On 11/08/2019 13.50, Rob Wilton (rwilton) wrote:
Hi Vladimir,
Thanks for the comments.
Regarding increasing the L2 MTU if 802.1Q tags are present, this is because of
how IEEE 802.3 defines the MTU of an Ethernet interface (at least for a single
802.1Q tag).
Where does IEEE 802.3 define "MTU of an Ethernet interface" (me being
positive IEEE 802.3 or 802.1Q do not use MTU definition at all)?
If you have a mix of single and double tagged sub-intefaces, it also means that
the IP MTU for all of those sub-interfaces can be the same.
E.g. if you set the L2 MTU of a physical interface to 1514 (excluding FCS
bytes) then the IP MTU for each of the sub-interfaces would be 1500 bytes
regardless of whether they are configured to match single or double VLAN tags.
Conversely, if you have a strict L2 MTU that doesn't have this flexibility then
a single tagged sub-interface would end up with an IP MTU of 1496, and a double
tagged sub-interface would end up with an IP MTU of 1492, complicating L3
configuration.
The problem can become very complicated if we introduce new definition
of MTU (I still do not agree L2 MTU is a thing).
Why the ifMtu object from IF-MIB is not mapped to config false
/ietf-interfaces:interfaces/interface/ietf-interfaces-common:mtu in
ietf-intefaces-common.yang and instead we need something else?
From RFC 2863:
ifMtu OBJECT-TYPE
SYNTAX Integer32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The size of the largest packet which can be sent/received
on the interface, specified in octets. For interfaces that
are used for transmitting network datagrams, this is the
size of the largest network datagram that can be sent on the
interface."
::= { ifEntry 4 }
IMO this sounds as useful today as it did before.
From RFC 8343 "4. Relationship to the IF-MIB":
The ifMtu object from the IF-MIB is not mapped to the
"ietf-interfaces" module. It is expected that interface-type-
specific YANG modules provide interface-type-specific MTU leafs by
augmenting the "ietf-interfaces" model.
I am aware of that text too but I do not agree mapping ifMtu which is
interface type independent to /interfaces/interface/mtu is not necessay
and can be replaced by introducing "interface-type-specific MTU leafs".
Vladimir
BTW, I'll be on PTO for a week, so please expect a delay in response.
Thanks,
Rob
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