MTU and Giant/Input Errors

[Jay.Gibson531]

Hello all,

there might already be a thread out there for this but I wanted to ask.  We are getting a bunch of Giants on a TenGigE interface on an ASR9K connected to an ASR5500.  I have 2 questions:

1. Do Giants cause any issue and if so, what?

2. The ASR9K is defaulted to 1518 on the phyiscal interface and 1514 on the Bundle-Ether interface, neither is configured with MTU command...how does that work?  Does the physical interface just inherit the MTU from the BE interface or are they really operating on different MTUs (which I doubt)?

          a. I read over the ASR9K MTU documenation but it only addresses sub-interfaces vs physical and not Bundle-Ether interfaces vs. Physical

Any help is appreciated

[LJ Gabrillo]

Giants are packets that are too large, larger than 1518 bytes. These packets are dropped

>Does it cause any issues? Well, it depends on your traffic:

-Does not your network expect giant packets to passthrough? If so, then yes, there could be an issue since its dropping it

-Did it just occur, and normally shouldnt be there? There could be an issue in between that port to the connected device: <1> Possible physical port/NIC issue. <2> Do you see the collision count increasing as well on the "show interface"?, collisions can contribute to Runts/Giants, there could be an issue with your fiber cable <3> It could also be an indication of an attack (where a malicious entity is sending you large packets to DDoS you or, already a man-in-the-middle attack where they malform your packet, and insert their own data) <4> It could be a bug on either device, the local device or the device connected to that port

Appreciate if you can mark my post "Helpful" if it helped you

[Jay.Gibson531]

so it's weird because we're seeing spikes of Giants and then it's stable for a time...then spikes again and it just started a day before we saw the issue.

We do allow Giants in a large portion of our network, just not on this link, and it's same in other devices in the same role in different locations EXCEPT no Giants are seen in the other locations.

Lastly, can anyone answer the below:

The ASR9K is defaulted to 1518 on the phyiscal interface and 1514 on the Bundle-Ether interface, neither is configured with MTU command...how does that work?  Does the physical interface just inherit the MTU from the BE interface or are they really operating on different MTUs (which I doubt)?

          a. I read over the ASR9K MTU documenation but it only addresses sub-interfaces vs physical and not Bundle-Ether interfaces vs. Physical

[Elliot Dierksen]

ethernet frame size and IP MTU are different things. Obviously they impact one another, but they aren't the same. That is one of the things I don't like about using jumbo frames is the entire network path has to support the jumbo frames or they get dropped. If the end points aren't doing something like IP MTU path discovery, then you can have a problem.More often than not, storage is the kind of traffic that wants to use jumbo frames. Is it possible you have some intermittent traffic from hosts that have jumbo frames enabled?