--- title: Dual-Stack (IPv4+IPv6) IS-IS Routing « IS-IS Labs description: A hands-on lab exercise exploring dual-stack IPv4+IPv6 IS-IS routing, covering the pitfalls of enabling IPv6 in an existing IS-IS domain. The lab demonstrates... canonical: https://daily.dev/posts/dual-stack-ipv4-ipv6-is-is-routing-is-is-labs-qb16mr0vl twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Dual-Stack (IPv4+IPv6) IS-IS Routing « IS-IS Labs | daily.dev og:description: A hands-on lab exercise exploring dual-stack IPv4+IPv6 IS-IS routing, covering the pitfalls of enabling IPv6 in an existing IS-IS domain. The lab demonstrates... og:url: https://daily.dev/posts/dual-stack-ipv4-ipv6-is-is-routing-is-is-labs-qb16mr0vl og:image: https://api.daily.dev/og/posts/Qb16mR0VL.png og:image:alt: Dual-Stack (IPv4+IPv6) IS-IS Routing « 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. # Dual-Stack (IPv4+IPv6) IS-IS Routing « IS-IS Labs **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 13 min read · 0 upvotes · 0 comments ## Summary A hands-on lab exercise exploring dual-stack IPv4+IPv6 IS-IS routing, covering the pitfalls of enabling IPv6 in an existing IS-IS domain. The lab demonstrates how a single IPv4-only link (R2-R3) can cause IPv6 black holes when routers outside that link incorrectly assume IPv6 reachability through it. The solution is multi-topology IS-IS (RFC 5120), which maintains separate topology databases per address family, allowing the SPF algorithm to correctly exclude IPv4-only links from IPv6 path calculations. Differences in strictness between FRRouting and Arista EOS implementations are highlighted, along with the LSP changes that multi-topology introduces. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does IS-IS create IPv6 black holes when some links are IPv4-only in a dual-stack network? Without multi-topology IS-IS, all routers share a single topology database. A router like R1 sees R3's IPv6 loopback advertised in the LSP and installs a route via R2, unaware that the R2-R3 link carries only IPv4. IS-IS hello packets advertise per-interface address families, but that information is local and never propagated network-wide, so remote routers cannot know which links are IPv6-capable. _Network engineers troubleshooting IS-IS dual-stack black holes find the relevant RFCs and lab configs on daily.dev._ ### How do I enable multi-topology IS-IS for IPv6 to prevent routing black holes? Enable multi-topology IS-IS with a command like 'multi-topology' inside the IPv6 address family, or 'topology ipv6-unicast' at the router level, on all routers. This causes each router to advertise per-topology link reachability in its LSP — Extended Reachability TLVs for IPv4 links and MT Reachability TLVs for IPv6 links — letting SPF compute separate shortest-path trees per address family. _Engineers rolling out IS-IS multi-topology across a production network track implementation differences between vendors on daily.dev._ ### Does Arista EOS form IS-IS adjacencies over IPv4-only links when IPv6 is configured in the IS-IS process? No. Arista EOS is strict and will not form an IS-IS adjacency over a link that does not carry all address families configured in the IS-IS routing process. If IPv6 is enabled in IS-IS but a link has no IPv6 address, Arista EOS drops the adjacency on that link. FRRouting takes a more relaxed approach and maintains the adjacency while simply excluding IPv6 from that neighbor's capabilities. _Vendor-specific IS-IS behavior like this surfaces regularly for network engineers following routing protocol news 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 - [IPv6 in 2025 – Transitioning to IPv6](https://daily.dev/posts/ipv6-in-2025-transitioning-to-ipv6-x1lwgorkb) · Cisco · 0 upvotes · 0 comments - [Multivendor SR-MPLS « ipSpace.net blog](https://daily.dev/posts/multivendor-sr-mpls-ipspace-net-blog-qekokwduk) · ipSpace.net Blog · 0 upvotes · 0 comments - [BGP-Free Network Core with SRv6 « ipSpace.net blog](https://daily.dev/posts/bgp-free-network-core-with-srv6-ipspace-net-blog-9bbn88wub) · ipSpace.net Blog · 0 upvotes · 0 comments --- [View this post on daily.dev](https://daily.dev/posts/dual-stack-ipv4-ipv6-is-is-routing-is-is-labs-qb16mr0vl) ```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":"Dual-Stack (IPv4+IPv6) IS-IS Routing « IS-IS Labs","url":"https://daily.dev/posts/dual-stack-ipv4-ipv6-is-is-routing-is-is-labs-qb16mr0vl","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/dual-stack-ipv4-ipv6-is-is-routing-is-is-labs-qb16mr0vl"},"datePublished":"2026-08-11T08:51:53.110Z","dateModified":"2026-08-11T08:59:28.265Z","description":"A hands-on lab exercise exploring dual-stack IPv4+IPv6 IS-IS routing, covering the pitfalls of enabling IPv6 in an existing IS-IS domain. The lab demonstrates...","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/dual-stack-ipv4-ipv6-is-is-routing-is-is-labs-qb16mr0vl","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"","timeRequired":"PT13M"} {"@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":"Dual-Stack (IPv4+IPv6) IS-IS Routing « IS-IS Labs"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/dual-stack-ipv4-ipv6-is-is-routing-is-is-labs-qb16mr0vl#faq","mainEntity":[{"@type":"Question","name":"Why does IS-IS create IPv6 black holes when some links are IPv4-only in a dual-stack network?","acceptedAnswer":{"@type":"Answer","text":"Without multi-topology IS-IS, all routers share a single topology database. A router like R1 sees R3's IPv6 loopback advertised in the LSP and installs a route via R2, unaware that the R2-R3 link carries only IPv4. IS-IS hello packets advertise per-interface address families, but that information is local and never propagated network-wide, so remote routers cannot know which links are IPv6-capable. Network engineers troubleshooting IS-IS dual-stack black holes find the relevant RFCs and lab configs on daily.dev."}},{"@type":"Question","name":"How do I enable multi-topology IS-IS for IPv6 to prevent routing black holes?","acceptedAnswer":{"@type":"Answer","text":"Enable multi-topology IS-IS with a command like 'multi-topology' inside the IPv6 address family, or 'topology ipv6-unicast' at the router level, on all routers. This causes each router to advertise per-topology link reachability in its LSP — Extended Reachability TLVs for IPv4 links and MT Reachability TLVs for IPv6 links — letting SPF compute separate shortest-path trees per address family. Engineers rolling out IS-IS multi-topology across a production network track implementation differences between vendors on daily.dev."}},{"@type":"Question","name":"Does Arista EOS form IS-IS adjacencies over IPv4-only links when IPv6 is configured in the IS-IS process?","acceptedAnswer":{"@type":"Answer","text":"No. Arista EOS is strict and will not form an IS-IS adjacency over a link that does not carry all address families configured in the IS-IS routing process. If IPv6 is enabled in IS-IS but a link has no IPv6 address, Arista EOS drops the adjacency on that link. FRRouting takes a more relaxed approach and maintains the adjacency while simply excluding IPv6 from that neighbor's capabilities. Vendor-specific IS-IS behavior like this surfaces regularly for network engineers following routing protocol news on daily.dev."}}]} ```