on 03-04-2011 11:32 AM
This section describes the equations on how the MTU is being calculated for interfaces in IOS-XR.
The MTU is important since it is used as part of the signaling for Pseudo Wires, hence it is important to understand the way XR calculates MTU.
Also OSPF will benefit from the right mtu tuning
This section describes the symptoms of the problem and the main issue the document resolves.
The L2 MTU of the sub-interface is calculated as follows:
The L2 payload MTU is calculated as follows:
sub-l2-mtu – (14 + (4 * (encaps-pop-tags-count – encaps-push-tags-count)))
The IM L2 payload MTU is calculated as follows:
Notes on the calculations above:
Notes related to specific EFP encapsulation commands:
dot1q vlan 10 20
interface GigabitEthernet0/0/0/0.2 l2transport
service instance ethernet
encapsulation dot1q 11
rewrite ingress tag push dot1ad 10 dot1q 20 symmetric
To further explain the example for GigabitEthernet0/0/0/0.2. If a 2026 byte frame was received from the PW on the disposition path then once the 2 tags have been popped off due to the rewrite policy the packets would be of size 2018, hence matching the MTU of the Egress interface.
 Note the MTU negotiated is the L2 payload MTU and hence the actual maximum size of the frame that may be carried is 14 bytes larger for a type 5 PW and 18 bytes larger for a type 4 PW, although in the case of the type 4 PW the PW 802.1Q tag is effectively discarded and ignored for the purpose of the L2VPN MTU calculations (since it isn’t part of the payload).
Starting in XR release 5.1.1 the ethernet interface driver configuration
for MTU and MRU is determined by the XR config.
Prior to these changes MTU/MRU calculation was simply based on the
configured MTU + 12 bytes for the addition of 2 Ethernet Tags and the CRC
field as mentioned above.
However from 511 onwards, the config determines the actual MTU/MRU.
If no VLAN tags are configured -
then driver MTU/MRU = configured MTU on interface + 4 CRC bytes, ie
If one VLAN is configured -
MTU/MRU = configured MTU + 8 bytes (2 tags + CRC)
If two VLAN tags are configured -
MTU/RMU = configured MTU + 12 bytes (3 tags + CRC)
Xander Thuijs, CCIE #6775
Principal Engineer ASR9000
Today I have the MTU configured on the vlan as shown below, but now I am changing it to the IOS-XR standard (EVC Framework), so my question is, where do I have to adjust the MTU in a way that it has the same effect that the old configuration on IOS?
interface vlan 200
:router(config)# interface bvi 100
:router(config-if)# vrf vod-management
:router(config-if-vrf)# ipv4 address <IP> <mask>
:router(config-l2vpn)# bridge group VL100
:router(config-l2vpn-bg)# bridge domain VL100
:router(config-l2vpn-bg-bd)# interface gigabitethernet 0/0/0/4
:router(config-l2vpn-bg-bd)# interface gigabitethernet 0/0/0/5.100
:router(config)# interface gigabitethernet Gi0/0/0/4
:router(config-if)# interface Gi0/0/0/5.100 l2transport
:router(config-if)# encapsulation dot1q 100
:router(config-if)# rewrite ingress tag pop 1 symmetric
Thanks in advance,
9016 is the value to choose, since you have one tag on the 0/0/0/5.100 EFP and you are popping the tag (which is necessary since you are linking it to a 0/0/0/4 main EFP and using BVI.
Can I get to know how to create an object group for monitoring the status of vrf interfaces in XR. Accept my apologies if it's not the right discussion to ask this. I am expecting your valid response.
Hi Rakesh, yeah this question is not really related to interface MTU and calculations , you can always raise a new question on the "root" of the XR OS forum, not a problem.
Any case what you are after is what I call Enhanced Object Tracking. There are possibilities, eg via EEM.
For that I have a write up too, you can find it as doc under my profile. I believe it has a section on EOT also, if not
I will post a separate document on establishing object tracking.
Thanks a lot.I have read the documentation. I need to ask one more question. Can we apply the configuration(which is specified the document) under vrf interfaces ? I have got another document about object tracking. Don't they specify about the abject group monitoring in vrf interfaces. Let me know if it is not the right document for achieving the required feature .Please go throuh the below link and suggst the right one. Please help me one more time.
As I understand the default MTU value for Gig / 10Gig interfaces on ASR's is 1514 bytes. This includes the 14 bytes for ethernet header.
Hence the IP MTU come to 1500 bytes.
We intend to use 1G and 10G interfaces as MPLS core interfaces on our ASR's. We will be having these MPLS links between ASR - ASR and ASR - Junipers.
We plan to get our transport links to support 1600 bytes of maximum frame-size. And also configure 1600 as interface MTU on the ASR and Juniper.
The effective IP MTU and MPLS MTU that we should get from the above configuration should be as below
ASR - Interface MTU : 1600 bytes
ASR - IP MTU : 1600 - 14 = 1586 bytes
ASR - MPLS MTU : 1600 - 14 = 1586 bytes
Juniper - Interface MTU : 1600 byte
Juniper - IP MTU : 1600 -14 = 1586 byte
Juniper - MPLS MTU : 1586 - 12 = 1574 byte
Our MPLS Core will have OSPF as IGP. Please suggest if the above understanding and MTU calculations are correct.
We don't intend to hard-code IP MTU and also don't want to use "ip ospf mtu ignore".
Many thanks !!
I have a customer with ASR9000 and IOS XR 5.1.3.The customer has deployed aTengigabitethernet link between ASR9K across a Cisco DWDM network.
The asr9000 HW is the following:
ASR9000 IOS XR 5.1.3 with A9K-MOD80-TR and A9K-MPA-4X10GE -------- <CISCO DWDM NETWORK> -------- ASR9000 IOS XR 5.1.3 with A9K-4T-B
We have configured hte MTU as 9216 in main interfaces in both sides.
Then we could observe that MTU in l3 subinterfaces is 9216 + 4 = 9220
We was waiting to pass across the link pings with a maximum MTU value of 9202 bytes ( 9220- 4 (DOT1Q ETHERNET HEADER) – 14 (ETHERNET HEADER = 9202 BYTES)
But the ping normal and extended with the donotfrag option have reply only for 9198
I am not sure if the problem is my understanding of the equipment or the DWDM network has a configuration problem.
We would be grateful if you could comment.
yeah you are exceeding the hw limit of the mtu at this configuration. you're 4 bytes over,
and that is precisely the problem you're hitting:
RP/0/RP0/CPU0:ASR9922(config)#int te 0/5/0/10
RP/0/RP0/CPU0:ASR9922(config-if)#int te 0/5/0/10.10
RP/0/RP0/CPU0:ASR9922(config-subif)#encap dot1q 10
RP/0/RP0/CPU0:ASR9922(config-subif)#ip add 184.108.40.206 255.255.255.0
Fri Jun 3 08:59:53.101 EDT
LC/0/5/CPU0:Jun 3 08:59:53.560 EDT: vic: %L2-ETHERNET-6-MTU_NEAR_HW_LIMIT : The MTU of 9220 on TenGigE0_5_0_10 is over the H/W limit, and hence large frames may be dropped as oversized frames
Thanks for detailed information.
I would like to know if all ios xr versions follow the same calculation.
Specifically if between 5.1.3 and 5.3.3 we had the MTU defaul value/calculation change.
After upgrade to 5.3.3 some services goes down because of the MTU value change. After manually configure the required MTU values the services back to work again.
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