Article Detail

Description

This article explains one of the reasons for ISIS adjacency failure over L2circuit and the solution to resolve this issue.

Symptoms

ISIS adjacencies are stuck in Initializing state on CPE1
root@CPE1_RE> show isis adjacency
Interface System L State Hold (secs) SNPA
ae100.3150 0001.0001.0012 2 Initializing 20

ISIS adjacencies are not listed on CPE2
root@CPE2_RE> show isis adjacency

root@CPE2_RE>

ISIS Statistics - IIH counters are not incrementing on CPE2
root@CPE1_RE> show isis statistics
IS-IS statistics for CPE1_RE:
PDU type Received Processed Drops Sent Rexmit
LSP 0 0 0 0 0
IIH 847 8 2 888 0
...

root@CPE2_RE> show isis statistics
IS-IS statistics for CPE2_RE:
PDU type Received Processed Drops Sent Rexmit
LSP 0 0 0 0 0
IIH 44 3 2 894 0
...

Solution

The issue was due to the incorrect MTU configuration on PE Routers and other routers in core path towards remote CPE Routers. Correcting MTU on interfaces resolved the ISIS adjacency issue.

From the below logs, we can see that ISIS Packets are getting dropped on PE1 due to MTU issues. Below outputs are collected from PE Routers.

root@PE01_RE> show pfe statistics traffic | match MTU |refresh 5
---(refreshed at 2024-03-04 22:00:54 PST)---
Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics:
Input Checksum : 0
Output MTU : 803

---(refreshed at 2024-03-04 22:00:59 PST)---
Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics:
Input Checksum : 0
Output MTU : 804

VMX-0(PE01_RE vty)# show jnh 0 exceptions terse

Reason Type Packets Bytes
==================================================================
...

Packet Exceptions
----------------------
mtu exceeded DISC( 21) 811 1227043

VMX-0(PE01_RE vty)# show jnh 0 exceptions terse

Reason Type Packets Bytes
==================================================================
...

Packet Exceptions
----------------------
mtu exceeded DISC( 21) 818 1237634

VMX-0(PE01_RE vty)# show jnh exceptions-trace
[Mar 5 06:05:59.717] [62336] jnh_exception_packet_trace:1456 ###############
[Mar 5 06:05:59.717] [62337] jnh_exception_packet_trace:1461 [iif:373,code/info:149 D(mtu exceeded)/0x5d0,score:(0x1),ptype:1/10,orig_ptype:1,offset:21,orig_offset:21,len:1513,l2iif:374,oif:347,BD 0,l2-off=0,token=670 ]
[Mar 5 06:05:59.717] [62338] jnh_exception_packet_trace:1485 0x00: 1a 01 95 00 5d 00 01 75 00 15 05 e9 80 01 76 10
[Mar 5 06:05:59.717] [62339] jnh_exception_packet_trace:1485 0x10: 15 00 00 40 02 00 00 02 9e 00 00 00 00 15 b0 09
[Mar 5 06:05:59.717] [62340] jnh_exception_packet_trace:1485 0x20: 00 2b 00 00 05 2c 6b f5 80 17 c0 81 00 cc 4e 05
[Mar 5 06:05:59.717] [62341] jnh_exception_packet_trace:1485 0x30: d7 fe fe 03 83 14 01 00 11 01 00 00 02 00 01 00
[Mar 5 06:05:59.717] [62342] jnh_exception_packet_trace:1485 0x40: 01 00 11 00 1b 05 d4 01 f0 0f 01 00 00 01 6b 00
[Mar 5 06:05:59.717] [62343] jnh_exception_packet_trace:1485 0x50: 01 00 01 00 12 00 00 01 6a 81 02 cc 8e 84 04 0b
[Mar 5 06:05:59.717] [62344] jnh_exception_packet_trace:1485 0x60: 0b 0b 01 01 04 03 49 00 11 d3 03 00 00 00 08 ff
[Mar 5 06:05:59.717] [62345] jnh_exception_packet_trace:1485 0x70: 52 45 87 12 00 00 00 00 20 ac 10 15 64 00 00 00


root@PE02_RE> show pfe statistics traffic | match MTU |refresh 5
---(refreshed at 2024-03-04 22:01:20 PST)---
Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics:
Input Checksum : 0
Output MTU : 42
---(refreshed at 2024-03-04 22:01:25 PST)---
Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics:
Input Checksum : 0
Output MTU : 42

VMX-0(PE02_RE vty)# show jnh 0 exceptions terse

Reason Type Packets Bytes
==================================================================
...

Packet Exceptions
----------------------
mtu exceeded DISC( 21) 42 63546

Ping packets from CE devices are failing when they send with do-not-fragment bit set
root@CPE1_RE> ping 11.11.11.2 size 1500 do-not-fragment
PING 11.11.11.2 (11.11.11.2): 1500 data bytes
ping: sendto: Message too long
ping: sendto: Message too long
ping: sendto: Message too long
^C
--- 11.11.11.2 ping statistics ---
3 packets transmitted, 0 packets received, 100% packet loss

Issue resolved after correcting MTU issues on PE Router and other Routers in the core path
root@PE01_RE> show system rollback compare 1 0
[edit interfaces ae1]
+ mtu 9192;

[edit interfaces ae100]
+ mtu 9192;

root@CPE2_RE> ping 11.11.11.1 size 1500 do-not-fragment
PING 11.11.11.1 (11.11.11.1): 1500 data bytes
1508 bytes from 11.11.11.1: icmp_seq=0 ttl=64 time=22.420 ms
1508 bytes from 11.11.11.1: icmp_seq=1 ttl=64 time=22.055 ms
^C
--- 11.11.11.1 ping statistics ---
3 packets transmitted, 2 packets received, 33% packet loss
round-trip min/avg/max/stddev = 22.055/22.238/22.420/0.183 ms

root@CPE1_RE> ping 11.11.11.2 size 1500 do-not-fragment
PING 11.11.11.2 (11.11.11.2): 1500 data bytes
1508 bytes from 11.11.11.2: icmp_seq=0 ttl=64 time=41.853 ms
1508 bytes from 11.11.11.2: icmp_seq=1 ttl=64 time=56.106 ms
1508 bytes from 11.11.11.2: icmp_seq=2 ttl=64 time=18.445 ms
^C
--- 11.11.11.2 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max/stddev = 18.445/38.801/56.106/15.526 ms

root@PE01_RE> show pfe statistics traffic | match MTU | refresh 30
---(refreshed at 2024-03-04 22:26:18 PST)---
Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics:
Input Checksum : 0
Output MTU : 913
---(refreshed at 2024-03-04 22:26:48 PST)---
Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics:
Input Checksum : 0
Output MTU : 913
---(refreshed at 2024-03-04 22:27:18 PST)---
Packet Forwarding Engine Input IPv4 Header Checksum Error and Output MTU Error statistics:
Input Checksum : 0
Output MTU : 913

root@PE01_RE> show isis statistics
IS-IS statistics for PE01_RE:
PDU type Received Processed Drops Sent Rexmit
LSP 9225 9225 0 5287 0
IIH 146413 77 0 146475 0
CSNP 86449 86449 0 115261 0
PSNP 5189 5189 0 9035 0
Unknown 0 0 0 0 0
Totals 247276 100940 0 276058 0

Total packets received: 247276 Sent: 276058

root@PE02_RE> show isis statistics
IS-IS statistics for PE02_RE:
PDU type Received Processed Drops Sent Rexmit
LSP 11459 11459 0 5923 0
IIH 146398 78 0 146377 0
CSNP 57619 57619 0 115263 0
PSNP 5833 5833 0 11215 0
Unknown 0 0 0 0 0
Totals 221309 74989 0 278778 0

Total packets received: 221309 Sent: 278778

Modification History

Updated KB with more detailed logs