Jumbo frame support under L2vpn sub-interface on ASR9K
I run jumbo frame support test for our l2vpn xconnect service on Cisco ASR 9K platfrom, XR OS is kind of old as 3.9.2
I setup AC interface under both end router sub-interface. 200, shown as below:
testing IOS router 1 int gi0.200---- int gi0/0/0/1.200 ASR9K router 1--MPLS enable --ASR9K router 2 int gi 0/0/0/0.200-----inter gi0/1.200 testing IOS router 2
I already enable jumbo frame on two ASR9K mpls link and also enable MTU size over 9000 on ASR 9K router1 main interace gi 0/0/0/1 and
ASR 9K router2 main interace gi 0/0/0/0, and also on both end sub-interface as well.
the large size PING test from testing IOS router 1 or 2 are show good, as ping through over 9000 packet size without fragment is OK.
the issue is: when I changed MTU size back to 1518 on ASR9K router 1 int gi0/0/0/1.200 and ASR9K router 2 int gi 0/0/0/0.200, I still can ping through large packet size over 9000 without fragment from both IOS testing router end. it is showing my sub-interface MTU size setting couldn't limit l2vpn packet size, even the L2vpn xconnect output showing MTU size is been changed back to 1500.
sh l2vpn xconnect detail
Fri Nov 29 20:35:21.203 UTC
Group test1, XC test1, state is up; Interworking none
AC: GigabitEthernet0/0/0/1.200, state is up
Type VLAN; Num Ranges: 1
VLAN ranges: [200, 200]
MTU 1500; XC ID 0x40001; interworking none
packets: received 84148, sent 507960
bytes: received 9043313, sent 37742100
drops: illegal VLAN 0, illegal length 0
PW: neighbor 10.10.100.2, PW ID 200, state is up ( established )
PW class not set, XC ID 0x40001
Encapsulation MPLS, protocol LDP
PW type Ethernet, control word disabled, interworking none
Re: Jumbo frame support under L2vpn sub-interface on ASR9K
yup I can see how that is a bit misleading, but this is expected, let me explain.
In L2(vpn) mode, thesub-interface MTU is not enforced and merely used for signaling only.
So eventhough a packet is larger then the MTU of the l2transport subinterface, it will still go through as MTU checking
is not performed on the L2 interface side. This was done for good reasons (performance), and conformance too sort-a
speak as in L2 mode we ought to be transparent (but ok to what level right).
Either case the performance gain we get out of this is a massive benefit.
If however the MTU of the main interface is smaller then the packet that wants to get out, then it will get dropped.
Remember that XR will never fragment in hardware.
In L3 mode (that is a (sub)interface with an ip address on it pretty much), MTU is enforced, but when an MTU is mismatched against the CEF forwarding entry the packet is punted to the egress LC cpu for fragmentation.
This is limited to 1000pps (or whatever that lpts policer value is these days), but either case features are NOT applied!
So if you have egress ACL or QOS, this will not get applied to the injected fragments and that is a big pain.
Few more examples that hopefully explain a couple of additional scenarios:
If you have a phy MTU defined, then the packet will get dropped when it is over
If you have a phy MTU defined of say 8000 and a subif MTU of 2000 and that
subif is an L2transport interface then you WILL be able to receive a 3000 byte packet on your AC.
If the AC is an xconnect with a PW going out a core interface, the core interface MTU on the egress LC will be enforced and may result in fragmentation.
If you receive a 3000 byte packet on your PW (assuming core can handel that)
and needs to go out the AC with the subif MTU of 2000, and phy MTU of 8000 this WILL go through. (basically your case you described)
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