isis retransmit-interval Vs. isis retransmit-throttle-interval

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 <seconds>” and “isis retransmit-throttle-interval <mseconds>“.

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.

Figure 1

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.

Figure 2

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.

Figure 3

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
interface FastEthernet1/0
ip address
ip router isis 1
speed auto
duplex auto
isis retransmit-throttle-interval 100
isis retransmit-interval 10

And on IOS-XR:

RP/0/0/CPU0:XR1(config-isis-if)#show config
Thu Jun 13 22:49:41.657 UTC
Building configuration...
!! IOS XR Configuration 3.9.1
router isis 1
interface POS0/6/0/0
retransmit-interval 10
retransmit-throttle-interval 100
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:

The complete IS-IS Routing Protocol:

  1. Great info and easy to understand!
    Now go on with spf-interval, prc-interval, lsp-gen-interval and csnp-interval 😉

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