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Beginner

MTU for out going interface.

Hi Expert,

I have ASR9010 with single incoming interface(GE) and two outgoing interfaces(TenG1 and Bundle-E(TenG2)). the routing/cef/label is configured to forwarding traffic out TenG1. All interfaces are configured with MTU 9194.

- When I inject traffic with 1500byte size, traffic has been forwarded out TenG1 as intended.

- Same traffic but change the size to 9000byte, traffic has been forwared out Bundle-E and it can not reach the destination.... routing /CEF still say that it should be forwared out TenG1.

Any possibility that I have done something wrong?

Regard,

Marit.

2 ACCEPTED SOLUTIONS

Accepted Solutions
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you may have loadbalancing going on in this case with 2 equal paths between the tenG and the bundle-e,

depending on the l3/l4 info, the hash distribution may be different hence different paths are selected.

there is no length parameter in the hash calculation, but I am thinking that if the egress interface had a smaller mtu then the packet size, meaning that the egress Lc would punt for fragmentation, it could be the sw injected the packets down into other paths then what you'd expect.

fragmentation should be prevented at all cost, because that is not working in hw and no features are applied.

you can check the np counters of your linecard to see if the packets were forwarded and or punted.

regards

xander

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If you can recreate this issue again, i suggest you sniff the packet that come out of Bundle-E and find the difference with the first stream. If you don't have ECMP, so there is no reason for same stream to take another path. I suspect the destination address change in the spirent setting.

thanks,

rivalino

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Cisco Employee

Hello Marit,

How did you find out that the traffic with size 9000byte was sent out of Bundle-E? I would like to see RIB and CEF output too.

regards,

Rivalino

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Hi Rivalino,

Thanks for help!

I used spirent to generate 200 mbps traffic through incoming interface and just used "show interface" to see the outgoing byte counter since it is only 2 exits. When I changed from 1500byte to 9000byte the counter of TenG1 went down from 200 while Bundle-E went up to 200mbps.

Tomorrow I will show you RIB/CEF.

This is last day that i have spirent in my lab tomorrrow i will use cpe-router to generate traffic.

Thanks and Regard,

Marit

Highlighted

you may have loadbalancing going on in this case with 2 equal paths between the tenG and the bundle-e,

depending on the l3/l4 info, the hash distribution may be different hence different paths are selected.

there is no length parameter in the hash calculation, but I am thinking that if the egress interface had a smaller mtu then the packet size, meaning that the egress Lc would punt for fragmentation, it could be the sw injected the packets down into other paths then what you'd expect.

fragmentation should be prevented at all cost, because that is not working in hw and no features are applied.

you can check the np counters of your linecard to see if the packets were forwarded and or punted.

regards

xander

View solution in original post

Highlighted

Thank guys,

After Spirent has been took out then I generated traffic by using ping with jumbo frame packet from CPE it suddenly reached the destination. So, there might be something wrong with Spirent configuration but I still wonder even wrongly configured spirent could effect routing decision on router ??

Xander,

I can confirm that the load balance had been took out and CEF clearly shown that the outgoing was TenG interface.

"if the egress interface had a smaller mtu then the packet size, meaning that the egress Lc would punt for fragmentation, it could be the sw injected the packets down into other paths then what you'd expect." <-- This is what I worry, could it possible to happen that way?

Highlighted

If you can recreate this issue again, i suggest you sniff the packet that come out of Bundle-E and find the difference with the first stream. If you don't have ECMP, so there is no reason for same stream to take another path. I suspect the destination address change in the spirent setting.

thanks,

rivalino

View solution in original post

Highlighted

Hi Rivalino,

I think so...I will leave it there till i have Spirent again which supposed to be the end of Jan.

Regard,

Marit

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