# Juniper MX Jumbo Frames Can Cause Fragmentation on the PFE-RE Ethernet Link

DevFeed: [Juniper MX Jumbo Frames Can Cause Fragmentation on the PFE-RE Ethernet Link](<https://devfeed.tech/articles/interesting-juniper-mx-and-jumbo-frames-11199.md>)

Original publisher: [Read original article](<https://blog.ipspace.net/2025/06/juniper-mx-jumbo-frames/>)

Published: 2025-06-10T05:41:00Z

Content type: opinion

Language: en

Sources: [ipSpace.net blog](<https://devfeed.tech/sources/ipspace-net-blog.md>)

Topics: [Ethernet](<https://devfeed.tech/topics/ethernet.md>), [networking](<https://devfeed.tech/topics/networking.md>), [Routing (disambiguation)](<https://devfeed.tech/topics/routing.md>)

Tags: [blog](<https://devfeed.tech/tags/blog.md>), [blog-post](<https://devfeed.tech/tags/blog-post.md>), [ethernet](<https://devfeed.tech/tags/ethernet.md>), [fragmentation](<https://devfeed.tech/tags/fragmentation.md>), [fragments](<https://devfeed.tech/tags/fragments.md>), [queue](<https://devfeed.tech/tags/queue.md>), [routing](<https://devfeed.tech/tags/routing.md>), [worth-reading](<https://devfeed.tech/tags/worth-reading.md>)

## AI overview

The article explains that the Ethernet link between the Packet Forwarding Engine and Routing Engine in Juniper MX has a fixed 1500-byte MTU. Jumbo frames can therefore be fragmented, and dropped fragments may remain in the reassembly queue when control-plane policing intervenes.

## Source excerpt

Did you know that there's an Ethernet link between the Packet Forwarding Engine (PFE - data plane) and Routing Engine (RE - control plane) in every Juniper MX? That's why you have to run two VMs to emulate it (sometimes conveniently packed into one larger VM, proving RFC 1925 rule 6a). That Ethernet link happens to have the MTU fixed at 1500 bytes. Guess what happens in the world where everyone uses jumbo frames? Did you say fragmentation? Bingo! And what do you think happens when one of those fragments gets dropped due to control-plane policing, and the rest of them are stuck in the reassembly queue? You'll find the gory details in a lengthy blog post by Nitzan Tzelniker.