09-02-2021 12:15 PM
Hi All,
I am a bit confused whenever I try to understand the loop prevention mechanism used by IS-IS or you can also broadly say Link-State Routing Protocols. Can anyone help me in understanding this?
Thanks in advance.
09-02-2021 01:19 PM
A link-state protocol, like OSPF ad IS-IS, create a graph of their (area) topology, and using Dijkstra's algorithm (https://en.wikipedia.org/wiki/Dijkstra%27s_algorithm), create a loop free "tree" (with computing router as root).
09-02-2021 02:06 PM
Hello @network_geek ,
just to add to what has been already noted by Joseph
Link state routing protocols use the same SPF algorithm on the SAME set of data the link state database within an area.
As a result of this the SPF calculations are loop free.
Most of the efforts in link state is in flooding data structures called LSP in IS-IS terms within a scope.
The LSPs and the LSPs built by DIS in each LAN segment ( the so called pseudo node) allows to actually build a topology map accurate that tells what routers are connected to a LAN segment also to devices that are not connected to it.
The use of the topology map is the great advantage of link state DB.
when a change occurs a timer is started and and a new SPF is not executed timmediately but only after that timer expires this is to give all nodes the time to receive and process the new data and eventually receive and process other new data structures.
if additonal changes happen the new SPF calculation may be delayed more in an attempt to avoid unnecessary cycles.
To be more accurate : link state is loop free in almost all cases but there are some temporary corner cases where a loop can form.
Hope to help
Giuseppe
10-24-2023 11:18 PM
Thank you Giuseppe for your information. Could you please tell me more about routing loops in case of IS-IS routing?
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