In this short post i want to try and shed some light on a couple of ISIS timers that had me confused at first. I think i got them down now, but please let me know if i have misunderstood them.
The timers in question is “isis retransmit-interval ” and “isis retransmit-throttle-interval “.
Both of these commands are only relevant on point-to-point links as no concept of a DIS is present here.
Lets start out with the simple topology of two routers in figure 1.
When R1 has some LSPs to send to R2 it sends them with a default interval of 33 ms (which is the “isis lsp-interval“).
This is shown in figure 2.
R1 expects an acknowledgement within the “isis retransmission-interval“, which is 5 seconds by default. The retransmission-interval specifies the interval between retransmissions of the same LSP.
In our example the 5 seconds pass and so R1 must retransmit the LSP’s to R2. R1 is now in its retransmission-window.
Now here’s where the second timer comes into play. Instead of sending the LSP’s which needs to be retransmitted with a 33ms delay between each of them, the “retransmit-throttle-interval” goes into effect and increase the time between the LSP’s from 33ms to 100ms.
This is shown in figure 3.
All of this is done in order to help R2 from staying over burdened. If the same LSP is not acknowledged within 5 seconds, the same LSP is retransmitted again per the retransmission-interval timer.
Configuration wise, it is very simple to configure. On IOS:
Current configuration : 179 bytes
ip address 10.1.2.1 255.255.255.0
ip router isis 1
isis retransmit-throttle-interval 100
isis retransmit-interval 10
And on IOS-XR:
Thu Jun 13 22:49:41.657 UTC
!! IOS XR Configuration 3.9.1
router isis 1
address-family ipv4 unicast
I hope that helped clear up some confusion on these two timers. And again, if i have misunderstood anything, please let me know. Thanks!
Cisco Documentation: http://www.cisco.com/en/US/docs/ios-xml/ios/iproute_isis/command/irs-a1.html#wp1754145253
The complete IS-IS Routing Protocol: Amazon.co.uk