Showing results for 
Search instead for 
Did you mean: 
Walkthrough Wednesdays

VCS X7 Software supports RFC 4821 for Packetization Layer Path MTU Discovery


With the release of X7 code for the Telepresence Video Communications Server (VCS), RFC 4821 is supported.  When enabled, this allows the VCS to be able to respond to  ICMP type 3, code 4 (Destination unreachable, fragmentation needed, DF  flag is set) messages.  The VCS will then readjust the frame size of the packet and retransmit.  Below is a brief description on how to enable and disable this feature on the VCS:

The VCS X7 software now supports RFC4821:

IP RFC4821 Mode: <Auto/Enabled/Disabled>

Determines when RFC4821 Packetization Layer Path MTU Discovery is used by the VCS network interface.

Enabled: Packetization layer MTU probing is always performed.

Auto: Disabled by default, enabled when an ICMP black hole is detected.

Disabled: Packetization layer MTU probing is not performed.

Default: Disabled

Example: xConfiguration IP RFC4821 Mode: Enabled

This feature is useful if you have a situation where fragmentation is occuring across a link or there is a potential for consistent congestion on the link.

For additional information on RFC 4821, please refer to the below link to the RFC:

Andrew Bell (Webex PM)
Cisco Employee

Note that in X8 this requires a reboot to take affect.

X7 was immediate