How to Configure MTU and TCP MSS on Cisco IOS

Maximum transmission unit (MTU) defines the largest size of packets that an interface can transmit without the need to fragment.
IP packets larger than the MTU must go through IP fragmentation procedures.

ip tcp mss
To enable a maximum segment size (MSS) for TCP connections originating or terminating on a router, use the ip tcp mss command in global configuration mode.
Device(config)#ip tcp mss (bytes)
Device#show ip tcp mss

ip tcp adjust-mss
Adjusts the MSS value of TCPSYN packets going through a router.
NOTE: The max-segment-size argument is the maximum segment size, in bytes. The range is from 500 to 1460.
Device(config)#interface GigabitEthernet 1/0/0
Device(config-if)#ip tcp adjust-mss 1452


How to Configure MTU

Configuring the System MTU
Device# configure terminal
Device(config)# system mtu 1900
Device# show system mtu

Configuring the Ethernet MTU
Device# configure terminal
Device# int g0/0
Device(config-if)mtu (bytes)

Configuring Protocol-Specific MTU
Device# configure terminal
Device(config)# interface gigabitethernet0/0
Device(config-if)# ip mtu 68

TASKS:
1. Check default Interface/Ethernet MTU.
NOTE: It’s 1500 bytes by default for Ethernet.
show interface gig0/0 | inc MTU
2. Check default IPv4 MTU.
NOTE: It’s also 1500 bytes by default. (1460 bytes for TCP MSS + 20 bytes for TCP header + 20 bytes for IP header)
show ip interface gig0/0 | inc MTU
3. Check TCP MSS value.
show ip tcp mss
4. Reduce IP MTU on R1:gigabitethernet0/0 to 1400 and perform an extended ping from PC1 with dont fragment set to 1.
R1:
conf t
int gig0/0
ip mtu 1400
PC1:
ping 10.10.1.254 size 1401 df-bit
5. From R1, lower MSS on the router itself.
conf t
R1(config)#ip tcp mss 1450
R1(config)#do show ip tcp mss
6. From PC1 and R1, enable debug ip tcp transactions
7. From PC1, telnet 10.10.1.254 and check MSS value in the debug output.

References:
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