I was working with a PPP Cisco lab yesterday in the network lab at uni. Three routers in a serial link triangle, running OSPF. The lab instructions said to break PPP (for some reason, it was a “challenge” CCNA lab, so.. the breaking of the link and observing the behavior was already done in the previous lab, but oh well), so we did.
After doing that, and then restoring the encapsulation on the interface to PPP, we noticed that OSPF did not function properly anymore. It took quite some time to figure out that it was this that was happening. For some reason, the interface had gotten an “ip mtu 576”, therefore causing the weird behavior of OSPF, the neighbours to the troublesome router were stuck in EXSTART. I am still not sure why it had that.. we did of course not set it. I thought at first it might have been because to break PPP, we set the encapsulation to “x25 dce” on that particular interface, though, checking it in dynamips today, it didn’t set “ip mtu 576” on the interface I tried to set it on.
Anyway, as suggested by the linked article above, fixing the MTU issues, (in my case “no ip mtu 576” on the particular interface) fixed that.
Cisco IOS hints and tricks seems to be a nice site/blog btw. I was reading a few more articles on it, and found this one, stating something we also noticed. Namely that when using PPP CHAP authentication, the passwords has to be in clear text. (something that seems logical now, but caused some confusion for us..)