We have 3 T1 configured in a multilink interface between a Cisco and a Juniper router. Everything works great except when 2 of the 3 links are down (i.e. only one T1 UP). When there is only one T1 in the multilink bundle working the multilink fails. I have been told to use:
ppp multilink fragment 30
command on the Cisco to correct this behavior. I also know that the fragmentation algorithm changes when this command is applied on the Cisco. Does anyone no why the link fails without this command? Does the Cisco router stop using the MLPPP header when there is only one link up in the bundle or something? Also I am not using any other commands to fine any sort of minimum links on the Cisco and the bundle stays up when it is Cisco to Cisco.
Last question, I believe the default fragmentation algorithm on the Cisco is based on the number of links in the bundle and that this deafult fragmentation only applies if you have all links of the same speed and have NOT applied interleaving or the fragmentation delay command. Does anyone know the details of this algorithm? Does it fragment all packets into n fragments where n=number of active links? I do not have any voice or delay sensitive traffic and the multilink is made up of only T-1 circuits so I would prefer to not fragment at all, but I need the bundle to stay up even if only a single T-1 is active.
