Re: Cisco8608-ASR9902 same clns MTU, but different isis interface MTU

Leo Gal 79 · ‎03-02-2025

Hello gents, I would need a help here.

I have a network with Cisco 8608 (XR 24.2.11) as a core device and ASR9902 (XR-24.2.2) and 9904 around it as PE devices.
Started configuring MTU on interfaces, but it bit me right away. Configured same interface MTU on both sides, resulting in
the same CLNS MTU on both sides, but different MTU that ISIS is using for the sessions, so obviously the sessions are down.
I configured the interface MTU (ethernet MTU) to 9646, which is max for asr9902.
I understand, why IP and CLNS mtu is 9632, it is 14bytes lower.
But why, if I have the SAME CLNS MTU(9632) on both sides, ISIS deduces DIFFERENT MTUs for the interface.
ISIS on 8608 deduces MTU 9629, while ASR9902 deduces MTU 9577.
I need to understand whats behind this. And needless to say, I would rather the config be very simple and uniform in the core, don't want to "tweak" it unless absolutely necessary.

Update: the behaviour is the same for ASR9902 and 9904, for TenG and FourHundredG

I would be very grateful if someone could shed some light on the problem.

Thank you in advance.

Here are outputs for Cisco 8608

RP/0/RP0/CPU0:DC2RTC25COR01P#show ver
Sun Mar 2 13:50:32.968 UTC
Cisco IOS XR Software, Version 24.2.11 LNT
Copyright (c) 2013-2024 by Cisco Systems, Inc.

Build Information:
Built By : cisco
Built On : Tue Jul 02 14:32:37 UTC 2024
Build Host : iox-ucs-037
Workspace : /auto/srcarchive11/prod/24.2.11/8000/ws/
Version : 24.2.11
Label : 24.2.11

cisco 8000 (Intel(R) Xeon(R) CPU D-1633N @ 2.50GHz)
cisco 8608 (Intel(R) Xeon(R) CPU D-1633N @ 2.50GHz) processor with 64GB of memory
DC2RTC25COR01P uptime is 2 weeks, 2 days, 18 hours, 43 minutes
Cisco 8600 - 8 Slot Centralized Chassis

RP/0/RP0/CPU0:DC2RTC25COR01P#

RP/0/RP0/CPU0:DC2RTC25COR01P#
RP/0/RP0/CPU0:DC2RTC25COR01P#sh lldp neighbors | i 0/2/0/10/0
Sun Mar 2 11:52:46.465 UTC
DC2RTC25RRC01P TenGigE0/2/0/10/0 120 R TenGigE0/0/0/0
RP/0/RP0/CPU0:DC2RTC25COR01P#
RP/0/RP0/CPU0:DC2RTC25COR01P#sh run int ten0/2/0/10/0

interface TenGigE0/2/0/10/0
description DC2RTC25RRC01P ROUTE REFLECTOR TenGigE0/0/0/0
mtu 9646
ipv4 address 10.242.5.17 255.255.255.252
!

RP/0/RP0/CPU0:DC2RTC25COR01P#show im database interface tenGigE 0/2/0/10/0
Sun Mar 2 11:53:32.515 UTC

View: OWN - Owner, L3P - Local 3rd Party, G3P - Global 3rd Party, LDP - Local Data Plane
GDP - Global Data Plane, RED - Redundancy, UL - UL

Node 0/RP0/CPU0 (0x2000)

Interface TenGigE0/2/0/10/0, ifh 0x0f0002c4 (up, 9646)
Interface flags: 0x00000000011284df (ROOT_IS_HW|IFCONNECTOR
|REPLICATED|DYN_REP|IFINDEX|BROADCAST|VIRTUAL
|CONFIG|HW|VIS|DATA|CONTROL)
Encapsulation: ether
Interface type: IFT_TENGETHERNET
Control parent: None
Data parent: None
Views: UL|GDP|LDP|G3P|L3P|OWN

Protocol Caps (state, mtu)
-------- -----------------
None hw_oor (up, 9646)
None spio (up, 9646)
None ether (up, 9646)
arp arp (up, 9632)
clns clns (up, 9632)
ipv4 ipv4 (up, 9632)
ether_sock ether_sock (up, 9632)

RP/0/RP0/CPU0:DC2RTC25COR01P#
RP/0/RP0/CPU0:DC2RTC25COR01P#show isis inter brief
Sun Mar 2 11:53:52.499 UTC

IS-IS CMSCORE Interfaces
Interface All Adjs Adj Topos Adv Topos CLNS MTU Prio
OK L1 L2 Run/Cfg Run/Cfg L1 L2
----------------- --- --------- --------- --------- ---- ---- --------
Lo0 Yes - 0 1/1 1/1 Up 1500 - -
Lo1 Yes - 0 1/1 1/1 Up 1500 - -
FH0/0/0/0 No - - 0/1 0/1 Down 9629 - -
FH0/0/0/1 Yes - 1 1/1 1/1 Up 9629 - -
FH0/0/0/2 Yes - 0 1/1 1/1 Up 9629 - -
Hu0/2/0/5 Yes - 0 1/1 1/1 Up 9629 - -
.
Te0/2/0/10/0 Yes - 0 1/1 1/1 Up 9629 - -
Te0/2/0/10/2 Yes - 1 1/1 1/1 Up 9629 - -
Te0/2/0/10/3 Yes - 0 1/1 1/1 Up 9629 - -

RP/0/RP0/CPU0:DC2RTC25COR01P#

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

here are outputs for ASR9902

RP/0/RP0/CPU0:DC2RTC25RRC01P#show version
Sun Mar 2 12:06:40.392 UTC
Cisco IOS XR Software, Version 24.2.2
Copyright (c) 2013-2024 by Cisco Systems, Inc.

Build Information:
Built By : swtools
Built On : Sun Sep 29 01:55:38 PDT 2024
Built Host : iox-ucs-001
Workspace : /auto/srcarchive10/prod/24.2.2/asr9k-x64/ws
Version : 24.2.2
Location : /opt/cisco/XR/packages/
Label : 24.2.2

cisco ASR9K () processor
System uptime is 2 weeks 2 days 23 hours 38 minutes

RP/0/RP0/CPU0:DC2RTC25RRC01P#
RP/0/RP0/CPU0:DC2RTC25RRC01P#show lldp neighbors
Sun Mar 2 12:07:19.895 UTC
Capability codes:
(R) Router, (B) Bridge, (T) Telephone, (C) DOCSIS Cable Device
(W) WLAN Access Point, (P) Repeater, (S) Station, (O) Other

Device ID Local Intf Hold-time Capability Port ID
DC2RTC25COR01P TenGigE0/0/0/0 120 R TenGigE0/2/0/10/0

Total entries displayed: 1

RP/0/RP0/CPU0:DC2RTC25RRC01P#
RP/0/RP0/CPU0:DC2RTC25RRC01P#sh run int tenGigE 0/0/0/0
Sun Mar 2 12:07:27.254 UTC
interface TenGigE0/0/0/0
description DC2RTC25COR01P
mtu 9646
ipv4 address 10.242.5.18 255.255.255.252
!

RP/0/RP0/CPU0:DC2RTC25RRC01P#show im database interface Ten0/0/0/0
Sun Mar 2 12:07:43.994 UTC

View: OWN - Owner, L3P - Local 3rd Party, G3P - Global 3rd Party, LDP - Local Data Plane
GDP - Global Data Plane, RED - Redundancy, UL - UL

Node 0/0/CPU0 (0x8220)

Protocol Caps (state, mtu)
-------- -----------------
None spio (up, 9646)
None ether (up, 9646)
arp arp (up, 9632)
clns clns (up, 9632)
ipv4 ipv4 (up, 9632)
ether_sock ether_sock (up, 9632)

RP/0/RP0/CPU0:DC2RTC25RRC01P#
RP/0/RP0/CPU0:DC2RTC25RRC01P#show isis inter brief
Sun Mar 2 12:07:49.080 UTC

IS-IS CMSCORE Interfaces
Interface All Adjs Adj Topos Adv Topos CLNS MTU Prio
OK L1 L2 Run/Cfg Run/Cfg L1 L2
----------------- --- --------- --------- --------- ---- ---- --------
Lo0 Yes - 0 1/1 1/1 Up 1500 - -
Lo1 Yes - 0 1/1 1/1 Up 1500 - -
Te0/0/0/0 Yes - 0 1/1 1/1 Up 9577 - -
Te0/0/0/24 No - - 0/1 0/1 Down 9577 - -
RP/0/RP0/CPU0:DC2RTC25RRC01P#

balaji.bandi · ‎03-02-2025

These might have been configured, depending on the requirement I am in guess. If you configured globally, that will be inherited for all interfaces. It looks like you are overridden with interface config. Maybe re-considering making all in the same MTU is a good option.

BB

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Leo Gal 79 · ‎03-02-2025

Hello BB
The config on both devices was clean, when I started. I am configuring whole network. The only configuration I want is to have the same MTU on both interfaces, but that does not work...that's why I opened up this conversation. On both sides on main interfaces I have
mtu 9646
And that leads to different number in MTU ISIS is using on both sides. see the outputs, if possible, please
Thank you for the efforts
Leo

balaji.bandi · ‎03-03-2025

Is the neighbor device connected directly or over any other link, (like a provider between?)

also look at the ISIS MTU behavior

:https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/ios-xr-software/116350-trouble-ios-xr-mtu-00.html

BB

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Leo Gal 79 · ‎03-03-2025

Hello BB
The devices are connected back to back, please see show lldp neighbor in my initial output.
In that link you sent there is 0 instances of word "ISIS". What exactly you had in mind to look at regarding ISIS please?
Thank you
Leo

M02@rt37 · ‎03-02-2025

Hello @Leo Gal 79

I suspect IOS XR that it might subtracting different values for ISIS LSP, padding, or other protocol overhead...

So, explicitly set it to 9577, the lower of the two values. I hope this should override any automatic deductions.

Best regards
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Leo Gal 79 · ‎03-03-2025

So far I did not do anything but disabling hello-padding on core devices. And I will probably disable it on all the devices. But this suffices to get it running.
Both sides have the same default lsp-mtu = 1492. Padding takes the PDU and adds balast up to the ISIS (i guess) intf MTU which on ASR990x(PEs) is 9577. That will be accepted by core devices, that have ISIS intf MTU at 9629.

On the other hand If core device creates LSP PDU 1492bytes big and adds balast up to the ISIS intf MTU (9629) and sends it....(it can send it cause clns MTU is 9632) it will be dropped on PE devices because of ISIS intf MTU being 9577.

Short story, default lsp-mtu 1492, hello-padding disabled, physical interface MTU the same on both sides 9646, IP and CLNS MTU at 9632 on both sides. ISIS intf MTU different, isis sessions up and I believe I am safe with this setup.

If anyone thinks about better solution, please let me know.
Thank you
LG

M02@rt37 · ‎03-03-2025

@Leo Gal 79

Sounds good. Disabling hello-padding prevents unnecessary large ISIS PDU from being sent ! which helps avoid fragmentation issues... Since both sides have the same lsp-mtu and physical interface MTU, and CLNS/IP MTU match, so your IS-IS sessions should remain stable.

But, one potental improvement would be to standardize the ISIS interface MTU across both core and PE devices. The mismatch (9629 on core vs. 9577 on PE) might not break adjacency but could lead to unexpected behavior under certain conditions, like when an LSP exceeds the PE's MTU as an example.

It should be better to align the ISIS interface MTU across all devices would in order to eliminate that risk.

Best regards
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Leo Gal 79 · ‎03-03-2025

Hi M02@rt37

well...if you can tell me how to do it, thats why I am here :-). And I hope (so far only hope)...if default lsp-mtu is 1492, that ISIS will not create bigger PDUs than that.
LG

Leo Gal 79 · ‎03-04-2025

Hey there for everyone that tried to help.
Thank you for the efforts. I\ve got news.

After setting interface MTU to 9214 I got the same ISIS interface MTU numbers on both platforms.

RP/0/RP0/CPU0:DC2RTC25COR01P#show isis interface brief
Tue Mar 4 09:59:39.337 UTC

IS-IS CMSCORE Interfaces
Interface All Adjs Adj Topos Adv Topos CLNS MTU Prio
OK L1 L2 Run/Cfg Run/Cfg L1 L2
----------------- --- --------- --------- --------- ---- ---- --------
Te0/2/0/10/0 Yes - 1 1/1 1/1 Up 9197 - -
RP/0/RP0/CPU0:DC2RTC25COR01P#

RP/0/RP0/CPU0:DC2RTC25RRC01P#show isis interface brief
Tue Mar 4 10:04:08.419 UTC

IS-IS CMSCORE Interfaces
Interface All Adjs Adj Topos Adv Topos CLNS MTU Prio
OK L1 L2 Run/Cfg Run/Cfg L1 L2
----------------- --- --------- --------- --------- ---- ---- --------
Te0/0/0/0 Yes - 1 1/1 1/1 Up 9197 - -
RP/0/RP0/CPU0:DC2RTC25RRC01P#

So the result is...do NOT use that high MTU. Going for 9214.

Cisco8608-ASR9902 same clns MTU, but different isis interface MTU why?