[c-nsp] MTU on XR

[Mikael Abrahamsson]

Hello.

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

What I can check personally with machines I have available, the above 
article is correct in saying that vlan tags does not count when it comes 
to configuring MTU. XR will automatically add number of vlan tags when 
programming MTU to the hardware, no care has to be taken to configuring 
it, at least not for L3 interfaces.

On an interface with MTU 9200 on the main interface, the subinterface has 
this:

#show im database interface Te0/3/0/0.101
Interface TenGigE0/3/0/0.101, ifh 0x06001340 (up, 9204)
   Interface flags:          0x0000000000800597 (ROOT_IS_HW|IFINDEX

|SUP_NAMED_SUB|BROADCAST|CONFIG|VIS|DATA|CONTROL)
   Encapsulation:            dot1q
   Interface type:           IFT_VLAN_SUBIF
   Control parent:           TenGigE0/3/0/0
   Data parent:              TenGigE0/3/0/0
   Views:                    GDP|LDP|L3P|OWN

   Protocol        Caps (state, mtu)
   --------        -----------------
   None            vlan_jump (up, 9204)
   None            dot1q (up, 9204)
   ipv6            ipv6_preswitch (up, 9186)
   ipv6            ipv6 (up, 9186)

so this single tagged subinterface has automatically had its MTU raised to 
9204 to accomodate the vlan tag, even though config says 9200. The IPv6 
MTU ends up being 9186.

This for instance contradicts what AMSIX has written on this page:

https://ams-ix.net/technical/specifications-descriptions/config-guide

"5.7. MTU Config

On Cisco IOS, the interface IP MTU should be set to 1500. MTU 
configurations for IOS-XR include the Layer-2 headers, and need to be 
adjusted when using VLAN tags."

So which one is correct? I have talked to numerous people and they have 
different experience and results are contradicting.

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se