IS-IS Routing Protocol Interview Questions

IS-IS Interview Questions - IS-IS is a link state routing protocol. Commonly used in Service Provider networks.

Back in old days, IS-IS routing protocol software was more stable and robust compare to OSPF, thus many service provider choose IS-IS as their interior routing protocol.

I collected the questions which I received from my students and readers related with IS-IS routing protocol and my answers in this post.

I am explaining this topic in deep detail in my CCDE Bootcamp and Self Paced CCDE course.

Below questions are commonly asked.

Question 1: In OSPF ASBR is used for external domain (external routes) injection, could u please let me know in ISIS who will be utilise to connect different routing protocol domain…. Is that L1/L2 or L2 or L1?

Answer 1: First of all, ASBR is not just an OSPF concept. Whichever device performs redistribution regardless of routing protocol, that device is called as an ASBR.

Redistribution can be performed in both Level 1 and Level 2 domains.

IS-IS L2 domain is an equivalent of OSPF backbone area.

L2 domain has to be contiguous.

L1 domain is similar to totally not so stubby areas.It doesn’t allow any external or summary information but allows redistribution into domain. Thus external information’s can be injected in L1 domain as well.

Question 2: Can L1 routes be redistributed into different protocol? Is that a part of good design to do so?

Answer 2: You can redistribute routes from both L1 and L2 domains into another routing protocol.

In general both are not a good design. You should carry the external routing information from another domain through BGP.

See the redistribution best practices post here.

Question 3: Is IS-IS routing protocol only for IP networks – Does it support CLNS routing?

Answer 3: IS-IS is not only for IP routing.

IS-IS was originally invented for CLNS. It runs top of layer 2. There is an ethertype for IS-IS packets.

It is not an IP based protocol, like EIGRP and OSPF.

For EIGRP and OSPF you need an IP protocol number.

OSPF for example use IP protocol number 89.

If it is layer 2 protocol such as IS-IS you need an ether type value.

Question 4: ISIS routing protocol can’t be use for DMVPN is that true?? So with DMVPN just EIGRP or is there any other protocol which we can use?

Answer 4: IS-IS routing protocol can’t support IP based tunnel; it is not supported with DMVPN. If you have point to point GRE tunnel, then IS-IS can run over GRE.

It is not just EIGRP over DMVPN ,in fact OSPF, BGP, even RIP can run over DMVPN.

In fact even RIP scales better than OSPF.

You need to manually configure OSPF to scale your network, but flooding will stay as a problem whatever you do.

Thus for large scale networks, EIGRP is the best for the DMVPN design.

Question 5: What is the benefit of set-overload bit in ISIS routing protocol?

Answer 5: It is used to avoid black holing.

Imagine a scenario where you have BGP on every router. Which mean you don’t run MPLS in the core.

In case of a link failure, IGP, as well as BGP neighborship goes down.

If you don’t set overload bit on the intermediate routers, since IS-IS will converge faster than BGP, BGP destinations will be blackhole.

If you set overload bit on a router, router is not used as a transit router; it waits to BGP to converge.

It is very similar to IGP LDP synchronisation. But overload bit is used for IGP BGP synchronisation in IS-IS routing protocol.

Question 6: Is there a designated router in IS-IS , similar to OSPF DR? When we should or shouldn’t use it ?

Answer 6: In a multi-access networks, such as Ethernet, IS-IS elects a DIS (Designated Intermediate System).

DIS is very similar to DR (Designated Router in OSPF),creates a pseudo node to represent multi-access segment.

Without DIS, every router would flood the LSPs to each other. For efficient flooding on a multi-access networks, you want to have DIS.

But from the convergence point of view, having a DIS is not good. Election takes time. But also LSP will traverse more hop to reach to every other router.

If you have only two routers which are connected back to back through Ethernet, since it will be broadcast, IS-IS elects a DIS.

If you have only two routers on a segment, having a DIS doesn’t improve flooding.

But if you change the type to point to point you will get better convergence since there will not be a DIS.

Question 7: External routes can be summarised within L1 area only,is that true?

Answer 7: No. In IS-IS external routes since they will be regenerated at the L1/L2 router, you can summarize on L1/L2 router as well.

You may want to check this RFC.

Question 8: Do you think that ISIS routing protocol flooding mechanism better than OSPF? What is the difference between full and partial SPF ?

Answer 8: Flooding is very similar in OSPF and IS-IS.

But IS-IS and OSPFv2 behave different for the route calculation. OSPF sends reachability and topology information within an area in Type 1 (Router LSA).

Which mean, if you add even loopback interface on one router in an area, all the routers in the same area will run full SPF.

In IS-IS reachability and topology information is carried in different TLV in the LSP.

So if you add a loopback on the router, the other routers will just run partial SPF.

Only the topology change triggers full SPF calculation is IS-IS,

In large scale network, full vs. partial SPF is important for the CPU.

Question 9: Full mesh – to reduce flooding use mesh-groups what does Mesh-groups mean?

Answer 9: In highly meshed networks this is the mechanism for link state protocols to control the flooding.

It is not specific to IS-IS routing protocol, same concept is used in OSPF as well.

Since MPLS traffic engineering for the distributed path computation use link state protocols, mesh group is applicable for the MPLS TE as well.

Basically you elect a router or two for redundancy and say that just those two routers will flood the LSP to the other routers.

Those routers has to have connection to the all the nodes in the topology of course.

Thus you eliminate flooding churn.

Question 10: Multi topology routing, what are the benefits in ISIS?

Answer 10: If you have IPv4 and IPv6 on your network and IPv6 routing topology is different than IPv4, then having a multi topology routing gives benefit.

Don’t forget that you will have 2 LSDB, one for IPv4 and one for IPv6.Y

You need to have extra memory and CPU on the routers and troubleshooting will be much harder.

Thus MTTR (Mean time to repair) increases.

If IPv6 topology follows the IPv4, You don’t need MTR.

Don’t complicate your network.

Question 11: Multi instance routing what are the benefits in ISIS routing protocol?

Answer 11: As RFC 6822 defines; Multi Instance IS-IS can be used to an alternate to Multi Topology Routing.

But Multi instance on a link is critical in some topology for the OSPF operation.

Those topologies often require putting the link in non-backbone area and creating a Virtual Link on the link.

Thus multi instance on a link is excellent solution for OSPF.

IS-IS routing protocol solves the same issue by default having L1/L2 router.

To have a great understanding of SP Networks, you can check my new published Service Provider Networks Design and Perspective Book. It covers the SP network Technologies with also explaining in detail a factious SP network.

If you are a service provider network engineer, what is your IGP ?

Do you have one or two level IS-IS on your network?

Is there any Enterprise Network engineer who runs IS-IS routing protocol on their network?

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Orhan Ergun

Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

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