CR16-F路由器与思科N9K对接ISIS邻居因为MTU不一致导致邻居无法建立问题

我司CR16-F路由器与Cisco N9K路由对接ISIS时，发现ISIS邻居一直无法建立成功，两边配置完全一致；现场收集了debug信息以及思科侧信息；

通过现场debug信息查看，II H报文收发正常，但是不断提示ISIS hello超时，如此反复不断；

*Mar  2 07:21:40:899 2016 H3C ISIS/7/ISISDBG:

ISIS-1-TMR: Level-1 hello timer expired on the circuit GigabitEthernet0/0.

*Mar  2 07:21:40:899 2016 H3C ISIS/7/ISISDBG:

ISIS-1-ADJ: Send a Lan L1 Hello packet on circuit(GigabitEthernet0/0)

*Mar  2 07:21:40:921 2016 H3C ISIS/7/ISISDBG:

ISIS-1-ADJ: Receive a Lan L1 Hello packet from(0000.0000.0001) on circuit(GigabitEthernet0/0)

*Mar  2 07:21:40:922 2016 H3C ISIS/7/ISISDBG:

ISIS-1-ADJ:

0000: 83 1b 01 06  0f 01 00 00  01 00 00 00  00 00 01 00 

0010: 1e 00 35 40  00 00 00 00  00 01 01 01  02 01 10 84 

0020: 04 0a 00 00  01 81 01 cc  d3 03 00 00  00 06 06 0e 

0030: cd 0a 5c 01  05

*Mar  2 07:21:40:922 2016 H3C ISIS/7/ISISDBG:

ISIS-1-ADJ: Level-1 NBR(0000.0000.0001) two way pass.

那么我们可以通过打印出来的hello报文查看一下，根据ISIS IIH报文结构，我们可以看出我们发出的ISIS报文为1541大小：

*Mar  2 12:20:19:597 2016 ZhangJiang-H3C-P2 ISIS/7/ISISDBG: -MDC=1;

ISIS-9812-ADJ:

0000: 83 1b 01 06  10 01 00 00  02 00 00 00  00 00 59 00 

0010: 1e 06 05 40  00 00 00 00  00 59 02 01  02 01 00 84    

 // 0x605 = 1541，也就是mtu=1544

0020: 04 db e9 2b  8e 81 01 cc  d3 01 00 06  06 6c 9c ed 

0030: 0d ed ea 08  ff 00 00 00  00 00 00 00  00 00 00 00 

但是我们收到对端的ISIS IIH报文长度只有1527：

接收的报文长度：

*Mar  2 12:20:22:397 2016 ZhangJiang-H3C-P2 ISIS/7/ISISDBG: -MDC=1;

ISIS-9812-ADJ: Receive a Lan L2 Hello packet from(0000.0002.0530) on circuit(Ten-GigabitEthernet2/0/4)

*Mar  2 12:20:22:399 2016 ZhangJiang-H3C-P2 ISIS/7/ISISDBG: -MDC=1;

ISIS-9812-ADJ:

0000: 83 1b 01 00  10 01 00 00  02 00 00 00  02 05 30 00 

0010: 1e 05 f7 40  00 00 00 02  05 30 3d 81  01 cc d3 03    

//但接收的报文长度为，0x5f7 = 1527

0020: 00 00 00 01  02 01 00 84  04 db e9 2b  8d 06 00 08 

此时查看Cisco侧的ISIS信息：

#show isis interface tenGigE 0/0/0/0

Wed Mar  2 17:20:39.287 CST

TenGigE0/0/0/0              Enabled

  Adjacency Formation:      Enabled

  Prefix Advertisement:     Enabled

  IPv4 BFD:                 Disabled

  IPv6 BFD:                 Disabled

  BFD Min Interval:         150

  BFD Multiplier:           3

  Circuit Type:             level-2-only (Interface circuit type is level-1-2)

  Media Type:               LAN

  Circuit Number:           37

  Level-2                  

    Adjacency Count:        1

    LAN ID:                

    Priority (Local/DIS):   64/64

    Next LAN IIH in:        226 ms

    LSP Pacing Interval:    33 ms

    PSNP Entry Queue Size:  0

  CLNS I/O

    Protocol State:         Up

MTU:                    1527   //发现对端只有1527

    SNPA:                   6c9c.ed0d.eda8

    Layer-2 MCast Groups Membership:

      All Level-2 ISs:      Yes

  IPv4 Unicast Topology:    Enabled

    Adjacency Formation:    Running

    Prefix Advertisement:   Running

    Metric (L1/L2):         10/10

    MPLS LDP Sync (L1/L2):  Enabled/Enabled

      Sync Status:          Achieved

  IPv4 Address Family:      Enabled

    Protocol State:         Up

    Forwarding Address(es): 219.233.13.249

    Global Prefix(es):      219.233.13.248/30

  LSP transmit timer expires in 0 ms

  LSP transmission is idle

  Can send up to 9 back-to-back LSPs in the next 288 ms

但是实际接口配置确实为1544

#sh int tenGigE 0/0/0/0

Wed Mar  2 17:20:50.379 CST

TenGigE0/0/0/0 is up, line protocol is up

  Interface state transitions: 27

  Hardware is TenGigE, address is

  Layer 1 Transport Mode is LAN

  Description:

  Internet address is

  MTU 1544 bytes, BW 10000000 Kbit (Max: 10000000 Kbit)

     reliability 254/255, txload 0/255, rxload 0/255

  Encapsulation ARPA,

  Full-duplex, 10000Mb/s, ZR, link type is force-up

  output flow control is off, input flow control is off

  loopback not set,

  ARP type ARPA, ARP timeout 04:00:00

  Last input 00:00:00, output 00:00:00

  Last clearing of "show interface" counters never

由此我们可以推断出思科的MTU算法与我司不一致，思科的MTU计算了以太头部而我司不计算以太头；

将我司接口MTU配置为1544-14=1530后，MTU就一样了，这样ISIS邻居就可以正常建立；