[j-nsp] Layer 2 Fragmentation

[Ala' Amira]

I believe it is not valid for Ethernet interfaces dear salbad 

-----Original Message-----
From: juniper-nsp-bounces at puck.nether.net
[mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of Mohammad Salbad
Sent: Sunday, March 06, 2011 1:41 PM
To: juniper-nsp at puck.nether.net
Subject: [j-nsp] Layer 2 Fragmentation

Hello All

 

I have two EX4200 switches connected back to back to each other with
interfaces in trunk mode as following:

Case(1) back to back connection using ethernet cable

(VL11 IP: 10.1.1.1/30) EX4200 -------------------TRUNK
(Ethernet)---------------------EX4200 (VL11 IP: 10.1.1.2/30)

è For this case I can ping with don’t fragment option up to 1554 byte as
following:

ping 10.1.1.2 size 1554 do-not-fragment    

PING 10.1.1.2 (10.1.1.2): 1554 data bytes

1562 bytes from 10.1.1.2: icmp_seq=0 ttl=64 time=3.639 ms

1562 bytes from 10.1.1.2: icmp_seq=1 ttl=64 time=3.550 ms

1562 bytes from 10.1.1.2: icmp_seq=2 ttl=64 time=3.556 ms

Case(2) back to back connection using wireless backhaul devices

(VL11 IP: 10.1.1.1/30) EX4200 -------------------TRUNK (wireless
backhaul)---------------------EX4200 (VL11 IP: 10.1.1.2/30)

è For this case I can ping with don’t fragment option up to 1476 byte as
following:

ping 10.1.1.2 size 1476 do-not-fragment    

PING 10.1.1.2 (10.1.1.2): 1476 data bytes

1484 bytes from 10.1.1.2: icmp_seq=0 ttl=64 time=4.096 ms

1484 bytes from 10.1.1.2: icmp_seq=1 ttl=64 time=4.094 ms

1484 bytes from 10.1.1.2: icmp_seq=2 ttl=64 time=5.496 ms

 

Switches configuration sample

ge-0/0/1 {

    mtu 1600;

    ether-options {

        link-mode full-duplex;

        speed {

            100m;

        }

    }

    unit 0 {

        family ethernet-switching {

            port-mode trunk;

            vlan {

               members [ TEST ];

        }

    }

}

vlan {

    mtu 1600;

    unit 11 {

        family inet {

            address 10.1.1.1/30;

    }

}

vlans {

    TEST {

        vlan-id 11;

    }

 

The case is that I didn’t change the configuration on the switches I just
replaced the Ethernet cable with wireless backhaul devices and then I can’t
ping with larger mtu sizes, and this is because the wireless devices I’m
using for backhaul are limited to 1500 MTU size, so I’m wondering if I there
is anything I can do on the switches to overcome the limitation on the
wireless backhaul devices, i.e. if I can fragment the packet into smaller
pieces before sending them to the wireless device and combining the packet
on the other end (something like link fragmentation and interleaving).

 

Thank you

Mohammad Salbad

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