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;
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