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Ask the Expert: Understanding, Configuring, and Troubleshooting Intermediate System-to-Intermediate System Routing Protocol for IP on Cisco Routers

            Read the bioWith Vignesh R.P.

Welcome to the Cisco Support Community Ask the Expert conversation. This is an opportunity to learn and ask questions about configuring and troubleshooting the Intermediate System-to-Intermediate System (IS-IS) routing protocol for IP on Cisco routers. 

The IS-IS routing protocol is an Interior Gateway Protocol (IGP) standardized by the Internet Engineering Task Force (IETF) and commonly used in large service provider networks. IS-IS may also be deployed in extremely large enterprise networks. IS-IS is a link-state routing protocol, providing fast convergence and excellent scalability. Like all link-state protocols, IS-IS is very efficient in its use of network bandwidth.

Vignesh R. P. is a customer support engineer in the Cisco High-Touch Technical Support center in Bangalore, India, supporting Cisco's major service provider customers in routing and Multiprotocol Label Switching (MPLS) technologies. His areas of expertise include routing, switching, and MPLS. Previously at Cisco he worked as a network consulting engineer for enterprise customers. He has been in the networking industry for eight years and holds CCIE certification in the routing and switching and service provider tracks. 

Remember to use the rating system to let Vignesh know if you have received an adequate response. 

Vignesh might not be able to answer each question due to the volume expected during this event. Remember that you can continue the conversation in Network Infrastructure,  sub-community WAN, Routing and Switching discussion forum shortly after the event. This event lasts through December 20, 2013. Visit this forum often to view responses to your questions and the questions of other community members.

23 REPLIES
New Member

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hi Vignesh,

I have not used ISIS much but would still like to know what are the different types of ISIS packets.

Regards,

Palani

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hello Palani,

In ISIS the packets are known as Protocol Data Units (PDUs) and the different type of PDUs are Hellos, LSP, CSNP & PSNP. A brief about them is as below.

1) Hello - Used establish & maintain adjacencies

2) LSP - Link state PDUs are used to advertise link state information.

3) CSNP - Complete sequence number PDU is a type of update which contains the complete list of list of LSPs known to the router.

4) PSNP - Partial sequence number PDU is used to acknowledge a routing update (LSP) on point-to-point links and to request missing information about a route after receiving a CSNP.

Hope this helps.

Thanks & Regards,

Vignesh R P

New Member

Ask the Expert: Understanding, Configuring, and Troubleshooting

Thanks a lot for the response Vignesh.

New Member

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hi Vignesh,

And what are the types of network topology supported by ISIS ?

Regards,

Palani

Re: Ask the Expert: Understanding, Configuring, and Troubleshoot

Hello Palani,

ISIS supports broadcast, unnumbered broadcast & point-to-point network types.

Thanks & Regards,

Vignesh R P

New Member

Ask the Expert: Understanding, Configuring, and Troubleshooting

Thanks for the prompt response.

Re: Ask the Expert: Understanding, Configuring, and Troubleshoot

Hi Vignesh,

I don't know almost anything about IS-IS so my first question is about its purpose: what new/unique feature(s) will it bring into my network so I can justify all new investments into people? I guess this is another way to ask the same question: why should I replace existing link state protocol (in my case OSPF) with IS-IS?

Thanks,

Tenaro

Re: Ask the Expert: Understanding, Configuring, and Troubleshoot

Hello Tenaro,

Usually ISIS is used in Service Provider networks & very large Enterprise Networks. Whether to use OSPF or ISIS as an IGP is purely a personal choice & depends on the network design.

Below are the common positives of using ISIS.

1) Proven to be stable and scalable, with very fast converegnce.

2) IS-IS rides directly above layer two, which offers a security advantage (IS-IS attacks cannot be routed).

3) ISIS supports both the IPv4 & IPv6 routed protocols in a single router process.

Thanks & Regards,

Vignesh R P

Re: Ask the Expert: Understanding, Configuring, and Troubleshoot

Hello Tenaro,

You can also read through the below link to understand the further differences between ISIS & OSPF whenever time permits.

http://tools.ietf.org/html/draft-bhatia-manral-diff-isis-ospf-01

Thanks & Regards,

Vignesh R P

Cisco Employee

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hi Vignesh,

Thank you for hosting this great topic!

I would like to ask - and perhaps raise a personal feature enhancement request - regarding the default-information originate command in IS-IS. Currently, it does not appear to support any kind of metric manipulation that would allow to set the initial metric of the default route injected by this command. This makes it difficult, if not impossible, to explicitly inject multiple default routes into an area with differing initial metrics, thereby making one of them preferred. See, for example, this thread where this was requested:

https://supportforums.cisco.com/message/4100005#4100005

Do you believe this feature could be added to IS-IS at some point in future? Thank you!

Best regards,

Peter

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hello Peter,

Kindly confirm if the below sample configuration would be helpful in acheiving your requirement.

router isis

net 49.0001.1720.1600.1001.00

default-information originate route-map sample

!

route-map sample permit 10

set metric 5

Thanks & Regards,

Vignesh R P

Cisco Employee

Ask the Expert: Understanding, Configuring, and Troubleshooting

Vignesh,

Thank you very much for your answer. I could swear I have tested the same approach using the route-map and it did not work but obviously I have made some mistake in my experiment. Right now, I have reproduced the configuration and it is working as expected. Seems I need to double-check my experiments before jumping to conclusions.

As we have opened the issue of metrics in IS-IS, though, may I have another set of questions?

  1. With the traditional narrow metrics, one of the bits in the metric expresses internal or external metric type. Is there any sensible usage for these bits in Cisco's implementation, i.e. can they be used in any way to influence best path selection?
  2. With the setting of metric-style transition, how exactly do the routers operate with respect to metric advertisements and path selection? Can a transition be made from narrow to wide metrics through the use of transition without causing outages in a network?

Thank you!

Best regards,

Peter

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hello Peter,

Redistribution from any other routing protocol, static configuration, or connected interfaces is allowed in any type of router (Level 1 and Level 2). By default the metric type will be set as internal, which means that the metric of the route will compete with all other internal routes. Metric type may be set to external. In that way the prefix will have a metric equal to the cost specified in the redistribution command plus a value of 64.

Although the metric is increased if the metric is flagged as external on redistribution, the internal/external bit used to increase the metric is actually ignored when calculating routes unless the use of external metric is specified in the configuration.

Hope the above explanation answers the first question.

Thanks & Regards,

Vignesh R P

Re: Ask the Expert: Understanding, Configuring, and Troubleshoot

Hello Peter,

When metric-style transition is used routers advertise both metrics. If there is a router that only understands narrow metrics, it uses that, and others that understand both use wide metrics for path selection. And I have not come across complaints from customers, that network outages happen while transitioning from narrow metrics to wide metrics through the use of "metric-style transition".

Hope this answers the second question.

Thanks & Regards,

Vignesh R P

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hi Vignesh,

Does ISIS have an automatic metric calculation like OSPF, which calculates metric based on the bandwidth?

Thanks,

Jerome

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hello Jerome,

ISIS interface cost is always 10 and is not dynamic like OSPF. But this can be maually changed by "isis metric" interface command.

Hope this helps.

Thanks & Regards,

Vignesh R P

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hi Vignesh,

Also let me know the differences with respect to backbone requirements between ISIS and OSPF.

Thanks,

Jerome

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hello Jerome,

OSPF requires that an area be defined as a backbone area and that each other area border that backbone area. Special configuration (a virtual link) is required for any area that does not border the backbone area. IS-IS backbone routers can reside in any area. There merely must be an unbroken chain of Level 2 or Level 1/2 routers in order for the backbone to function.

Hope this helps.

Thanks & Regards,

Vignesh R P

New Member

Ask the Expert: Understanding, Configuring, and Troubleshooting


Hi Vignesh,

Kindly throw some light on the ATT Bit used in ISIS.

Thanks & Regards,

Karthi.S

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hello Karthi,

The ATT bit is set in the Level 1 LSP by a L1-L2 router if it has connectivity to another area. It indicates to the area routers (Level 1) that it is a potential exit point of the area.Level 1 routers select the closest (best metric) Level 2 router with the ATT-bit set.ATT bit has no meaning in L2 LSP.

Hope this provides a good understanding.

Thanks & Regards,

Vignesh R P

New Member

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hi Vignesh,

Also let me know when SPF & PRC would be executed in case of ISIS

Thanks & Regards,

Karthi.S

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hello Karthi,

The SPF is run only when the whole topology has to be calculated & the SPF tree need to built/re-built.

The PRC (Partial Route Calculation) is executed when IP routing information has to be calculated. If an IS recieves an LSP where only IP information has changed then it will run only PRC and not the whole SPF saving a lot of CPU.Since IP prefixes are just the leaves of the SPF tree running the PRC alone suffices.

Thanks & Regards,

Vignesh R P

New Member

Ask the Expert: Understanding, Configuring, and Troubleshooting

Hi Vignesh,

I read through the whole event and found it very useful. Could you also touch upon the usage of Overload Bit in ISIS.

Regards,

Sridhar

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