Introduction to IS-IS

Thanks Laz.

Looking at the packet captures you supplied, is it significant that IS-IS is built upon ‘IEEE 802.3 Ethernet’ whilst OSPF and RIP use ‘Ethernet II’?

Thanks,

Gareth.

Hello Gareth

Yes, actually, that is very significant. IS-IS is a protocol that has been developed to strictly adhere to the OSI model and not the TCP/IP model. IEEE 802.3 adheres to the OSI model whereas Ethernet II, strictly speaking, does not. Remember that the OSI model separates the Data Link Layer into two sublayers, the Media Access Control (MAC) and the Logical Link Control (LLC). IEEE 802.3 adheres to this, providing both a MAC and LLC sublayer component, which can be seen in the Wireshark output. Ethernet II does not, and is thus not compatible with IS-IS.

I hope this has been helpful!

Laz

Hi Rene,

R2 and R4 form L2 adjacency but why do they install 192.168.24.0 in their L1 database. Aren’t they supposed to maintain it in their L2 database only?

Hello Raj

It is true that R4 and R2 are L2 neighbors, but the 192.168.24.0/24 network must also be advertised to the other L1 routers such as R3 and R4 so they can know how to route to it, and must thus be included in the L1 LSP.

I hope this has been helpful!

Laz

Hii Rene,

i am working in ISP . ISIS & OSPF & BGP troubleshooting is my basic day to day Activites.

planning to shift Job. Kindly provide your suggestions whether to learn Phyton or Datacentre firewalls or any…

Hello Chandrasekhar

Your job sounds like a great opportunity to learn a lot. Troubleshooting for an ISP and working with these protocols means that you get a lot of hands-on experience. If you’re looking to move into another area and enrich your skill set, there are a lot of things that you can do. Learning Python, moving into more in-depth data center issues, or focusing on security and firewalls are all good choices. All three are very “in demand” and you will find jobs easily with these specializations.

My suggestion would be to choose what you like best. It is so important to love what you do, because you will be spending more time in your job than with your family! So since all of these areas are in high demand, choose the thing that will give you the most pleasure. That way, you will like your job, and automatically you will be better at it.

I hope this has been helpful!

Laz

Hi all,
Do you know how to test this feature by show result …
Thanks,

Hello Quy

Applying prioritization for prefixes within ISIS imply involves the tagging of your choice of prefixes, and then configuring the device to give priority to those tagged prefixes, so they are updated first in the RIB. More on this process can be found at this Cisco Documentation.

Now in order to see the tags that have been assigned, you can use the show isis rib command. This will output the prefix and include the assigned tag. By default, such tags are set to 0. You can find out more about this command at the following link:

I hope this has been helpful!

Laz

Great ! Your suggestion is very userful. I can see the result.

1 Like

Hi Rene,
I see a lot of clns command like

sh clns neig
sh clns nei detail
sh clns interface

I didn’t find these commands in ISIS topic. Can you please explain their command outputs along-with topology?

Hello Bhupesh

CLNS stands for Connectionless-mode Network Service. This is an OSI Network layer service and is similar to the services provided by a combination of IP and UDP. IS-IS is compatible with this service and can be used to route CLNS traffic. However, CLNS is not as popular as TCP/IP, and has since been on the decline. Even so, it is supported by Cisco equipment, as you have seen in the commands you mentioned.

Because it is not part of the Cisco certification topics, it has not been included in the lessons.

I hope this has been helpful!

Laz

Hi Rene,

What is the difference between LSP and CSNP .When I did packet capture there are NLRI info in the both packet so I am bit confuse to understand the difference .

Thanks in advance

Hello BGP

ISIS uses various types of Protocol Data Unit (PDU) types in order to exchange routing information with peers. These include:

Link State PDUs (LSPs) - which advertise its neighbors and the destinations that are directly connected to the router itself.
Sequence Number PDUs (SNPs) - which contain a summary description of one or more LSPs. There are two types of SNPs, one of which is:

  • Complete Sequence Number PDU (CSNP) - which is used to send a summary of the LSP Database that a router has for a specific level.

You can find out more detailed information about these and other PDUs that are used by ISIS at the following CIsco documentation:

I hope this has been helpful!

Laz

Hi Rene,

This is the config that i am trying to understand . i tried to find on lines highlighted in bold. But, not finding much details on these. Please add these subtopics also.
Can you please help me to understand why do we use these commands ?

router isis IGN
 is-type level-2-only
 net xx.xxbb.00xx.000x.00xx.00
 log adjacency changes
 lsp-gen-interval maximum-wait 5000 initial-wait 1 secondary-wait 50
 lsp-refresh-interval 64000
 max-lsp-lifetime 65500
 purge-transmit strict
 address-family ipv4 unicast
  metric-style wide
  mpls traffic-eng level-2-only
  mpls traffic-eng router-id Loopback99
  mpls traffic-eng multicast-intact
  spf-interval maximum-wait 1000 initial-wait 150 secondary-wait 150
  distance 115 0.0.0.0/0 GEN-ACL
  distance 255
  segment-routing mpls
...

Thanks,
Bhupesh

Hello Bhupesh

  • lsp-gen-interval - This command is used to throttle, or regulate the frequency with which LSPs are generated
  • lsp-refresh-interval- The refresh interval determines the rate at which Cisco IOS software periodically transmits in LSPs the route topology information that it originates. This is done to keep the database information from becoming too old. Reducing the refresh interval reduces the amount of time that undetected link-state database corruption can persist at the cost of increased link utilization.
  • max-lsp-lifetime - Sets the maximum time for which LSPs persist without being refreshed. Values are in hours
  • metric-style wide - used to configure an IS-IS router to use the new-style type length value objects (TLVs)
  • mpls traffic-eng level-2-only - To configure a router running IS-IS so that it floods MPLS-TE link information into IS-IS level 2.
  • mpls traffic-eng router-id - To specify that the traffic engineering router identifier for the node is the IP address associated with a given interface
  • mpls traffic-eng multicast-intact - Allows PIM and MPLS to work together.
  • spf-interval - used to throttle or regulate SPF (shortest path first) calculations for IS-IS convergence
  • segment-routing - enables segment routing for IPv4 addresses within the MPLS data plane.

These are commands that deal with the use of IS-IS within an environment where MPLS is used. For more information, take a look at these Cisco documents:

If you’re interested in seeing a lesson on the site that has to do with one or more of these types of topics, feel free to make a suggestion at the Member Ideas page below. You may find that others have made similar suggestions to yours, and you can add your voice to theirs.

I hope this has been helpful!

Laz

Thanks Laz for the perfect response.I will take a look. Thanks for the document links also.

Thanking you,
Bhupesh Saini

1 Like

Why IS-IS mostly used in SP environment?? OSPF in enterprise network. Please mention some points

1 Like

Hello Muhammad

The fast answer is that IS-IS is more suited for larger networks. Indeed, it has been called the “de facto standard” for large service provider backbone networks. IS-IS outperforms OSPF as networks get larger. Conversely, OSPF was designed to work well in smaller networks and it works better than IS-IS.

There are several reasons for this, and it all has to do with the design of each routing protocol.

Both are link-state protocols and both use the Dijkstra algorithm. However:

  • OSPF was natively built to route IP, and actually relies on IP to function, while IS-IS is a purely Layer 2 protocol. This means that IS-IS is neutral regarding the type of network addresses that it can route. This fact made it easy for IS-IS to route IPv6, whereas OSPF had to be completely redesigned to support it.
  • The use of Level 1 and Level 2 routers as opposed to the various Areas used by OSPF makes IS-IS more scalable for extensively large networks. IS-IS does not require an Area 0 backbone, which is a limiting factor for OSPF when it comes to extensive scalability.

Now part of the fact that ISPs are using IS-IS also has to do with history. In the 1990s, ISPs generally chose IS-IS over OSPF due to the fact that Cisco’s implementation of IS-IS was much more stable at the time. It was kind of a no-brainer that you would choose IS-IS. Since then, OSPF has improved its stability for large networks, but the use of IS-IS still stuck. In addition, IS-IS seems to be more “tunable” in certain aspects that make it preferable by larger networks. And since the 1990s, IS-IS has been further developed to support the needs of larger networks.

So all of that together has caused ISPs to lean more towards IS-IS than OSPF for large networks.

I hope this has been helpful!

Laz

Mostly ISIS is used in SP and OSPF is used in enterprise networks
Wherever i read there is a statement made that ISIS is more scalable than OSPF
Can u explain how is ISIS more scalable than ospf with a real scenario
Thanks

Hello Anoop

Take a look at this post:

I hope this has been helpful!

Laz