Defining service availability using the famous X nines (and all the hacks like “planned downtime doesn’t count”) is pretty useless in a highly distributed system where the only thing that really matters is the user experience, not ping response times. One should ask what precisely should we be measuring, and how could we make sure we can act on the measurements
This is a guest blog post by Philippe Jounin, Senior Network Architect at Orange Business Services.
You could use track objects in Cisco IOS to track route reachability or metric, the status of an interface, or IP SLA compliance for a long time. Initially you could use them to implement reliable static routing (or even shut down a BGP session) or trigger EEM scripts. With a bit more work (and a few more EEM scripts) you could use object tracking to create time-dependent static routes.
Cisco IOS 15 has introduced Enhanced Object Tracking that allows first-hop router protocols like VRRP or HSRP to use tracking state to modify their behavior.
Unveiling of the Cisco IOS release 15.1(1)T was the extreme opposite of the CRS-3 and Catalyst 3750-X splashes; the next release of one of the foundations of Cisco’s core business deserved a modest two-paragraph mention in the What's New in Cisco Product Documentation page.
If you’re a voice guru, you’ll probably enjoy the list of 20+ voice-related new features, including the all-important Enhanced Music on Hold. For the rest of us, here’s what I found particularly interesting:
Every now and then, a really interesting question appears on the cisco-nsp mailing list. A while ago I’ve seen this one:
I've heard that Cisco devices handle ICMP at a low priority. I found one post describing it handled in process-switching and not fast-switching. Does anyone have an article that explains that process and is it configurable?
Most packets sent to the router are handled in process switching (the packet is queued in the input queue of one of the IOS processes), the obvious exceptions being GRE and IPSec packets (unless they're fragmented).
Packets sent to the router can also be rate-limited with a control plane policy.
The IOS processes perform their job between interrupts (packets being CEF- or fast switched). A reply to an ICMP packet is therefore a lower-priority task than regular packet forwarding.
Jeremy Stretch was kind enough to publish his IP SLA monitoring article in the CT3 wiki, at the same time changing the command syntax from the ip rtr command set to the newer ip sla command set. Thanks, Stretch!