IOS XE, IOS XR ISIS MTU AND ISIS Adjacency Problems due to MTU

CLNS MTU or LSP MTU ? :

On IOS XR you can not change CLNS MTU. It’s derived from interface mtu;

interface GigabitEthernet0/0/0/0
mtu 9000
ipv4 mtu 1500
!
sh im database interface gigabitEthernet 0/0/0/0
.......
.......
Protocol Caps (state, mtu)
-------- -----------------
None spio (up, 9000)
None ether (up, 9000)
arp arp (up, 8986)
clns clns (up, 8986) # interface mtu 9000 - ether header 14
ipv4 ipv4 (up, 1500)
ether_sock ether_sock (up, 8986)

But you can change LSP MTU

router isis XXXXX
lsp-mtu 1500

On IOS XE, clns mtu can be changed under interface parameters;

interface GigabitEthernet0/1
clns mtu 1500

 

Padding : Default behaviour on both platforms to pad ISIS LSP to interface MTU.

1 – You can adjsut CLNS MTU on IOS XE via interface level command clns mtu or on IOS XR under isis process level lsp-mtu command.

2 – Or you can disable padding of lsp on IOS XE via interface level command no isis hello padding always or on IOS-XR isis process interface level command hello-padding disable