Discussion on MPLS MTU problem

Source: Internet
Author: User
Tags bit set header

MPLS default MTU is 1500, do not make any changes, I ping full package does not fragment, why can pass! It adds MPLS information altogether is 1504 bytes, how can transmit?

Here is my reply:

It has been verified, what you need to do is to study your network topology again, if you have a ready-made environment, put your detailed topology map, as well as the configuration of each device sent to me, I can help you study it, you said the situation should not be established. My email address is: hny2000@gmail.com, or we'll discuss it on this blog.

Here you need to be aware of the ping on what equipment. The following are the differences between the MTU and the packets that are ping on the PC and on the router. Be sure to pay attention to these two concepts.

1. PC Ping X.x.x.x-l 1000 This 1000 refers to the data, so this ping packet in the network card when the size of 1000+20 (IP header) +8 (ICMP header) +14 (ether) = 1042

2. Network Device ping X.x.x.x-l 1000 This 1000 refers to L3 packets, including ICMP, except L2 all packet, add 14=1514 when out of the interface

Here I have a topology map that has been tested.

My environment is like this:

R1 ( R2 ( R3 ( R5 ( R6 (VRF R7

This column more highlights: http://www.bianceng.cn/Network/basis/

First of all, in r1ping to R7, you can only use the 1492来 ping, if you use 1493来 ping not fragmented words will lose the package.


Protocol [IP]:

Target IP address:

Repeat count [5]:

Datagram size [100]: 1492

Timeout in seconds [2]:

Extended commands [n]: Y

Source address or interface:

Type of service [0]:

Set DF bit in IP header? [No]: Yes

Validate reply data? [No]:

Data pattern [0xABCD]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Sweep range of sizes [n]:

Type escape sequence to abort.

Sending 5, 1492-byte ICMP Echos to, timeout is 2 seconds:

Packet sent with a source address of

Packet sent with the DF bit set


Success rate is percent (5/5), round-trip Min/avg/max = 192/256/328 ms

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