ISIS
Last updated
Last updated
IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol like OSPF. It builds a map of the network and calculates the best path using the Shortest Path First (SPF) algorithm.
But IS-IS has its quirk;
It was originally designed for OSI (not TCP/IP) but adapted for IP.
It runs directly over Layer 2 (no IP headers like OSPF's use of IP protocol 89)
It does not have Area 0 drama. - just levels ( Level 1 for intra area, Level 2 for inter area)
think of IS-IS as OSPF'' cousin - the one that's quieter ,efficient, and doesn't make a fuss about areas.
IS-IS vs OSPF - What's Differences ?
So if OSPF is a strict schoolteacher demanding Area 0, IS-IS is the laid-back professor who says, “Just call them Levels, and we’ll be fine.”
IS-IS AT THE PACKET TYPES
IS-IS does not use traditional IP packets. Nope, it is cool for that. Instead, it has its own special PDUs(Protocol Data Units) Think of these like different types of email in your inbox - some are useful, some are annoying, and some are just pain weird.
Now, if you’re thinking, “Wait, why does IS-IS have a whole system that sounds like postal mail?”—well, that’s because it’s like an old-school librarian. Everything must be logged, checked, and cross-checked.
Since IS-IS skips IP altogether (yup, it doesn’t even use IP headers), it has a unique packet structure. Here’s what a typical IS-IS packet looks like:
It’s like a VIP pass—without it, your IS-IS updates aren’t getting past the bouncer
IS-IS relies on TLVs to carry the actual routing info. It’s like a buffet—only take what you need. OSPF, on the other hand, is that restaurant where you must order a full meal even if you just wanted fries.
EXAMPLE OF AN IS-IS LSP TLV
Inside an IS-IS LSP, you might see:
Translation: "I can get you to 192.168.1.0/24 in 10 hops. Also, I’m cool enough for Segment Routing!
• TLV 135 advertises reachability for 192.168.1.0/24. • Sub-TLV 31 (Prefix-SID) indicates that this prefix has a Segment Routing MPLS Label 16010. • Flags (0x40, No-PHP) means Penultimate Hop Popping (PHP) is disabled (i.e., the label stays until the final destination). • Algorithm 0 means the label is assigned using default SPF routing.
Ever wondered what the NET address in IS-IS actually means?
If you're coming from OSPF, you’re probably thinking, "Just give me a router ID and be done with it!" But IS-IS is a bit more... sophisticated. Instead of a simple IP-based identity, it has its own special Network Entity Title (NET)—think of it as a router’s secret superhero identity that makes IS-IS work.
The NET (Network Entity Title) is a 10-byte (or longer) address that uniquely identifies a router in an IS-IS network. It’s like a router’s passport number, except instead of boring digits, it follows a cool hierarchical format. The NET address is made up of three parts: Area Address → "Which neighborhood am I in?" System ID → "Who am I, really?" NSEL (NSAP Selector) → "What’s my role at the party?" (Always 00 for IS-IS routers)
Let’s say you have the following NET address:
49.0001.1921.6800.1001.00 Here’s what each part means:
Now, if routers were in a club, the Area Address would be their table number, the System ID would be their name tag, and the NSEL would tell the bouncer if they were a VIP or just a regular guest (spoiler: routers are always "00").
Unlike OSPF, where the Router ID is just a single IP address, IS-IS wants a 6-byte System ID. You can pick any unique value, but a common trick is: Use an IP address (without dots!) • Example: If the loopback IP is 192.168.1.1, you can format it as 1921.6800.1001 Use a simple numbering scheme • Example: R1 = 0000.0000.0001, R2 = 0000.0000.0002 Use MAC addresses (if you like long and messy things) Whatever you pick, just make sure it's unique across the IS-IS domain!
OSPF floods LSAs like a panicked intern sending mass emails. IS-IS? Nah, it’s way more relaxed.
New LSPs are generated when topology changes. (Think: “Hey, new route available!”)
LSPs have sequence numbers to keep track of updates. (Like version numbers on a document.)
CSNPs act like a full index of known LSPs. (Sent periodically to ensure everyone is on the same page.)
PSNPs help routers request missing LSPs. (Like asking for a re-send of an email you missed.)
LSP Aging makes sure expired routes get tossed. (No hoarding of old data!)
END RESULT? IS-IS is more efficient and doesn’t have the same LSA flooding headaches as OSPF.
If you fire up Wireshark and capture an IS-IS Hello packet, here’s what you might see:
What this means: • "Hey, I’m sending an IS-IS Hello!" • "I belong to Area 49.0001!" • "I want to be friends with 00:1A:2B:3C:4D:5E" If the neighbor likes your back, an adjacency forms. If not? Well, it’s not you, it’s them.
No LSA Types Mess – OSPF has 11 types of LSAs, IS-IS has just LSPs.
No IP Overhead – Runs directly on Layer 2, skipping unnecessary IP processing.
Faster SPF Computation – Thanks to its streamlined database and simple flooding mechanism.
Smaller Packet Sizes – TLVs allow compact updates instead of rigid, bloated LSAs.
All routers will run Cisco IOS XR and form an IS-IS Level 2 network (like OSPF Area 0).
STEP 1: ENABLE IS-IS ON ALL ROUTERS Go to each router and enable IS-IS in Level-2 mode (like OSPF Area 0).
R1 CONFIGURATION
R2 CONFIGURATION
R3 CONFIGURATION
R4 CONFIGURATION
STEP 2: IS-IS VERIFICATION
IS-IS KEY TAKEAWAYS FOR OSPF USERS
If OSPF is a dramatic soap opera (with Area 0 politics, ABRs, and LSAs causing drama), IS-IS is a chill, organized documentary—clean, structured, and free of unnecessary emotions. IS-IS may not be as common as OSPF, but it’s widely used in service provider networks due to its simplicity, scalability, and stability. If OSPF is your first love, IS-IS might just be your cool, lowmaintenance second relationship. Now that you know how IS-IS works at the packet level, go ahead and fire up Wireshark! Capture some IS-IS packets, analyze them, and see just how efficient it really is. Next time, we’ll dive into ECMP, Traffic Engineering, and Segment Routing—because why settle for basic routing when you can optimize the path like a pro? Stay tuned
