Configuring Ip Path Mtu Discovery; Enabling Pmtu Discovery - Juniper IGP - CONFIGURATION GUIDE V11.1.X Configuration Manual

JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide
By default, the router uses an MSS value of 536 bytes and the advertised MSS is
derived from the MTU of the transmitting interface. However, you can use the tcp
mss command to set the MSS for TCP advertisements.
tcp mss
NOTE: The MSS value is equal to the MTU value minus the IP and TCP headers, so
the MSS value is generally 40 bytes less than the MTU.

Configuring IP Path MTU Discovery

IP hosts transmit large amounts of data to other hosts using a series of IP datagrams.
To best use resources, increase performance, and avoid difficult reassembly, hosts
try to send datagrams that are as large as possible without requiring fragmentation
anywhere along the path from the source to the destination. This datagram size is
referred to as the path MTU (PMTU), and it is equal to the smallest MTU for each hop
in the path.
Path MTU discovery is the process of discovering the PMTU value and using that
value when transmitting TCP packets in datagrams.

Enabling PMTU Discovery

Use the tcp path-mtu-discovery command to enable PMTU discovery on the active
virtual router.
tcp path-mtu-discovery
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IP Routing
Use to specify the MSS value for TCP to advertise.
Use the vrfName variable to specify a VRF to which you want to assign the TCP
MSS value.
Example
host1(config-if)#tcp mss 1000
Use the no version to remove the MSS value so that the router uses the advertised
MSS derived from the MTU of the output interface.
See tcp mss
Use to enable and configure path MTU discovery on the virtual router.
Issue the command without any keywords to enable path MTU discovery.
Issue the age-timer keyword to set the time (minutes) that TCP waits before
attempting to increase the path MTU after receiving an ICMP Too Big message
or after previously increasing the PMTU successfully (minutes2). The range of
these two timers is 1–30 minutes. The timer defaults to 10 minutes.
Issue the age-timer infinite keyword to disable PMTU aging functions.
Example 1 Enables path MTU discovery