MTU SIZE Issue with JUNIPER Router In MPLS

[moorthy1977]

Hi,

I have Juniper router & Next Hop router is Connetd Via Metro Ethernet switch Atrica & then It is given L2 MPLS Connectivity.Issue is when, i am not able to use more than 1472 packet size in Juniper.How to go ahead with MTU Issue.My Metro Atrica Switch is set 1500 MTU Size & I do not know about MPLS since it is through provider & Provider is saying no issue from there side.Find the Ping Respnse From Juniper.

ping do-not-fragment 172.16.8.10 size 1472

PING 172.16.8.10 (172.16.8.10): 1472 data bytes

1480 bytes from v: icmp_seq=0 ttl=251 time=7.704 ms

--- 172.16.8.10 ping statistics ---

1 packets transmitted, 1 packets received, 0% packet loss

round-trip min/avg/max/stddev = 7.329/7.719/8.125/0.325 ms

------------------------------------------------------------------------

ping do-not-fragment 172.16.8.10 size 1473

PING 172.16.8.10 (172.16.8.10): 1473 data bytes

ping: sendto: Message too long

ping: sendto: Message too long

ping: sendto: Message too long

^C

--- 172.16.8.10 ping statistics ---

3 packets transmitted, 0 packets received, 100% packet loss

Regards,

Moorthy

[Harold Ritter]

This is normal behavior.With JunOS, the size you specify on the ping command is actually the size of the ICMP payload. In this case, 1472 of payload + 8 bytes of ICMP header + 20 bytes of IP header results in a 1500 bytes IP packet, which is indeed the largest packet you can send without fragmentation.

Let me know if I answered your question,

[moorthy1977]

Hi,

Thanks for your reply but i had given command

ping do-not-fragment 172.16.8.10 size 1472

Ping with Do-not-fragment coomand.

regards,

[Harold Ritter]

That is exactly my point. You are trying to pass packets larger than 1500 bytes (1473 + 8 + 20 = 1501) but you are also specifying that you don't want fragmentation to take place (do-not-fragment).

According to the ouput you provided, it seemed to work quite well with a packet size of 1500 bytes (1472 bytes payload).

Hope this helps,

[AnandJalawadi]

Hi,

if i m not confused ,icmp header will be of 4 bytes??right

[Harold Ritter]

The ICMP header would actually consists of eight bytes in the case of echo request/request.

type 1 byte

code 1 byte

chksum 2 bytes

identifier 2 bytes

sequence number 2 bytes

Hope this helps,

[moorthy1977]

Hi,

Still i have not got my answer.I need to check MTU size on MPLS network.how to check MTU size above 1472 on cisco or on Juniper.

Hope for the answer

regards,

[Harold Ritter]

I think I might not understand the question as I think I already answered that question. Pinging with a size of 1472 and the do-not-fragment on the Juniper side will verify that the path from source to destination has an MTU of 1500 bytes (1472 payload + 8 ICMP header + 20 bytes IP header).

On the Cisco side, you need to specify a size of 1500 bytes while using the extended ping as this value includes the size of IP and ICMP headers.

Let me know if I answered your question,

[AnandJalawadi]

Hi,

On my juniper router i am able to ping loopback of other PE with size of 1492 with do-not-fragment set

r0> ping 10.10.10.1 do-not-fragment size 1492

PING 10.10.10.1 (10.10.10.1): 1492 data bytes

1500 bytes from 10.10.10.1: icmp_seq=0 ttl=253 time=8.396 ms

1500 bytes from 10.10.10.1: icmp_seq=1 ttl=253 time=57.736 ms

1500 bytes from 10.10.10.1: icmp_seq=2 ttl=253 time=8.405 ms

1500 bytes from 10.10.10.1: icmp_seq=3 ttl=253 time=8.955 ms

1500 bytes from 10.10.10.1: icmp_seq=4 ttl=253 time=8.398 ms

1500 bytes from 10.10.10.1: icmp_seq=5 ttl=253 time=8.326 ms

[AnandJalawadi]

Dear Herold,

I completly agree the above statement made by you(i.e 1472+8+20=1500),as i m able to ping the CE' s IP under vrf with size of 1472 (do-not-fragment set),however as soon as i try to ping any of my juniper PE loopbacks the ultimate size goes to 1492,with this observation we cant say that this ping is not carrying an IP header,is it right??

thanks and regards

Anand

[Harold Ritter]

What do you mean by "the ultimate size goes to 1492"? What size value do you specify on the ping command? The other thing to take in consideration is the MPLS overhead.

Also, What is the MTU value on your core links? If you use Sonet in the core you should be able to ping with a size much bigger than 1500 bytes.

Hope this helps,

[AnandJalawadi]

Herold,

The 1492 is the maximum size with which i m able to ping loopbacks of other PEs with do-not-fragment bit set,Juniper PEs are connected to Cisco cores through gigabit links with mtu size of 1600,and all core links are of sonet with mtu 4470 bytes.

thanks and regards

Anand

[moorthy1977]

Hi,

you mean to say that if i ping with 1472 size actually i am pinging size of 1500 ?

if i need to check above 1472 what cooman i need to give on cisco.

Regards,

[AnandJalawadi]

Hi,

The mpls mtu on our outgoing interfaces is 1524,so its 20 bytes for tcp header,8 bytes for ICMP,4 bytes of mpls label ,so it accounts to 1524-32=1492 bytes of payload

thanks

Anand

[Jim.Mills]

You mean 20 bytes for IP header, not TCP.