Routing

Can anyone explain the ISIS command "lsp-interval"? The Juniper documentation states "By default, the routing device sends one link-state PDU packet out an interface every 100 milliseconds." but ISIS sends hello packets every 9 seconds and CSNPs are also periodically sent to ensure the LSP database is up to date. Why are the routers sending an LSP every 100ms? And what information is in this particular LSP?



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PAUL HERBERT
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Greetings, it seems like you'd appreciate a better understanding of IS-IS.
David Roy has an excellent guide on IS-IS but the site seems to be down:  http://junosandme.over-blog.com/article-inetzero-blog-isis-training-and-configuration-122458793.html
This one is also pretty good, despite its age:  https://www.scribd.com/document/71500902/Isis-Design-Guide

The short answer is that hellos are just to keep the adjacency alive, they don't contain any routing information.
IS-IS Link State PDUs are messages containing reachability information.
The 100ms interval was originally designed to prevent overloading adjacent routers with updates 30+ years ago.   These days 50ms is a commonly accepted value.

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BLAKE WILLIS
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Original Message:
Sent: 02-04-2022 05:16
From: PAUL HERBERT
Subject: ISIS LSP interval

Can anyone explain the ISIS command "lsp-interval"? The Juniper documentation states "By default, the routing device sends one link-state PDU packet out an interface every 100 milliseconds." but ISIS sends hello packets every 9 seconds and CSNPs are also periodically sent to ensure the LSP database is up to date. Why are the routers sending an LSP every 100ms? And what information is in this particular LSP?



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PAUL HERBERT
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Hi Blake,

Thank you for your response. 

My understanding is that LSPs are sent to routers when there changes. CSNPs are periodically sent to allow routers to compare this summary to their own. Should there be missing or outdated LSP information, a PSNP can be sent requesting the LSP. It therefore surprises me that a router sends a LSP every 100ms as it seems to make all these efficiencies rather pointless. 

Thanks

------------------------------
PAUL HERBERT
------------------------------

Original Message:
Sent: 02-04-2022 09:09
From: BLAKE WILLIS
Subject: ISIS LSP interval

Greetings, it seems like you'd appreciate a better understanding of IS-IS.
David Roy has an excellent guide on IS-IS but the site seems to be down:  http://junosandme.over-blog.com/article-inetzero-blog-isis-training-and-configuration-122458793.html
This one is also pretty good, despite its age:  https://www.scribd.com/document/71500902/Isis-Design-Guide

The short answer is that hellos are just to keep the adjacency alive, they don't contain any routing information.
IS-IS Link State PDUs are messages containing reachability information.
The 100ms interval was originally designed to prevent overloading adjacent routers with updates 30+ years ago.   These days 50ms is a commonly accepted value.

------------------------------
BLAKE WILLIS

Original Message:
Sent: 02-04-2022 05:16
From: PAUL HERBERT
Subject: ISIS LSP interval

Can anyone explain the ISIS command "lsp-interval"? The Juniper documentation states "By default, the routing device sends one link-state PDU packet out an interface every 100 milliseconds." but ISIS sends hello packets every 9 seconds and CSNPs are also periodically sent to ensure the LSP database is up to date. Why are the routers sending an LSP every 100ms? And what information is in this particular LSP?



------------------------------
PAUL HERBERT
------------------------------