1. MTU
Amit Dhamija

2. MTU Calculation in IOS
In cisco IOS physical MTU consists of L2 headers.
Example :-
MTU 1500 on Ethernet links includes 14 bytes of L2 header and pass traffic upto 1514 bytes or
IP-MTU = Physical-MTU
In Cisco IOS when we ping and give the total payload size then it accommodates the (IP+ICMP Header)
If we ping 1500 bytes as payload in IOS it means payload is 1472 as it includes
MRU in IOS is – physical-MTU so if it receives packet higher then MTU it will drop it, However in egress it will fragment automatically.
IP-MTU command works only in egress direction

3. MTU Calculation in IOS-XR
In cisco IOS-XR physical MTU is excluding the L2 headers.
Example :-
MTU 1500 on Ethernet links excludes 14 bytes of L2 header and pass traffic upto 1486 bytes or
IP-MTU =Physical-MTU-14 Bytes ( Ethernet Header)
RP/0/RP0/CPU0:crs1.BLB#sh interfaces Te0/1/1/0 | in MTU
Fri May 23 15:45: IST
MTU 4484 bytes, BW Kbit (Max: Kbit)
RP/0/RP0/CPU0:crs1.BLB#sh ipv4 interface Te0/1/1/0 | in MTU
Fri May 23 15:45: IST
MTU is 4484 (4470 is available to IP)
In Cisco IOS-XR when we ping and give the total payload size then it accommodates the (IP+ICMP Header)

4. MTU Calculation in JUNOS
In JUNOS physical MTU is excluding the L2 headers.
Example :-
MTU 1500 on Ethernet links excludes 14 bytes of L2 header and pass traffic upto 1486 bytes or
IP-MTU =Physical-MTU-14 Bytes ( Ethernet Header)
show interfaces xe-0/0/0 | match MTU
Link-level type: Ethernet, MTU: 1600, LAN-PHY mode, Speed: 10Gbps,
Protocol inet, MTU:  Including one Vlan TAG
In JUNOS adds icmp and ip header to the payload size you specify. So 1500 becomes bytes of ip packet.
MRU in JUNOS is – physical-MTU so if it receives packet higher then MTU it will drop it, However in egress it will fragment automatically.

5. MTU Calculation in SR-OS
In SR-OS physical MTU is excluding the L2 headers.
Example :-
MTU 1500 on Ethernet links excludes 14 bytes of L2 header and pass traffic upto 1486 bytes or
IP-MTU =Physical-MTU-14 Bytes ( Ethernet Header)
*A:KJC_PE_00001# show port 5/1/2 | match MTU
Physical Link : Yes MTU : 9212
*A:KJC_PE_00001# show router interface "To_FSS_PE_00004_IPE-4" detail | match MTU
IP Oper MTU : ICMP Mask Reply : True
In SR-OS it adds icmp and ip header to the payload size you specify. So 1500 becomes bytes of ip packet.
Service MRU in SR-OS has no limit so if it receives packet higher then MTU it will pass it, However in egress it will fragment automatically and ip-mtu is similar to IOS/IOS-XR

6. Fragmentation in Egress – Router
If IP packet is longer than the MTU, the router breaks packet into smaller packets
Called IP fragments
Fragments are still IP packets
IP Packet 2 1
IP Packets
Fragmentation
MTU 3

7. Original packet may be fragmented multiple times along its route
Multiple Fragmentations
Original packet may be fragmented multiple times along its route
Source
Host
Internet
Process
Destination
Host
Internet
Process
Fragmentation

8. IP Defragmentation only occurs once
Internet layer process on destination host defragments, restoring the original packet
IP Defragmentation only occurs once
Destination
Host
Internet
Process
Defragmentation
Source

9. Fragmentation and IP Fields
More Fragments field (1 bit)
1 if more fragments
0 if not
Source host internet process sets to 0
If router fragments, sets More Fragments field in last fragment to 0
In all other fragments, sets to 1 1
Original IP Packet
Fragments

10. Identification Fields & Fragment Offset
IP packet has a 16-bit Identification field
Version
(4)
Hdr Len
(4)
TOS (8)
Total Length in bytes (16)
Indication (16 bits)
Flags (3)
Fragment Offset (13)
Time to Live (8)
Protocol (8)
Header Checksum (16)
Source IP Address
Destination IP Address
Options (if any)
PAD
Data Field

11. Identification Fields
Purpose
Allows receiving host’s internet layer process know what fragments belong to each original packet
Works even if an IP packet is fragmented several times 47
Original IP Packet
Fragments

12. Fragment Offset
Fragment offset field (13 bits) is used to reorder fragments with the same Identification field
Contains the data field’s starting point (in octets) from the start of the data field in the original IP packet
Value of offset is measured in 8 bytes. This is done because length of offset is only 13 bits long and cannot be represent a sequence of bytes greater then 8191.This force host/router that fragment datagrams to choose the size of each fragment so that first byte number is divisible by 8

13. This works even if fragments arrive out of order!
Fragment Offset Field
Receiving host’s internet layer process assembles fragments in order of increasing fragment offset field value
This works even if fragments arrive out of order!
Works even if fragmentation occurs multiple times
212
730
Fragment Offset Field

14. Fragmentation Rule
First fragment will be equal to ip-mtu – 20 bytes and then rest of fragments can be summed up.
Example below a link with ip-mtu 1300 with a payload of 5008 fragments into 4
1280 ( 0 – 1279) frag offset – 0 - 0
1280 ( 1280 – 2559) frag offset – 1280
1280 ( 2560 – 3839) frag offset – 2560
1168 ( 3840 – 1279) frag offset – 3480

18. Thanks