level-1路由器收到overload-bit置位的LSP后，在链路状态数据库里，会将OL位置1。路由器清除指向该level-1-2路由器的ISIS默认路由。

扩展应用二：set-overload-bit on-startup 120

R2上修改配置

router isis
 net 49.0001.2222.2222.2222.00
 set-overload-bit on-startup 120

R2#write
Building configuration...
[OK]
R2#relo
R2#reload

路由器运行时间小于120秒，查看结果

R2#show version
Cisco IOS Software, Linux Software (I86BI_LINUX-ADVENTERPRISEK9-M), Version 15.4(1)T, DEVELOPMENT TEST SOFTWARE
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2013 by Cisco Systems, Inc.
Compiled Sat 23-Nov-13 03:28 by prod_rel_team

ROM: Bootstrap program is Linux

R2 uptime is 0 minutes  //路由器运行小于120秒
System returned to ROM by reload at 0
System image file is "unix:/home/gns3/GNS3/images/i86bi-linux-l3-adventerprisek9-15.4.1T.b"
Last reload reason: Unknown reason

查看R1上链路状态数据库

R1#show isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x00000007   0xA9A2        378               0/0/0
R2.00-00              0x0000000B   0xC057        1119              0/0/1  //R1收到Overload-bit置位的LSP
R2.01-00              0x00000001   0x939A        0 (1121)          0/0/0
R3.00-00              0x00000006   0x7A25        619               1/0/0
R3.01-00              0x00000006   0xF931        537               0/0/0
R1#

路由器运行时间大于120秒，查看结果

R2#show version
Cisco IOS Software, Linux Software (I86BI_LINUX-ADVENTERPRISEK9-M), Version 15.4(1)T, DEVELOPMENT TEST SOFTWARE
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2013 by Cisco Systems, Inc.
Compiled Sat 23-Nov-13 03:28 by prod_rel_team

ROM: Bootstrap program is Linux

R2 uptime is 2 minutes  //路由器运行时间大于120秒
System returned to ROM by reload at 0
System image file is "unix:/home/gns3/GNS3/images/i86bi-linux-l3-adventerprisek9-15.4.1T.b"
Last reload reason: Unknown reason

R1#show isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x00000008   0xA7A3        1184              0/0/0
R2.00-00              0x0000000C   0xC250        1192              1/0/0   //R1上收到的R2 Overload-bit置位清除
R2.01-00              0x00000001   0x939A        0 (1078)          0/0/0
R3.00-00              0x00000006   0x7A25        576               1/0/0
R3.01-00              0x00000006   0xF931        494               0/0/0
R1#

小结：配置set-overload-bit on-startup 120后，重启level-1-2路由器，level-1路由器会将LSP的OL位置1，在规定的时间120S时level-1-2产生overload-bit没有置位的LSP，level-1路由器会收到此LSP，立即将LSP的OL位置0。

扩展应用三：set-overload-bit on-startup wait-for-bgp

修改R2的配置

R2
router isis
 net 49.0001.2222.2222.2222.00
 set-overload-bit on-startup wait-for-bgp

router bgp 123
 bgp log-neighbor-changes
 neighbor 10.1.24.4 remote-as 45

修改R4的配置

R4
router bgp 45
 bgp log-neighbor-changes
 neighbor 10.1.24.2 remote-as 123

R2#write
Building configuration...
[OK]
R2#reload

在BGP收敛之前查看R1上ISIS的链路状态数据库

R1#show isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x0000000A   0xA3A5        852               0/0/0
R2.00-00              0x00000019   0xA465        1160              0/0/1  //Overload-bit置位
R3.00-00              0x00000009   0x7428        926               1/0/0
R3.01-00              0x00000010   0xE53B        1164              0/0/0

在BGP收敛支护查看R1上ISIS的链路状态数据库

R2#debug isis update-packets
*Apr 26 10:57:37.084: %BGP-5-ADJCHANGE: neighbor 10.1.24.4 Up
*Apr 26 10:57:38.148: ISIS-Upd: Building L1 LSP
*Apr 26 10:57:38.148: ISIS-Upd: Important fields changed
*Apr 26 10:57:38.148: ISIS-Upd: TID 0 full SPF required
*Apr 26 10:57:38.148: ISIS-Upd: Building L2 LSP
*Apr 26 10:57:38.148: ISIS-Upd: Important fields changed
*Apr 26 10:57:38.148: ISIS-Upd: TID 0 full SPF required
*Apr 26 10:57:38.179: ISIS-Upd: Sending L1 LSP 2222.2222.2222.00-00, seq 1A, ht 1199 on Ethernet0/0 via high priority queue

R1#show isis database

IS-IS Level-1 Link State Database:
LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime      ATT/P/OL
R1.00-00            * 0x0000000A   0xA3A5        783               0/0/0
R2.00-00              0x0000001A   0xA65E        1141              1/0/0  //清除了Overload-bit置位
R3.00-00              0x00000009   0x7428        857               1/0/0
R3.01-00              0x00000010   0xE53B        1095              0/0/0

小结：配置set-overload-bit on-startup wait-for-bgp后，重启level-1-2路由器，在BGP收敛之前level-1-2会将LSP的OL位置1，在BGP收敛之后level-1-2会将LSP的OL位置0。

五、实验总结

overload-bit置位在level-1-2路由器上面配置
overload-bit置位为1之后，该level-1-2路由器将不再作为到目的网络的下一跳。
level-1路由器收到overload-bit置位的LSP后，链路状态数据库中OL变为1。


set-overload-bit
本地路由器产生的LSP overload-bit置位1。这台路由器将不会被其它路由器作为去往目的地的下一跳，除了其本地直连网段。

set-overload-bit on-startup <5-86400 sencondes>
on-startup后面跟的参数是秒，用来设置重启设备后多少秒内overload-bit会被置位，超过这个时间后overload-bit置位被清除。

set-overload-bit on-startup wait-for-bgp
在BGP收敛完成后，overload-bit置位被清除。