--- title: Running IS-IS Over Unnumbered/LLA-only Interfaces «... description: A hands-on lab exercise exploring how IS-IS routing protocol behaves over unnumbered IPv4 and link-local-only IPv6 interfaces. IS-IS uses its own layer-3... canonical: https://daily.dev/posts/running-is-is-over-unnumbered-lla-only-interfaces-is-is-labs-txhp3oiv3 twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Running IS-IS Over Unnumbered/LLA-only Interfaces « IS-IS Labs | daily.dev og:description: A hands-on lab exercise exploring how IS-IS routing protocol behaves over unnumbered IPv4 and link-local-only IPv6 interfaces. IS-IS uses its own layer-3... og:url: https://daily.dev/posts/running-is-is-over-unnumbered-lla-only-interfaces-is-is-labs-txhp3oiv3 og:image: https://api.daily.dev/og/posts/tXhP3OIv3.png og:image:alt: Running IS-IS Over Unnumbered/LLA-only Interfaces « IS-IS Labs og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Running IS-IS Over Unnumbered/LLA-only Interfaces « IS-IS Labs **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 8 min read · 0 upvotes · 0 comments ## Summary A hands-on lab exercise exploring how IS-IS routing protocol behaves over unnumbered IPv4 and link-local-only IPv6 interfaces. IS-IS uses its own layer-3 protocol (not IP), so it can form adjacencies without numbered IP addresses. The lab walks through migrating a multi-router topology from numbered to unnumbered interfaces, examining how IS-IS hello messages advertise loopback addresses as next-hops, how ARP resolves MAC addresses for unnumbered IPv4, and how IPv6 LLA is used as next-hop in routing tables. Includes warnings about platform-specific limitations on multi-access (LAN) unnumbered links. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How does IS-IS determine next-hop addresses when interfaces are unnumbered or LLA-only? IS-IS hello messages advertise the neighbor's IPv4 and IPv6 interface addresses, and these are used as next-hops in the IP routing table — not the addresses in LSPs. On unnumbered IPv4 interfaces, routers advertise their loopback address as the interface address in hellos, so the loopback becomes the next-hop. For IPv6, the link-local address (LLA) from hellos is always used as the next-hop. _Network engineers migrating to unnumbered designs track IS-IS behavior across platforms on daily.dev._ ### Can IS-IS form adjacencies over unnumbered multi-access LAN interfaces? Not all IS-IS implementations support unnumbered multi-access (LAN) interfaces. Some devices can run IS-IS over unnumbered point-to-point IPv4 links but fail to form adjacencies over unnumbered LAN segments, or may form adjacencies but fail to install some IPv4 routes in the routing table. Point-to-point unnumbered support is more broadly available than LAN unnumbered support. _Keeping up with platform-specific IS-IS quirks like this is easier when routing protocol news surfaces on daily.dev._ ## Similar posts on daily.dev - [The Never-Ending IPv6 Loopback Prefix Saga « ipSpace.net blog](https://daily.dev/posts/the-never-ending-ipv6-loopback-prefix-saga-ipspace-net-blog-4nlhgh10l) · ipSpace.net Blog · 0 upvotes · 0 comments - [Configuring SRv6 on Junos \(Locator and IGP\) « ipSpace.net blog](https://daily.dev/posts/configuring-srv6-on-junos-locator-and-igp-ipspace-net-blog-bhavnkj81) · ipSpace.net Blog · 0 upvotes · 0 comments --- [View this post on daily.dev](https://daily.dev/posts/running-is-is-over-unnumbered-lla-only-interfaces-is-is-labs-txhp3oiv3) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Running IS-IS Over Unnumbered/LLA-only Interfaces « IS-IS Labs","url":"https://daily.dev/posts/running-is-is-over-unnumbered-lla-only-interfaces-is-is-labs-txhp3oiv3","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/running-is-is-over-unnumbered-lla-only-interfaces-is-is-labs-txhp3oiv3"},"datePublished":"2026-08-11T08:54:53.551Z","dateModified":"2026-08-11T09:14:52.013Z","description":"A hands-on lab exercise exploring how IS-IS routing protocol behaves over unnumbered IPv4 and link-local-only IPv6 interfaces. IS-IS uses its own layer-3...","image":"https://media.daily.dev/image/upload/s--ZrL_HSsR--/f_auto/v1722860399/public/Placeholder%2006","thumbnailUrl":"https://media.daily.dev/image/upload/s--ZrL_HSsR--/f_auto/v1722860399/public/Placeholder%2006","isAccessibleForFree":true,"articleSection":"ipSpace.net Blog","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"ipSpace.net Blog","logo":"https://media.daily.dev/image/upload/s--xS7E82MT--/f_auto,q_auto/v1786438303/logos/ipspace?_a=BAMAMicg0","url":"https://daily.dev/sources/ipspace"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/running-is-is-over-unnumbered-lla-only-interfaces-is-is-labs-txhp3oiv3","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"","timeRequired":"PT8M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"ipSpace.net Blog","item":"https://daily.dev/sources/ipspace"},{"@type":"ListItem","position":3,"name":"Running IS-IS Over Unnumbered/LLA-only Interfaces « IS-IS Labs"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/running-is-is-over-unnumbered-lla-only-interfaces-is-is-labs-txhp3oiv3#faq","mainEntity":[{"@type":"Question","name":"How does IS-IS determine next-hop addresses when interfaces are unnumbered or LLA-only?","acceptedAnswer":{"@type":"Answer","text":"IS-IS hello messages advertise the neighbor's IPv4 and IPv6 interface addresses, and these are used as next-hops in the IP routing table — not the addresses in LSPs. On unnumbered IPv4 interfaces, routers advertise their loopback address as the interface address in hellos, so the loopback becomes the next-hop. For IPv6, the link-local address (LLA) from hellos is always used as the next-hop. Network engineers migrating to unnumbered designs track IS-IS behavior across platforms on daily.dev."}},{"@type":"Question","name":"Can IS-IS form adjacencies over unnumbered multi-access LAN interfaces?","acceptedAnswer":{"@type":"Answer","text":"Not all IS-IS implementations support unnumbered multi-access (LAN) interfaces. Some devices can run IS-IS over unnumbered point-to-point IPv4 links but fail to form adjacencies over unnumbered LAN segments, or may form adjacencies but fail to install some IPv4 routes in the routing table. Point-to-point unnumbered support is more broadly available than LAN unnumbered support. Keeping up with platform-specific IS-IS quirks like this is easier when routing protocol news surfaces on daily.dev."}}]} ```