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does IOS XR have an equivalent of the IOS interface setting 'clns mtu' ?

RP/0/RSP1/CPU0:router#sh run int Te0/0/0/0 mtu     

interface TenGigE0/0/0/0

mtu 9192


I've set the 'lsp-mtu' value under the isis process, but that doesn't seem to affect the interface' CLNS MTU;

RP/0/RSP1/CPU0:router#sh run router isis | inc mtu

lsp-mtu 1497

RP/0/RSP1/CPU0:router#sh isis interface brief Te0/0/0/0

IS-IS AS1 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      2/2        2/2     Up    9175    -    -

Thanks in advance,


Everyone's tags (3)
Cisco Employee


Hi Daniel,

If i understand correctly, "clns-mtu" value is used by both CLNS and IS-IS. For CLNS, this is used to determine the maximum sized network layer PDU that can be sent over that interface (this includes payload and L3 header). PDUs which are larger will be segmented (ISO speak for fragmentation) to clns-mtu size.

For IS-IS, clns-mtu has both a direct and indirect usage.

Directly, clns-mtu controls the maximum size PDU IS-IS will send on that interface. This directly affects the size of IIHs if hello padding is enabled (which is the default setting).

Indirectly, clns-mtu needs to be greater than or equal to the maximum size LSP that any IS in the area/domain may generate. So clns-mtu must be>= lsp-mtu for all links in the rea/domain which are used forflooding of LSPs."

Now, in IOS XR, you can't change the clns mtu, but you can change the lsp mtu.  Unlike IOS, where you can change the clns mtu.  The clns mtu will affect the hello packets, and the lsp mtu will not so that is a difference.  So you can keep using the clns mtu on the IOS router. 

The 'lsp-mtu' option in the IS-IS configuration now affects the size of CSN(Complete Sequence Number) and PSN (Partial Sequence Number) PDUs.  The size of these PDUs will be limited to the value specified by 'lsp-mtu'.  If 'lsp-mtu'is set to a value larger than the CLNS MTU of an interface running IS-IS, the CLNS MTU will be used instead.

Hope that helps.


Community Member

Re: IS-IS CLNS mtu

ISIS between ASR9K and ASR1K

You may set up the"clns mtu" on ASR1K to match the  ASR9K's isis MTU, like "clns mtu 8983".

Or, to calculate the gap of MTU, then configure the MTU in interface(not sub-interface) to match the


nterface TenGigabitEthernet0/1/0

 mtu 8986

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