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

[ciscomoderator]

            With 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.

[Jerome Chandrahasa]

Hi Vignesh,

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

Thanks,

Jerome

[Vignesh Rajendran Praveen]

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

[Jerome Chandrahasa]

Hi Vignesh,

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

Thanks,

Jerome

[Vignesh Rajendran Praveen]

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

[Karthi Sivalingam]

Hi Vignesh,

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

Thanks & Regards,

Karthi.S

[Vignesh Rajendran Praveen]

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

[Karthi Sivalingam]

Hi Vignesh,

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

Thanks & Regards,

Karthi.S

[Vignesh Rajendran Praveen]

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

[Sridhar Anbalagan]

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