ISIS需要注意到的几个问题

一、IS-IS router type 和 interface Circuit Type、 adjacency type 
router level-type 决定了路由器利用 lsp 建立、维护什么样的 lsdb，如果是 L1 维护本 area 的 LSDB , 如果是 L2，则维护全局 L2 LSDB，如果是 L1-l2 则维护所有的 LSDB，同时对通告给 L1 的 LSP 设置 ATT 位。

Circuit Type(接口线路类型)：默认 L1-L2， L1，L2

adjacency type： 决定了接口通告 L1，或者L2 ，或者全部区域的LSP，有 L1 或者 L2，L1-L2 。

Adjacency Building
Neighbors on point-to-point networks always become adjacent unless they do not see themselves in their neighbors' hello PDU and match on certain parameters. On broadcast networks and nonbroadcast multiaccess (NBMA) networks, the DIS (Designated Intermediate System) will become adjacent with its neighbors.

Two routers will become neighbors if the following parameters are agreed upon:
Level 1—The two routers sharing a common network segment must have their interfaces configured to be in the same area if they are to have a Level 1 adjacency.
Level 2—The two routers sharing a common network segment must be configured as Level 2 if they are in different areas and want to become neighbors.
Authentication—IS-IS allows for configuration of a password for a specified link, for an area, or for an entire domain. 

A1/L1 -- A1/L1 -> L1
A1/L1 -- A1/L1L2 -> L1
A1/L1 -- A2/L1L2 -> 

A1/L2 -- A1/L2 ->L2 
A1/L2 -- A2/L1L2 -> L2
A1/L2 -- A2/L2 ->L2 

A1/L1L2 -- A1/L1L2 -> L1L2
Comment: 不同 area 的 L1 接口不能形成邻接关系， L2 接口形成邻接关系与区域无关。一边说来只有修改 L1L2 接口的 circuit type 才有实际意义。

查看 router type
R2#sh clns protocol 

IS-IS Router: <Null Tag>
System Id: 0020.0200.2002.00  IS-Type: level-1-2
Manual area address(es): 
86.4809.0021
Routing for area address(es): 
86.4809.0021
Interfaces supported by IS-IS:
Serial1/1 - IP
Serial1/0 - IP
Loopback0 - IP
Redistribute:
static (on by default)
Distance for L2 CLNS routes: 110
RRR level: none
Generate narrow metrics: level-1-2
Accept narrow metrics:   level-1-2
Generate wide metrics:   none
Accept wide metrics:     none
R2#

查看接口 circuit type
R2#sh clns interface   
FastEthernet0/0 is administratively down, line protocol is down
CLNS protocol processing disabled
Serial1/0 is up, line protocol is up
Checksums enabled, MTU 1500, Encapsulation HDLC
ERPDUs enabled, min. interval 10 msec.
CLNS fast switching enabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 19 seconds
Routing Protocol: IS-IS
Circuit Type: level-1-2
Interface number 0x1, local circuit ID 0x100
Neighbor System-ID: 0010.0100.1001
Level-1 Metric: 10, Priority: 64, Circuit ID: 0020.0200.2002.00
Level-1 IPv6 Metric: 10

查看邻居 type
R2#   sh clns neighbors 
System Id      Interface   SNPA                State  Holdtime  Type Protocol
0010.0100.1001 Se1/0    *HDLC*              Up     26               L2     IS-IS
0030.0300.3003 Se1/1    *HDLC*              Up     25               L1     IS-IS
R2#

二、IS-IS 的 metric
Cisco IOS 支持的 metric style
R4(config-router)#metric-style ?
narrow      Use old style of TLVs with narrow metric
transition  Send and accept both styles of TLVs during transition
wide        Use new style of TLVs to carry wider metric
R4(config-router)#metric-style 

Narrow style: 接口的 metric (6bit) 默认为 10 （1-63），route/path 的 metric (10bit)，最大为 1023 .
wide style: interface 24bit， route/path metric 32bit

Comment: 一般建议使用 wide metric , 方便网络扩展。

三、DIS(Designated IS)
DIS 是指一个接口，在MA网络中， DIS 接口和其他路由器接口形成邻接关系，非DIS接口间不形成邻接关系。

R4#sh clns interface 
FastEthernet0/0 is up, line protocol is up
Checksums enabled, MTU 1497, Encapsulation SAP
ERPDUs enabled, min. interval 10 msec.
CLNS fast switching enabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 8 seconds
Routing Protocol: IS-IS
Circuit Type: level-1-2
Interface number 0x1, local circuit ID 0x2
Level-2 Metric: 10, Priority: 64, Circuit ID: 0040.0400.4004.02
DR ID: 0040.0400.4004.02
Level-2 IPv6 Metric: 10
Number of active level-2 adjacencies: 1
Next IS-IS LAN Level-2 Hello in 1 seconds
Loopback0 is up, line protocol is up
Checksums enabled, MTU 1514, Encapsulation LOOPBACK
ERPDUs enabled, min. interval 10 msec.
CLNS fast switching disabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 54 seconds
Routing Protocol: IS-IS
Circuit Type: level-1-2
Interface number 0x0, local circuit ID 0x1
Level-2 Metric: 10, Priority: 64, Circuit ID: 0040.0400.4004.01
DR ID: 0000.0000.0000.00
Level-2 IPv6 Metric: 10
Number of active level-2 adjacencies: 0
Next IS-IS LAN Level-2 Hello in 5 seconds
R4#
DIS 选举原则
1、接口优先级别
2、SNPA（MAC地址）的高低 (优先级相同选个子高的)

R4(config-if)#isis priority 10
R4(config-if)#do show clns int f0/0
FastEthernet0/0 is up, line protocol is up
Checksums enabled, MTU 1497, Encapsulation SAP
ERPDUs enabled, min. interval 10 msec.
CLNS fast switching enabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 50 seconds
Routing Protocol: IS-IS
Circuit Type: level-1-2
Interface number 0x1, local circuit ID 0x2
Level-2 Metric: 10, Priority: 10, Circuit ID: 0010.0100.1001.02
DR ID: 0010.0100.1001.02
Level-2 IPv6 Metric: 10
Number of active level-2 adjacencies: 1
Next IS-IS LAN Level-2 Hello in 4 seconds
R4(config-if)#

DIS 没有 backup DIS , 相应的解决备份问题的机制是DIS抢夺，代价是每抢夺一次会引起一组LSP的泛洪。同时IS-IS LSDB在LAN上不断同步，即CSNP的周期性发送，其实设计成周期性发送还有一个原因，那就是CSNP不要求有确认应答。

四、backbone area 
IS-IS是通过连续的L2路由器来组成骨干CSNA网络分析社区4 @- U5 w4 }" T区，IS-IS的骨干区域是由连续的L2，L1/L2路由器构成的。L1 area 只要与一个L1/L2 连接就可以了。如果 L2 ， L1/L2 router不连续的话，也会出现网络分裂的问题。只能依靠网络设计避免，避免单链路，单路由器故障造成骨干区域分裂。 IS-IS在设计时应先定义好区域，骨干区域全为Level-2 Router，边界为Level-1-2 Router（最好规划到Level-1区域），末节区域为Level-1 Router。