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New Member

IS-IS System-ID to Hostname Mapping



When we configure NET for every router running IS-IS, NET consists of (Area ID, Sys ID and NSel)

Question is when i do 

# show clns is-neighbors


#show isis neighbors,


I expect to see the System ID which i used on all routers' NET.

For eg if my NET is  49.0003.1720.2500.0042.00 as per I configured, I expect the System ID to be 1720.2500.0042 when I use the above verification commands.

But, what i instead see is the hostname I have given to the Router. 

R1#show clns is-neighbors

System Id      Interface   State  Type Priority  Circuit Id         Format
R2             Se0/0       Up     L1   0         00                 Phase V


R1's neighbor is R2 with System ID (1720.2500.0042), BUT I am not able to see the same under System ID column and I see instead the hostname.


Why is this? 

In TCP/IP Routing -Volume 1book, I see the actual System ID in the given examples.


But, not here.


Can anyone shed some light on this?




Everyone's tags (1)
Hall of Fame Super Silver

Hello Chinar,there is a

Hello Chinar,

there is a specific TLV in the ISIS LSP that tells what the hostname is, output of the show isis neig and other show commands have been modified to show hostnames instead of system-ids as a way to help us in troubleshooting.

probably at the time of Jeff Doyle's book writing this automatic mapping was not performed by IOS


You can still see the details when you look at the ISIS database.

Hope to help




New Member

That was Spot on Giuseppe.Got

That was Spot on Giuseppe.

Got my answer.

Actually, new to IS-IS in a SP environment, hence i have queries.


Must say, IS-IS is very underrated. It's neat and scalable.



New Member

Hello Chinar, you can change

Hello Chinar,


you can change this default behaviour under the router isis configuration. Enter "no hostname dynamic"; after this change you will see

R3#show isis neighbors

Tag null:
System Id      Type Interface   IP Address      State Holdtime Circuit Id
0000.0000.0001 L2   Gi3/1/1.171 UP    22       00
0000.0000.0002 L2   Te4/2.2000 UP    22       00

or alternatively you can use "show isis hostname", which will display the System IDs as well.





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