Category: NTP

The importance of having accurate time on distributed servers (and even personal workstations) has been recognized long time ago by the IT managers, but it hasn’t been applied consistently to the networking devices. In this article, I’ll describe the importance of time synchronization for networking devices, the basics of Network Time Protocol (NTP) that is commonly used to synchronize IP hosts and routers, how to use it on Cisco routers and IOS-based switches and how to implement it in a highly scalable way.

A few months ago Johannes Weber sent me a short email saying “hey, I plan to write a few NTP posts” and I replied “well, ping me when you have something ready”.

In the meantime he wrote a veritable NTP bible - a series of NTP-related blog posts covering everything from Why Should I Run My Own NTP Servers to authentication, security and monitoring - definitely a MUST READ if you care about knowing what time it is.

In his comment to the announcement of my NTP article, Joe said:

This is part of the problem with NTP. It's way more complicated then it needs to be. You shouldn't have to understand so much of it to use it on your routers. Take a look at openntpd. It's free and runs on bsd or linux.

I have to disagree with him on several counts:

If you're running NTP and SNTP on the same router, SNTP will never synchronize with a configured SNTP server, as the NTP process captures the reply packet before SNTP has the chance to react.

SNTP multicast/broadcast client mode works in combination with NTP

NTP process could be running even if your running configuration has no NTP-related commands. It starts automatically whenever you enter NTP-related configuration (ntp logging configuration command is enough) and is not stopped when the last NTP-related configuration command is removed. You have to reload the router to kill it.

I almost started writing an EEM applet that would detect and log the changes in router’s system time caused by NTP synchronizations, but then I’ve decided to check the IOS documentation first and found the ntp logging command.

A while ago a reader has asked me whether you could modify an IP access-list when the interface IP address changes. While that's definitely doable with Tcl and Embedded Event Manager, it's not a trivial task, so I've tried to understand why he would need such a functionality.

The answer was quite interesting: he's running NTP on his firewall router and thus needs to accept incoming NTP responses from an external NTP server. While that could be easily achieved with the following configuration (only the relevant bits-and-pieces are shown), he didn't want to make the access-list too generic (allowing NTP from the external server to any IP address).

ip inspect name DEFAULT100 tcp
ip inspect name DEFAULT100 udp
!
interface Dialer0
 description $FW_OUTSIDE$
 ip access-group 102 in
 ip inspect DEFAULT100 out
!
access-list 102 remark #### Dialer0 incoming ####
access-list 102 remark #### non-relevant lines deleted
access-list 102 permit udp host 1.2.3.4 eq ntp any eq ntp

This problem nicely illustrates a broader issues: the router does not inspect it's own traffic and thus does not prepare conduits for the return packets; you have to specify all the return traffic you're expecting in the incoming access list. This drawback has been fixed in IOS release 12.3(14)T with the introduction of the Inspection of Router-Generated Traffic feature. In our scenario you only need to change the inspect rules:

ip inspect name DEFAULT100 tcp router-traffic
ip inspect name DEFAULT100 udp router-traffic

... and the router synchronizes to an external NTP server:

sp#show ip inspect sessions
Established Sessions
 Session 474032B4 (192.168.1.3:123)=>(10.0.0.1:123) udp SIS_OPEN
sp#
01:04:34: %NTP-5-PEERSYNC: NTP synced to peer 10.0.0.1
01:04:34: %NTP-6-PEERREACH: Peer 10.0.0.1 is reachable

Note: This article is part of You've asked for it series.