[j-nsp] Jumbo frames / mismatch MTU

[H I Baysal]

Hi,

Juniper counts layer 2 the header as well for MTU, unlike most of the vendors.

Juniper side MTU has to be set 14 bytes bigger and your OSPF and LDP should come up

Good luck!!

Sent from my iPhone

> On 23 Apr 2021, at 15:36, Matthew Crocker <matthew at corp.crocker.com> wrote:
> 
> 
> Hello,
> 
> I have a network configured with a QFX core, 10 ACX2200 & 10 SRX300s.  The ACXs are connected to the QFX via 10G links and the SRX300s are connected via a Transparent LAN type service.    The ACX & SRX have LDP,OSPF & BGP running to support MPLS on the QFX facing interfaces.  The QFX is setup as a simple layer 2 device and is not running MPLS.
> 
> Each ACX & SRX has a 10G or 1G interface configured, connected to the QFX with an IP from 10.200.1.0/24.  Each router has a loopback lo0 with a /32 from 10.200.2.0/24.   All routers dynamically share loopback IPs via LDP & OSPF.  All routers have BGP full mesh and establish BGP sessions to the loopback addresses once they are learned.
> 
> The SRX devices are limited to an MTU of 1600 due to the TLS carrier they are using to connect back to the  QFX.
> 
> I need to support 9K frames from one ACX to another over this network.  The QFX is configured for MTU of 9192 on all interfaces.  When I configure a couple ACXs with 9192 MTU the OSPF & LDP sessions go away.
> 
> I can ping ACX to ACX with 9k packets just fine.
> 
> For some reason LDP or OSPF don’t get established the ACXs never learn the lo0 IPs of their peers which causes BGP to fail.
> 
> I’ve enabled mtu-discovery in OSPF & LDP to no avail
> Is there some other setting I’m missing?
> 
> Am I allowed to have most routers running MTU 1600 and a couple running 9k?  I only really need one ACX <-> QFX <-> ACX path to support 9k for SAN replication between buildings
> 
> Currently the ACX interface is:
> 
> xe-0/3/0
> 
> mtu 1600;
> 
> unit 0 {
> 
>    family inet {
> 
>        sampling {
> 
>            input;
> 
>            output;
> 
>        }
> 
>        address 10.200.1.1/24;
> 
>    }
> 
>    family mpls;
> 
> }
> 
> 
> 
> lo0
> 
> unit 0 {
> 
>    family inet {
> 
>        address 10.200.2.1/32;
> 
>    }
> 
> }
> 
> 
> protocols ldp
> 
> interface ge-0/0/0.0;
> 
> interface xe-0/3/0.0;
> 
> interface lo0.0;
> 
> 
> protocols ospf
> 
> traffic-engineering;
> 
> export [ export-direct export-statics ];
> 
> area 0.0.0.0 {
> 
>    interface lo0.0;
> 
>    interface xe-0/3/0.0;
> 
>    interface ge-0/0/0.0;
> 
> }
> 
> I’ve backed out the mtu-discovery for LDP & OSPF.
> 
> Everything is working.  If I ‘set mtu 9192’ everything breaks
> 
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