{"id":3416,"date":"2026-10-27T12:24:07","date_gmt":"2026-10-27T19:24:07","guid":{"rendered":"https:\/\/cyber.mag212.com\/learn\/isis-cheat-sheet\/"},"modified":"2026-10-27T12:24:07","modified_gmt":"2026-10-27T19:24:07","slug":"isis-cheat-sheet","status":"publish","type":"page","link":"https:\/\/cyber.mag212.com\/fr\/learn\/isis-cheat-sheet\/","title":{"rendered":"Aide-m\u00e9moire IS-IS"},"content":{"rendered":"<style>.m212is{--bg:#07111c;--card:#10283a;--green:#46db82;--red:#f25c5c;--cyan:#3cc8e8;--text:#e7f2fa;background:var(--bg);color:var(--text);font:16px\/1.65 system-ui,Arial,sans-serif;border:1px solid #1f3b4d;border-radius:14px;padding:26px 24px}\n.m212is *{box-sizing:border-box}\n.m212is .is-hero{padding:36px 30px;border:1px solid #2e5269;border-radius:12px;background:linear-gradient(115deg,#102638,#07111c)}\n.m212is .is-title{font-size:clamp(30px,5vw,46px);line-height:1.15;margin:14px 0;color:#fff;border:0;padding:0}\n.m212is h2{color:var(--green);border-bottom:1px solid #27445b;padding-bottom:8px;margin:36px 0 12px;font-size:26px}\n.m212is h3{color:var(--cyan);margin:22px 0 8px}\n.m212is p{margin:0 0 12px}\n.m212is .eyebrow{letter-spacing:.16em;font-weight:800;color:var(--cyan)}\n.m212is .brand{font-size:25px;font-weight:900;letter-spacing:2px}.m212is .brand span{color:var(--green)}\n.m212is .tag{display:inline-block;border:1px solid var(--green);border-radius:6px;padding:4px 10px;color:var(--green);font-size:12px;margin-bottom:10px}\n.m212is .note{background:#143040;border-left:5px solid var(--green);padding:16px 22px;margin:20px 0}\n.m212is .warn{border-color:var(--red);background:#30202c}\n.m212is .card{background:var(--card);border:1px solid #31516b;border-radius:11px;padding:18px;margin:18px 0}\n.m212is pre{white-space:pre-wrap;overflow-wrap:anywhere;background:#030b12;border:1px solid #2b5260;border-left:4px solid var(--cyan);padding:18px;border-radius:6px;color:#e2f8ff;font:13px\/1.6 ui-monospace,SFMono-Regular,Consolas,monospace;margin:8px 0 14px}\n.m212is code{color:#9bedb7;font-family:ui-monospace,Consolas,monospace}\n.m212is table{width:100%;border-collapse:collapse;margin:20px 0}\n.m212is td,.m212is th{padding:10px;border:1px solid #355166;text-align:left;vertical-align:top;color:var(--text)}\n.m212is th{background:#17384c;color:#46db82}\n.m212is figure{margin:20px 0}.m212is figure svg{width:100%;height:auto;display:block;border-radius:9px;border:1px solid #355166}\n.m212is figcaption{font-size:13px;color:#afd2e4;margin-top:6px}\n.m212is .pill{font-weight:700;color:#46db82}\n.m212is a{color:#72d8ff}\n.m212is .is-foot{border-top:1px solid #345;margin-top:42px;padding:20px 0 0;color:#a2c4d6;font-size:13px}\n.m212is .is-dl{display:inline-block;margin-top:16px;background:#46db82;color:#04130c;font-weight:800;text-decoration:none;border-radius:8px;padding:11px 18px}\n@media(max-width:620px){.m212is{padding:16px 12px}.m212is .is-hero{padding:24px 18px}.m212is table{font-size:13px}.m212is td,.m212is th{padding:7px}}<\/style>\n<div class=\"m212is\" id=\"m212-isis\"><div class=\"is-hero\"><div class=\"brand\"><span>MAG212<\/span> CYBER<\/div><div class=\"eyebrow\">NETWORK ENGINEERING \/ ROUTING PROTOCOL FIELD GUIDE<\/div><h2 class=\"is-title\">IS-IS Ultimate Cheat Sheet<\/h2><p style=\"font-size:20px\">Configuration \u2022 Commands \u2022 Multi-Area Design \u2022 Troubleshooting \u2022 Security<\/p><div class=\"tag\">ORIGINAL LAB &amp; DIAGRAMS \u00b7 2026 EDITION<\/div><p>Published by MAG212 CYBER \u00b7 Technical reference for administrators, CCNP learners and network engineers<\/p><a class=\"is-dl\" href=\"https:\/\/cyber.mag212.com\/wp-content\/uploads\/2026\/10\/MAG212_CYBER_ISIS_Ultimate_Cheat_Sheet.pdf\" target=\"_blank\" rel=\"noopener\">\u2b07 T\u00e9l\u00e9charger l&#039;\u00e9dition PDF<\/a><\/div><div class=\"is-sec\"><h2>01 \/ What is IS-IS?<\/h2><p>Intermediate System to Intermediate System (IS-IS) is an interior gateway link-state routing protocol. Routers exchange link-state PDUs (LSPs), synchronize their link-state databases (LSDBs) and run shortest-path-first (SPF) calculations. Integrated IS-IS carries IPv4 information in TLVs; extensions allow IPv6 as well.<\/p><table><tr><th>Quick fact<\/th><th>Explanation<\/th><\/tr><tr><td>Algorithm<\/td><td>Dijkstra SPF<\/td><\/tr><tr><td>Transport<\/td><td>Runs directly over Layer 2 (OSI CLNS), not over IP \u2014 unlike OSPF (IP protocol 89) or BGP (TCP port 179)<\/td><\/tr><tr><td>Administrative distance<\/td><td>Cisco IOS default: 115 for IS-IS routes<\/td><\/tr><tr><td>Levels<\/td><td>L1 (inside an area); L2 (between areas); L1\/L2 router participates in both<\/td><\/tr><tr><td>Identifiers<\/td><td>NET includes area ID, 6-byte system ID, and selector 00<\/td><\/tr><tr><td>M\u00e9trique<\/td><td>Modern networks generally use wide metrics; interface cost can be set manually<\/td><\/tr><\/table><div class=\"note\">Important: IS-IS does not use IP multicast 224.0.0.5\/6 like OSPF. It uses link-local multicast MAC addresses on Ethernet and forms adjacency at Layer 2.<\/div><\/div><div class=\"is-sec\"><h2>02 \/ L1, L2, and area architecture<\/h2><table><tr><th>R\u00f4le<\/th><th>Learning scope<\/th><th>Utilisation typique<\/th><\/tr><tr><td>Level 1<\/td><td>Intra-area LSDB and routes<\/td><td>Access\/area-internal forwarding<\/td><\/tr><tr><td>Level 2<\/td><td>Inter-area backbone topology<\/td><td>Connecting areas<\/td><\/tr><tr><td>Level 1-2<\/td><td>Maintains a separate L1 and L2 database<\/td><td>Area border connectivity<\/td><\/tr><\/table><p>Level 1 routers must use matching area addresses to form an L1 adjacency. Level 2 peers can have different area addresses. A Level 1 router typically depends on an attached L1\/L2 router for routes beyond its area.<\/p><\/div><div class=\"is-sec\"><h2>03 \/ Original three-area enterprise lab<\/h2><figure><svg xmlns=\"http:\/\/www.w3.org\/2000\/svg\" viewbox=\"0 0 1800 1130\" role=\"img\" aria-labelledby=\"isT isD\"><title id=\"isT\">IS-IS three-area lab topology<\/title><desc id=\"isD\">Six routers in three IS-IS areas. A2, B2 and C2 are Level-1 access routers. A1, B1 and C1 are Level-1-2 border routers joined by a Level-2 backbone, with a dashed backup Level-2 link between A1 and C1.<\/desc><defs><lineargradient id=\"isbg\" x2=\"1\" y2=\"1\"><stop stop-color=\"#05121e\"\/><stop offset=\"1\" stop-color=\"#102b3e\"\/><\/lineargradient><lineargradient id=\"isrt\" x2=\"0\" y2=\"1\"><stop stop-color=\"#5ca6c5\"\/><stop offset=\"1\" stop-color=\"#126181\"\/><\/lineargradient><\/defs><rect width=\"1800\" height=\"1130\" fill=\"url(#isbg)\"\/><g stroke=\"#1f3d52\" opacity=\".45\"><path d=\"M0 0V1130\"\/><path d=\"M50 0V1130\"\/><path d=\"M100 0V1130\"\/><path d=\"M150 0V1130\"\/><path d=\"M200 0V1130\"\/><path d=\"M250 0V1130\"\/><path d=\"M300 0V1130\"\/><path d=\"M350 0V1130\"\/><path d=\"M400 0V1130\"\/><path d=\"M450 0V1130\"\/><path d=\"M500 0V1130\"\/><path d=\"M550 0V1130\"\/><path d=\"M600 0V1130\"\/><path d=\"M650 0V1130\"\/><path d=\"M700 0V1130\"\/><path d=\"M750 0V1130\"\/><path d=\"M800 0V1130\"\/><path d=\"M850 0V1130\"\/><path 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900H1800\"\/><path d=\"M0 950H1800\"\/><path d=\"M0 1000H1800\"\/><path d=\"M0 1050H1800\"\/><path d=\"M0 1100H1800\"\/><\/g><text x=\"60\" y=\"78\" font-family=\"DejaVu Sans,Arial,sans-serif\" font-size=\"44\" font-weight=\"900\" fill=\"#46db82\">MAG212 CYBER<\/text><text x=\"470\" y=\"78\" font-family=\"DejaVu Sans,Arial,sans-serif\" font-size=\"36\" font-weight=\"800\" fill=\"#e3f4fc\">\u00b7 IS-IS ENTERPRISE LAB TOPOLOGY<\/text><text x=\"62\" y=\"118\" font-family=\"DejaVu Sans,Arial,sans-serif\" font-size=\"21\" fill=\"#9cc3d9\">3 areas \u00b7 Level-1 access \u00b7 Level-2 backbone \u00b7 point-to-point Ethernet \u00b7 wide metrics<\/text><rect x=\"60\" y=\"160\" width=\"500\" height=\"740\" rx=\"24\" fill=\"#0b2335\" fill-opacity=\".75\" stroke=\"#2f88b0\" stroke-width=\"3\"\/><text x=\"310\" y=\"208\" font-family=\"DejaVu Sans,Arial,sans-serif\" font-size=\"30\" font-weight=\"800\" fill=\"#3cc8e8\" text-anchor=\"middle\">AREA 49.0001<\/text><text x=\"310\" y=\"238\" 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stroke-width=\"8\" stroke-linecap=\"round\"\/><text x=\"150\" y=\"1092\" font-family=\"DejaVu Sans,Arial,sans-serif\" font-size=\"20\" font-weight=\"600\" fill=\"#d7e8f3\">Level-1 adjacency<\/text><path d=\"M470 1085h70\" stroke=\"#f25c5c\" stroke-width=\"8\" stroke-linecap=\"round\"\/><text x=\"560\" y=\"1092\" font-family=\"DejaVu Sans,Arial,sans-serif\" font-size=\"20\" font-weight=\"600\" fill=\"#d7e8f3\">Level-2 backbone<\/text><path d=\"M880 1085h70\" stroke=\"#f6a54a\" stroke-width=\"8\" stroke-linecap=\"round\" stroke-dasharray=\"20 12\"\/><text x=\"970\" y=\"1092\" font-family=\"DejaVu Sans,Arial,sans-serif\" font-size=\"20\" font-weight=\"600\" fill=\"#d7e8f3\">Backup Level-2 path<\/text><path d=\"M1300 1085h70\" stroke=\"#9fe6ff\" stroke-width=\"8\" stroke-linecap=\"round\"\/><text x=\"1390\" y=\"1092\" font-family=\"DejaVu Sans,Arial,sans-serif\" font-size=\"20\" font-weight=\"600\" fill=\"#d7e8f3\">Lo0 = router loopback<\/text><\/svg><figcaption>Six-router original lab. Green links are Level-1 adjacencies inside each area; red links form the Level-2 backbone; the dashed amber link is the backup Level-2 path (isis metric 40). Interface names and Lo0 addresses match the configurations below.<\/figcaption><\/figure><div class=\"card\"><b>Subnet allocation \u2014 lab example<\/b><table><tr><th>Link<\/th><th>Left IP<\/th><th>Right IP<\/th><\/tr><tr><td>A1 \u2194 A2 (L1)<\/td><td>10.40.1.1\/30<\/td><td>10.40.1.2\/30<\/td><\/tr><tr><td>B1 \u2194 B2 (L1)<\/td><td>10.40.2.1\/30<\/td><td>10.40.2.2\/30<\/td><\/tr><tr><td>C1 \u2194 C2 (L1)<\/td><td>10.40.3.1\/30<\/td><td>10.40.3.2\/30<\/td><\/tr><tr><td>A1 \u2194 B1 (L2)<\/td><td>10.40.12.1\/30<\/td><td>10.40.12.2\/30<\/td><\/tr><tr><td>B1 \u2194 C1 (L2)<\/td><td>10.40.23.1\/30<\/td><td>10.40.23.2\/30<\/td><\/tr><tr><td>A1 \u2194 C1 backup (L2)<\/td><td>10.40.13.1\/30<\/td><td>10.40.13.2\/30<\/td><\/tr><\/table><\/div><\/div><div class=\"is-sec\"><h2>04 \/ NET addressing explained<\/h2><p>A Network Entity Title (NET) is an NSAP-style identifier. A typical lab NET is <code data-no-translation=\"\">49.0001.0000.0000.0001.00<\/code>.<\/p><table><tr><th>Component<\/th><th>Example<\/th><th>Signification<\/th><\/tr><tr><td>AFI + area<\/td><td>49.0001<\/td><td>Private address allocation and Area 1<\/td><\/tr><tr><td>System ID<\/td><td>0000.0000.0001<\/td><td>Unique six-byte router identifier<\/td><\/tr><tr><td>N-selector<\/td><td>00<\/td><td>Required for router&#8217;s NET<\/td><\/tr><\/table><p>Use Area 49.0002 for B1\/B2 and Area 49.0003 for C1\/C2. Within each area, both routers use the same area ID but unique system IDs.<\/p><\/div><div class=\"is-sec\"><h2>05 \/ Working Cisco IOS XE configurations<\/h2><p>The following sample is an educational IOS XE lab. Interface names vary by platform; ensure interfaces are up and connected as mapped. The examples use point-to-point IS-IS on routed Ethernet ports and wide metrics.<\/p><h3>A1 \u2014 Area 1 border<\/h3><pre data-no-translation=\"\">hostname A1\nrouter isis CORE\n net 49.0001.0000.0000.0001.00\n is-type level-1-2\n metric-style wide\n log-adjacency-changes\n!\ninterface GigabitEthernet0\/0\n description L1 to A2\n ip address 10.40.1.1 255.255.255.252\n ip router isis CORE\n isis circuit-type level-1\n isis network point-to-point\n!\ninterface GigabitEthernet0\/1\n description L2 to B1\n ip address 10.40.12.1 255.255.255.252\n ip router isis CORE\n isis circuit-type level-2-only\n isis network point-to-point\n!\ninterface GigabitEthernet0\/2\n description Backup L2 to C1\n ip address 10.40.13.1 255.255.255.252\n ip router isis CORE\n isis circuit-type level-2-only\n isis network point-to-point\n isis metric 40 level-2\n!\ninterface Loopback0\n ip address 172.20.1.1 255.255.255.255\n ip router isis CORE<\/pre><h3>B1 \u2014 Area 2 border<\/h3><pre data-no-translation=\"\">hostname B1\nrouter isis CORE\n net 49.0002.0000.0000.0002.00\n is-type level-1-2\n metric-style wide\n!\ninterface GigabitEthernet0\/0\n description L1 to B2\n ip address 10.40.2.1 255.255.255.252\n ip router isis CORE\n isis circuit-type level-1\n isis network point-to-point\n!\ninterface GigabitEthernet0\/1\n description L2 to A1\n ip address 10.40.12.2 255.255.255.252\n ip router isis CORE\n isis circuit-type level-2-only\n isis network point-to-point\n!\ninterface GigabitEthernet0\/2\n description L2 to C1\n ip address 10.40.23.1 255.255.255.252\n ip router isis CORE\n isis circuit-type level-2-only\n isis network point-to-point\n!\ninterface Loopback0\n ip address 172.20.2.1 255.255.255.255\n ip router isis CORE<\/pre><h3>C1 \u2014 Area 3 border<\/h3><pre data-no-translation=\"\">hostname C1\nrouter isis CORE\n net 49.0003.0000.0000.0003.00\n is-type level-1-2\n metric-style wide\n!\ninterface GigabitEthernet0\/0\n description L1 to C2\n ip address 10.40.3.1 255.255.255.252\n ip router isis CORE\n isis circuit-type level-1\n isis network point-to-point\n!\ninterface GigabitEthernet0\/1\n description L2 to B1\n ip address 10.40.23.2 255.255.255.252\n ip router isis CORE\n isis circuit-type level-2-only\n isis network point-to-point\n!\ninterface GigabitEthernet0\/2\n description Backup L2 to A1\n ip address 10.40.13.2 255.255.255.252\n ip router isis CORE\n isis circuit-type level-2-only\n isis network point-to-point\n isis metric 40 level-2\n!\ninterface Loopback0\n ip address 172.20.3.1 255.255.255.255\n ip router isis CORE<\/pre><h3>A2, B2, C2 \u2014 access routers<\/h3><pre data-no-translation=\"\">! A2 example; B2 and C2 substitute their area NET\/IPs\nhostname A2\nrouter isis CORE\n net 49.0001.0000.0000.0012.00\n is-type level-1\n metric-style wide\n!\ninterface GigabitEthernet0\/0\n description Uplink to A1\n ip address 10.40.1.2 255.255.255.252\n ip router isis CORE\n isis circuit-type level-1\n isis network point-to-point\n!\ninterface Loopback0\n ip address 172.20.11.1 255.255.255.255\n ip router isis CORE<\/pre><div class=\"note\">Lab behavior depends on the L1\/L2 attached-bit and platform route-leaking defaults. For a complete inter-area reachability exercise, test return routes and configure supported prefix leaking or summarization where necessary. Do not assume all loopbacks in separate areas are automatically visible to all L1-only routers.<\/div><\/div><div class=\"is-sec\"><h2>06 \/ IS-IS PDUs and TLVs<\/h2><table><tr><th>PDU<\/th><th>Function<\/th><th>Field example<\/th><\/tr><tr><td>IIH (Hello)<\/td><td>Neighbor discovery \/ adjacency<\/td><td>Neighbor IDs, area addresses, capabilities<\/td><\/tr><tr><td>LSP<\/td><td>Topology and prefix advertisements<\/td><td>Reachability TLVs<\/td><\/tr><tr><td>CSNP<\/td><td>Complete LSDB summary<\/td><td>LSP IDs and sequence numbers<\/td><\/tr><tr><td>PSNP<\/td><td>Partial summary \/ acknowledgment or request<\/td><td>Missing LSP identifiers<\/td><\/tr><\/table><p>Examples of IPv4-related TLVs: 129 (protocols supported), 132 (IP interface address), 135 (extended IP reachability). RFC 5308 adds TLVs 232 and 236 for IPv6 addressing and reachability. LSP information is carried in extensible TLV structures rather than in a fixed IP routing-specific packet header.<\/p><\/div><div class=\"is-sec\"><h2>07 \/ DIS elections and circuit types<\/h2><p>On a multi-access LAN, IS-IS elects a Designated Intermediate System (DIS) to represent the segment via a pseudonode. Point-to-point Ethernet circuits do not elect a DIS.<\/p><pre data-no-translation=\"\">interface GigabitEthernet0\/3\n description Broadcast LAN example\n ip router isis CORE\n isis priority 90 level-1\n!\n! On a routed Ethernet point-to-point connection instead:\ninterface GigabitEthernet0\/1\n isis network point-to-point<\/pre><div class=\"note\">Unlike OSPF DR\/BDR, IS-IS has no backup DIS; DIS can be preempted. Confirm timer defaults for your exact interface type and software release rather than applying 10\/30 universally.<\/div><\/div><div class=\"is-sec\"><h2>08 \/ SPF and wide metric walkthrough<\/h2><p>For example, assume L2 cost from A1 to B1 is 10 and from B1 to C1 is 10, while the direct A1\u2013C1 link has cost 40. SPF prefers the two-hop backbone path with cost 20. If B1\u2013C1 fails, the direct link becomes a usable alternate with cost 40. This is an illustrative link-cost comparison, not live routing output.<\/p><pre data-no-translation=\"\">router isis CORE\n metric-style wide\n!\ninterface GigabitEthernet0\/1\n isis metric 10 level-2\n!\ninterface GigabitEthernet0\/2\n isis metric 40 level-2<\/pre><p>Wide metrics improve cost range and support more modern extensions. Keep metric policy consistent across the routing domain, and check direction-specific interface costs on both ends.<\/p><\/div><div class=\"is-sec\"><h2>09 \/ Troubleshooting command reference<\/h2><table><tr><th>Commande<\/th><th>Utiliser<\/th><\/tr><tr><td><code data-no-translation=\"\">show isis neighbors<\/code><\/td><td>Neighbor adjacency state<\/td><\/tr><tr><td><code data-no-translation=\"\">show clns neighbors<\/code><\/td><td>CLNS-level neighbors on supported IOS versions<\/td><\/tr><tr><td><code data-no-translation=\"\">show isis interface<\/code><\/td><td>Levels, timers, circuit type, priorities<\/td><\/tr><tr><td><code data-no-translation=\"\">show clns interface<\/code><\/td><td>CLNS \/ IS-IS per-interface information<\/td><\/tr><tr><td><code data-no-translation=\"\">show isis database<\/code><\/td><td>LSDB and LSPs<\/td><\/tr><tr><td><code data-no-translation=\"\">show isis topology<\/code><\/td><td>SPF-derived topological view<\/td><\/tr><tr><td><code data-no-translation=\"\">show ip route isis<\/code><\/td><td>Installed IPv4 IS-IS routes<\/td><\/tr><tr><td><code data-no-translation=\"\">show isis spf-log<\/code><\/td><td>SPF recalculation history<\/td><\/tr><tr><td><code data-no-translation=\"\">show running-config | section router isis<\/code><\/td><td>Process config \/ NET<\/td><\/tr><\/table><p>Command output syntax can differ between IOS XE, IOS XR and NX-OS; these examples target IOS XE-style syntax.<\/p><\/div><div class=\"is-sec\"><h2>10 \/ Failure scenarios and triage<\/h2><table><tr><th>Symptom<\/th><th>Likely check<\/th><th>Next action<\/th><\/tr><tr><td>No neighbor<\/td><td>Layer 2, interface shutdown, mismatched circuit types<\/td><td>Verify link state and hello reception<\/td><\/tr><tr><td>L1 neighbor fails<\/td><td>NET area IDs differ<\/td><td>Align L1 area addresses<\/td><\/tr><tr><td>L2 neighbor fails<\/td><td>Levels \/ authentication \/ MTU<\/td><td>Check matching L2 capability and authentication<\/td><\/tr><tr><td>Neighbor up, no route<\/td><td>Prefix TLVs, metric style, route filters, attached behavior<\/td><td>Check database then RIB<\/td><\/tr><tr><td>Frequent SPF events<\/td><td>Flapping links \/ unstable adjacency<\/td><td>Correlate logs and physical events<\/td><\/tr><tr><td>Routes not preferred<\/td><td>Administrative distance \/ metric \/ competing protocol<\/td><td>Inspect RIB and policy<\/td><\/tr><\/table><pre data-no-translation=\"\">show logging | include ISIS|CLNS|LINEPROTO\nshow isis neighbors detail\nshow isis database detail\nshow ip route 172.20.3.1\nshow interfaces GigabitEthernet0\/1<\/pre><div class=\"warn note\">Use debugging only with a change window and output constraints; uncontrolled protocol debugging can load a production device.<\/div><\/div><div class=\"is-sec\"><h2>11 \/ Authentication and security controls<\/h2><p>Enable authenticated adjacencies where supported, restrict physical\/L2 access to routing segments, validate allowed peers, protect the control plane, set passive or non-routing behavior where appropriate, log adjacency changes, and apply prefix control at redistribution points. Never use simple plaintext keys for production. Authentication configuration varies by release and device.<\/p><pre data-no-translation=\"\">! Example of classic IOS XE interface-level MD5 key-chain\nkey chain ISIS_KEYS\n key 1\n  key-string REPLACE_WITH_STRONG_SECRET\n!\ninterface GigabitEthernet0\/1\n isis authentication mode md5 level-2\n isis authentication key-chain ISIS_KEYS level-2\n!<\/pre><p>Replace demonstration secrets, consult release support and your organization&#8217;s secret management controls, and roll keys simultaneously across neighbors. IS-IS adjacency authentication does not itself encrypt traffic.<\/p><\/div><div class=\"is-sec\"><h2>12 \/ Quick lab checks and expected results<\/h2><pre data-no-translation=\"\">! On A1:\nshow isis neighbors\n! Expect L1 adjacency to A2, L2 to B1 and C1\n\nshow isis database\n! Inspect separate Level-1 and Level-2 LSDBs\n\nshow ip route isis\n! Look for learned routes with IOS administrative distance 115\n\nshow isis spf-log\n! Compare changes before and after controlled link failure<\/pre><p>Practice: shut down the B1-to-C1 laboratory link (not production) and check whether SPF switches to A1\u2013C1. Restore and verify all adjacencies. Record neighbor uptime, LSP sequence changes, and installed routes.<\/p><\/div><div class=\"is-sec\"><h2>13 \/ Questions with explanations<\/h2><div class=\"card\"><b>Q1: Does Level 1 require the same area address?<\/b><p><span class=\"pill\">Answer:<\/span> Yes, for an L1 adjacency. L2 adjacency does not have that same area-matching requirement.<\/p><b>Q2: What is the DIS on point-to-point Ethernet?<\/b><p><span class=\"pill\">Answer:<\/span> None; a DIS election is for broadcast multi-access networks.<\/p><b>Q3: Is IS-IS IP protocol 88?<\/b><p><span class=\"pill\">Answer:<\/span> No. EIGRP uses IP protocol 88. IS-IS operates directly over Layer 2.<\/p><b>Q4: Why can a neighbor be up without an installed route?<\/b><p><span class=\"pill\">Answer:<\/span> Adjacency confirms peering, not route selection. Inspect the LSDB, prefix advertisement, hierarchy, filter policy and RIB competition.<\/p><b>Q5: Why wide metrics?<\/b><p><span class=\"pill\">Answer:<\/span> They support larger metric ranges and modern traffic engineering \/ protocol extensions.<\/p><b>Q6: Is an LSP identical to an IP packet?<\/b><p><span class=\"pill\">Answer:<\/span> No. LSPs are IS-IS control-plane PDUs with TLVs describing topology and reachability.<\/p><\/div><\/div><div class=\"is-sec\"><h2>14 \/ Source verification and editorial notes<\/h2><p>This is an original MAG212 CYBER guide. The six-router Ethernet topology, IP plan, IOS XE lab, failure walkthrough, hardening guidance and practice questions were written independently and checked against the IETF standards and Cisco documentation below.<\/p><p><b>Primary technical references:<\/b><br><a href=\"https:\/\/www.cisco.com\/c\/en\/us\/td\/docs\/switches\/lan\/c9000\/lyr3-fwd\/isis\/is-is-configuration-guide\/is-is.html\" target=\"_blank\" rel=\"noopener\">Cisco IS-IS Configuration Guide (IOS XE 17)<\/a><br><a href=\"https:\/\/netascode.cisco.com\/docs\/data_models\/iosxe\/device\/isis\/\" target=\"_blank\" rel=\"noopener\">Cisco Network as Code \u2014 IOS XE IS-IS data model<\/a><br><a href=\"https:\/\/www.rfc-editor.org\/rfc\/rfc1195.html\" target=\"_blank\" rel=\"noopener\">RFC 1195 \u2014 integrated IS-IS<\/a><br><a href=\"https:\/\/www.rfc-editor.org\/rfc\/rfc5308.html\" target=\"_blank\" rel=\"noopener\">RFC 5308 \u2014 routing IPv6 with IS-IS<\/a><br><a href=\"https:\/\/www.rfc-editor.org\/rfc\/rfc5305.html\" target=\"_blank\" rel=\"noopener\">RFC 5305 \u2014 IS-IS TE extensions<\/a>.<\/p><p>All IP addresses and device configurations are lab examples, not a representation of a production network. Validate platform compatibility and approved changes before deployment.<\/p><\/div><div class=\"is-foot\">MAG212 CYBER \u00b7 NETWORK ENGINEERING \/ IS-IS ULTIMATE CHEAT SHEET<br>Published for educational use on cyber.mag212.com \u00b7 Original topology and editorial text<br><a href=\"\/fr\/learn\/\">Plus de guides dans Apprendre<\/a> \u00b7 <a href=\"\/fr\/learn\/eigrp-cheat-sheet\/\">EIGRP<\/a> \u00b7 <a href=\"\/fr\/learn\/bgp-cheat-sheet\/\">BGP<\/a> \u00b7 <a href=\"\/fr\/services\/\">Conseil en r\u00e9seau et s\u00e9minaires<\/a><\/div><\/div>","protected":false},"excerpt":{"rendered":"<p>IS-IS field guide: multi-area L1\/L2 design, NET addressing, six-router IOS XE lab, SPF, troubleshooting and authentication.<\/p>","protected":false},"author":1,"featured_media":0,"parent":3385,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-3416","page","type-page","status-publish","hentry"],"_hostinger_reach_plugin_has_subscription_block":false,"_hostinger_reach_plugin_is_elementor":false,"_links":{"self":[{"href":"https:\/\/cyber.mag212.com\/fr\/wp-json\/wp\/v2\/pages\/3416","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cyber.mag212.com\/fr\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/cyber.mag212.com\/fr\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/cyber.mag212.com\/fr\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/cyber.mag212.com\/fr\/wp-json\/wp\/v2\/comments?post=3416"}],"version-history":[{"count":0,"href":"https:\/\/cyber.mag212.com\/fr\/wp-json\/wp\/v2\/pages\/3416\/revisions"}],"up":[{"embeddable":true,"href":"https:\/\/cyber.mag212.com\/fr\/wp-json\/wp\/v2\/pages\/3385"}],"wp:attachment":[{"href":"https:\/\/cyber.mag212.com\/fr\/wp-json\/wp\/v2\/media?parent=3416"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}