From: Brian McGahan (brian@cyscoexpert.com)
Date: Mon Mar 17 2003 - 16:53:12 GMT-3
Stefan,
The DIS in IS-IS is like the DR/BDR in OSPF. However, the DIS
election is preemptive, and there is no backup DIS. There is one DIS
per broadcast segment per level.
To determine which router is the DIS, 'show clns is-neighbors'.
On a broadcast segment, the Circuit ID is the system ID of the DIS. In
addition to this, you can see the Pseudonode LSP in the ISIS database.
In the following example, there are two routers on a broadcast segment,
R1 & R6. DIS election is on a per level basis. Since R6 has a level-1
priority of 60, R1 is elected DIS for level-1 (R1 has default of 64).
However, since R6 has a level-2 priority of 70, it is elected DIS for
level-2.
R6#sh run int e0/0
Building configuration...
Current configuration : 142 bytes
!
interface Ethernet0/0
ip address 16.0.0.6 255.0.0.0
ip router isis
half-duplex
isis priority 60 level-1
isis priority 70 level-2
end
R6#sh isis databas
IS-IS Level-1 Link State Database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime
ATT/P/OL
R6.00-00 * 0x00000002 0xCA67 1083 0/0/0
R1.00-00 0x00000004 0x9670 1054 0/0/0
R1.01-00 0x00000001 0xF2C9 1077 0/0/0
IS-IS Level-2 Link State Database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime
ATT/P/OL
R6.00-00 * 0x00000001 0xF075 1078 0/0/0
R6.01-00 * 0x00000001 0x4F29 1079 0/0/0
R1.00-00 0x00000006 0xB681 1081 0/0/0
R6#
For the level-1 link state database, you can see that R1
generates is own LSP as well as an LSP for the pseudonode, denoted by
the R1.01-00 LSPID. For level-2, R6 is the DIS, hence the LSPID of
R6.00-00 and R6.01-00.
As for Frame-Relay, IS-IS does not run over hub and spoke,
therefore DIS election is not really an issue.
For more info on DIS election:
http://www.cisco.com/en/US/tech/tk472/tk474/technologies_tech_note09186a
0080094b42.shtml
HTH
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
Toll Free: 866.CyscoXP
Fax: 847.674.2625
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> stefan vogt
> Sent: Monday, March 17, 2003 9:20 AM
> To: ccielab@groupstudy.com
> Subject: ISIS and designated IS (DIS)
>
> Hi group,
>
> I have some questions regarding ISIS and designated IS (DIS).
>
> 1) I know that I can change the isis priority per interface. Is the
DIS
> eleceted per LAN segment or per area?
> 2) Is a DIS elected/required also for FR point-to-multipoint
connections?
> Which implications does this have to hub-and-spoke type
configurations?
> 3) What's the easiest way to figure out which router is elected as
DIS?
>
> Any input or link is appreciated.
>
> -Stefan
>
This archive was generated by hypermail 2.1.4 : Sat Apr 05 2003 - 08:51:41 GMT-3