Ethan Banks On productivity.

OECG – Chapter 5


We’re getting ready to wrap up OECG chapter 5 with a discussion of NTP, or network time protocol. That which keeps your router’s date and time synchronized with some master time server. Having everyone on your network keep time synched to a common source is a critical function to have, for logging & event correlation, if nothing else.

  • NTP – network time protocol, version 3, RFC 1305. Cisco devices can be NTP clients (getting their time from some other server) or servers (capable of serving time to clients). NTP servers have stratums, with a stratum 1 implying that he’s the top of the NTP heap – he’s getting his time from an atomic clock, GPS, etc.
  • Routers acting as clients can obtain their time information in several ways:
    • client mode, where the router talks to a specific peer or peers, and queries those peers for time information.
    • symmetric active mode, where the router will synchronize with another time server, and that time server will synchronize with the router. Stratum 1 or 2 level time servers on the Internet tend to cluster in this way.
    • broadcast mode, where the router listens for a time server broadcasting the time on the local subnet.
  • A router can serve the time as well, as an NTP server or broadcast server.
  • In interface mode “ntp broadcast” will cause the router to SEND NTP broadcasts out that interface.
  • “ntp authenticate” with “ntp authentication-key” and “ntp trusted-key” are IOS commands to configure NTP authentication, where routers will not synchronize time unless the credentials match.
  • “ntp master 4” would make the router an NTP server with a stratum of 4, implying that he’s 3 NTP hops away from a stratum 1 server. (IOW, he’s getting his time from a stratum 3 NTP server, who’s getting his time from a stratum 2 NTP server, who’s getting his time from a stratum 1.)
  • “show ntp associations” will show you who you’re synched up with.
  • “show ntp status” gives you even more ntp information.
  • You should not execute the “ntp clock-period” command yourself. The router will do this automatically when you do a “copy run start”. If you put it in, you’ll disturb the NTP, since the router computes the correct value to place.
  • “ntp server” sets the router to be a static client – IOW, this tells the router what NTP server to use, not to BE an NTP server.
  • In interface mode, “ntp broadcast client” tells the router to listen for NTP broadcasts on that interface.
  • “ntp peer” makes the router an NTP client in symmetric active mode.
  • More NTP info here.
By Ethan Banks
Ethan Banks On productivity.

You probably know Ethan Banks because he writes & podcasts about IT. For example, he co-authored "Computer Networks Problems & Solutions" with Russ White.

This site is Ethan on productivity--not tech so much.

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