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: [email protected]
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