This topic is to discuss the following lesson:
Hi Rene,
Thx for your very nice article as always 
If there is multiple Level1-2 Router on a area then due to default Route asymmetric Routing will be occurred. I saw your topic about Route leaking but I think its not scalable more . So is there any other scaleble option here ??
Hi @Zaman.rubd,
Since IS-IS routers only have a default route to get to other areas, sub-optimal routing can be an issue yes. It will depend on your network design. There’s not really an alternative to route leaking as far as I know…somehow you have to get those more specific prefixes in the routing table of your L1 routers.
For anyone else that is reading this and wondering what this is about, here is an example for IS-IS route leaking and sub-optimal routing:
https://networklessons.com/cisco/ccie-routing-switching-written/is-is-route-leaking/
Hello René,
Thank you a lot for this lesson. But I don’t see this command circuit-type level anywhere in the course. Normal?
Hello Djan,
I haven’t added this command indeed. It is pretty straight-forward though, you can use it to tell the router which hello packets to send on the interface: L1, L2 or L1 and L2. You can do this per interface so on Gi0/1 you could send L1 hello packets and on Gi0/2 only L2 hello packets.
Rene
Hello Rene,
I’ve a question about the is-type in the R2, before it had the is-type level-1 because we had one area only, after we added another area and R2 became a L1-L2 router, right. You have typed the is-type level-1-2 within router isis. My question is, Could I omit the is-type in router isis configuration because the router cisco has a L1-L2 behavior as default?
Thanks
Hello Eduardo
Since the default is-type is level 1-2, then you don’t actually have to explicitly configure the router as a level 1-2 router. This command can indeed be deleted. However, as is the case with many such commands, it’s a good idea to verify that the configuration is correct by implementing the command. This is all the more important when you configure devices that have been previously configured and may have some other configuration already set up.
I hope this has been helpful!
Laz
Excellent explaining. We learned a lot
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Hello,
I have a question, just to be sure: the 2 backbone routers become l2 neighbors because they’ve exchanged l2 LSP and found the directly connected net between them in the other router LSP and the other router has another area. Is this correct?
Thank you,
Stefanita
Hello Staut
Yes, this is correct.
Laz
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Thank you! One more question: if R1 would be set to L1-L2 he would see the R2 neighbor in the same area and would not establish a L2 neighborship thus not processing L2 LSP from R2. I guess R2 sends L2 LSP to R1 too, but since there is a L1 neighborship, R1 would not process them. Is this also correct?
Hello Stuat
Yes, if two routers are set to be L1-L2, and they are in the same area, they will only establish an L1 neighborship because they detect that they are in the same area.
I hope this has been helpful!
Laz
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Thank you! Just wanted to make sure that it’s not necessarily to set is-type level1 command if routers are in the same area.
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As per the configuration example I notice that there is a IS-IS adjacency established between Area 12 with Area 34 where i see the R2 is set to is-type level-1-2 and R4 set to is-type level-1. Once there is IS-IS enabled on the directly connected interfaces of R4 & R2 i get to see that R4 is able to view the link state database of both Level 1 and Level 2 in it although we did not set the R4 as is-type level-2. Is this a normal behaviour in IS-IS routing protocol, where if one end is set to is-type level-1-2 the other end would pick up the same is-type level-1-2 automatically ? Please clarifiy.
Hello Raja
To be honest, I didn’t know the answer to this right off, so I went in and labbed it it. It seems that you don’t need to configure R4 to function at Level-1-2 in order for R2 to create an inter area neighbor relationship. Looking deeper into it, it turns out that the default setting of IS-IS on Cisco routers is a Level-1-2 router.
So to summarize, by default, Cisco IS-IS routers function as Level-1-2 IS-IS routers. So the fact that R4 was not configured as such, does not change the results.
I hope this has been helpful!
Laz
Hello
Can you explain to me the difference between is-type level-1 and isis circuit-type level-1
Thanks
Iraklis
Hello Iraklis
The is-type command is used to configure the routing level for an instance of the IS-IS routing process. The command is applied under the ISIS routing config mode like so:
R1(config-router)#is-type level-1
The circuit-type command is used to specify the type of adjacency that will be created and is configured using this syntax in interface configuration mode:
R1(config)#interface GigabitEthernet 0/1
R1(config-if)#isis circuit-type level-2-only
The circuit-type command essentially allows you to configure specific interfaces to send out and accept only particular types of hellos that will create the desired adjacencies. For example, you can configure a router to have some interfaces to be level 2-only to prevent wasting bandwidth by sending out unused level 1 hello packets. The circuit-type command should only be used on ISIS routers that are between areas, that is Level1-2 routers.
The is-type and circuit-type commands are related, but they do two different things.
For more info on these commands, take a look at these Cisco command references:
https://www.cisco.com/c/en/us/td/docs/ios/iproute_isis/command/reference/irs_book/irs_is1.html#wp1013280
https://www.cisco.com/c/en/us/td/docs/ios/iproute_isis/command/reference/irs_book/irs_is1.html#wp1014983
I hope this has been helpful!
Laz
Hello Lazaros
Thanks for the explanation.
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Hello @lagapidis
This is my 1st approach to this routing protocol.
My 1st impressions
Compared to OSPF it seems to be more “flexible” since it hasn’t the limitations OSPF has regarding transit through backbone area 0.
For large networks such as SP , is better IS-IS than OSPF ?
Hello Juan
Both IS-IS and OSPF are excellent routing protocols, and each has its own strengths.
IS-IS is indeed more flexible than OSPF in terms of topology because it doesn’t have the backbone area 0 restriction. This makes IS-IS easier to manage in large, complex networks. It also has a slight advantage in terms of scalability due to its more efficient handling of link-state advertisements (LSAs).
Secondly, IS-IS is designed to function at the Network Layer of the OSI model, but unlike OSPF, it operates independently of the IP protocol. In other words, it doesn’t use IP to exchange IS-IS messages between routers.
However, OSPF is more widely used and understood, which can make it easier to find support and resources for troubleshooting. OSPF also integrates better with IP, which is the dominant network protocol and is therefore often a more convenient choice.
In terms of performance, both protocols are very similar, and the choice between them often comes down to the specific needs and constraints of your network, as well as personal preference and familiarity.
It is indeed true that forl large ISPs, IS-IS is the preferred IGP due to its scalability and flexibility advantages, but OSPF is also widely used and perfectly capable.
It’s also worth noting that both of these protocols can coexist in the same network, so you don’t necessarily have to choose one or the other.
I hope this has been helpful!
Laz