ISIS adjacency goes down when changing to isis point-to-point mode.

[krahmani323]

Hello,

I am facing an issue where I loose an isis adjacency between router A and B as soon as we change interface type to point-to-point (when in BROADCAST it is ok) and the 'show clns neighbor' outputs nothing.

The 'debug isis adj-packet' outputs the following when in isis BROADCAST type (isis neighborship UP)

On router A :

ISIS-Adj: Sending L1 LAN IIH on TenGigabitEthernet1/2.10, length 91
ISIS-Adj: Rec L1 IIH from bbbb.bbbb.bbbb (TenGigabitEthernet1/2.10), cir type L1, cir id 0000.0000.0034.01, length 91

On router B :

ISIS-Adj: Rec L1 IIH from aaaa.aaaa.aaaa (TenGigabitEthernet1/2.10), cir type L1, cir id 0000.0000.0034.01, length 91

ISIS-Adj: Sending L1 LAN IIH on TenGigabitEthernet1/2.10, length 91

The 'debug isis adj-packet' outputs the following when in isis POINT-TO-POINT type (isis neighborship DOWN)

On router A :

ISIS-Adj: Sending L1 LAN IIH on TenGigabitEthernet1/2.10, length 9197

ISIS-Adj: Sending serial IIH on TenGigabitEthernet1/2.10, length 9196

ISIS-Adj: Rec L1 IIH from bbbb.bbbb.bbbb (TenGigabitEthernet1/2.10), cir type L1, cir id 0000.0000.0034.01, length 9197

On router B :

ISIS-Adj: Sending L1 LAN IIH on TenGigabitEthernet1/2.630, length 9197

ISIS-Adj: Sending serial IIH on TenGigabitEthernet1/2.10, length 9196

ISIS-Adj: Rec L1 IIH from aaaa.aaaa.aaaa (TenGigabitEthernet1/2.10), cir type L1, cir id 0000.0000.0034.01, length 9197

--------------

Does someone have a clue about the potential cause of this isis adjacency loss when changing isis mode to point-to-point specifically ? Can it be related to MTU ??

Here the config of both interfaces which are the same.

interface TenGigabitEthernet1/2.10

encapsulation dot1Q 630
ip address x.x.x.x x.x.x.x
no ip redirects
no ip unreachables
no ip proxy-arp
ip mtu 9180
ip router isis
ip flow ingress
ip pim sparse-mode
ipv6 address xxxx:xxx:xxxx:xxx:x::x/xx
ipv6 enable
ipv6 mtu 9180
ipv6 router isis
no ipv6 mld router
mpls ip
bfd interval 50 min_rx 50 multiplier 3
no cdp enable
isis metric 30 level-1
isis password <removed>
isis ipv6 metric 30 level-1

( isis network point-to-point )

Any help would be appreciated.

Thanks a lot

Karim

[Giuseppe Larosa · Accepted solution]

Hello Karim,

first of all I would reduce the clns mtu to 1497 bytes there have been issues with so big mtu and ISIS and the problem was actually that it is not capable or recovering after a flap

The adjacency has to flap as changing network type may mean moving from L1 LAN hellos to serial hellos

Here we see that both routers send out an L1 LAN hello and a serial hello and only the lan hello is received

you can also disable hello-padding as part of the solution

Hope to help

Giuseppe

[krahmani323]

Hello community,

Indeed the Serial Hellos were sent BUT not received on both sides.

The problem was finally due to the interconnecting third-party switch which was blocking the isis point-to-point hello MAC due to a bug (Unregistered multicast frames may be dropped on the Switch).

*********************************************

The MAC-level broadcast addresses are:
      01-80-C2-00-00-15 for Level 2 adjacencies (AllL2ISs)
      01-80-C2-00-00-14 for Level 1 adjacencies (AllL1ISs)
  
The MAC-level point-to-point addresses are:
      09-00-2B-00-00-04 (AllEndSystems)     
      09-00-2B-00-00-05 (AllIntermediateSystems)   
*********************************************

We applied a workaround by adding a firewall filter on the switch allowing it to forward the multicast frames. From this action, the ISIS point-to-point adjacency changed to up and is now stable.

Thanks for the help.

Karim