Veryifying IS-IS DIS / Pseudonode

This has been really unclear to me until now, I haven’t seen it well explained in any resource I’ve seen so I’ll put it up here for others to see. IS-IS uses the DIS (Designated IS) to perform a similar function to the OSPF DR. The reason this is necessary is on multiaccess networks (eg ethernet) there would be a requirement for every IS to have a neighbor and send SNPs for every other neighbor. This causes a similar problem in BGP where n(n-1) neighbors / updates are required.

For example If I have 3 routers on an ethernet LAN, 3(3-1) = 6 neighbor sessions and 6 updates every interval and so on

Now the router elected DIS (by whatever means..) acts as a pseudonode. This means that all routers on the multiaccess network have a neighborship with the pseudonode and send updates to the pseudonode. This goes for the DIS as well. This way there is a fake logical IS router in the middle of the multiaccess network and it becomes a group of point-to-point neighbors over a multiaccess network (this is how it is explained in RFC1195).

Now onto the point of this, how to verify the DIS/pseudonode

The router elected as the DIS can be identified like this;

R0#show clns int fa0/0
FastEthernet0/0 is up, line protocol is up
Checksums enabled, MTU 1497, Encapsulation SAP
ERPDUs enabled, min. interval 10 msec.
CLNS fast switching enabled
CLNS SSE switching disabled
DEC compatibility mode OFF for this interface
Next ESH/ISH in 35 seconds
Routing Protocol: IS-IS
Circuit Type: level-1-2
Interface number 0×0, local circuit ID 0×1
Level-2 Metric: 10, Priority: 127, Circuit ID: R0.01
DR ID: R0.00
Level-2 IPv6 Metric: 10
Number of active level-2 adjacencies: 2
Next IS-IS LAN Level-2 Hello in 1 seconds

The pseudonode can be verified in the database because there is a duplicate IS router with a modified name;

R0#show isis database

IS-IS Level-2 Link State Database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
R0.00-00 * 0×00000001 0x3E0B 444 0/0/0
R0.01-00 * 0×00000002 0xE66A 472 0/0/0
R2.00-00 0×00000002 0xACC6 469 0/0/0
R3.00-00 0×00000003 0x2BC3 469 0/0/0
R4.00-00 0×00000002 0xF840 449 0/0/0

To interrogate this further you can look at the detailed output and see that there are only ISIS neighbors between each IS router and the pseudonode (note that R4 is not on the ethernet segment but lies on a point-to-point link off R3;

R0#show isis database detail

IS-IS Level-2 Link State Database:
LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL
R0.00-00 * 0×00000002 0x3C0C 1128 0/0/0
Area Address: 49.0000
NLPID: 0xCC
Hostname: R0
IP Address: 10.1.1.1
Metric: 10 IS R0.01
Metric: 0 IP 10.1.1.1 255.255.255.255
Metric: 10 IP 10.23.0.0 255.255.0.0
R0.01-00 * 0×00000002 0xE66A 366 0/0/0
Metric: 0 IS R0.00
Metric: 0 IS R2.00
Metric: 0 IS R3.00
R2.00-00 0×00000003 0xAAC7 1115 0/0/0
Area Address: 49.0000
NLPID: 0xCC
Hostname: R2
IP Address: 10.2.2.2
Metric: 10 IS R0.01
Metric: 0 IP 10.2.2.2 255.255.255.255
Metric: 10 IP 10.23.0.0 255.255.0.0
R3.00-00 0×00000004 0x29C4 1117 0/0/0
Area Address: 49.0000
NLPID: 0xCC
Hostname: R3
IP Address: 10.3.3.3
Metric: 10 IS R0.01
Metric: 10 IS R4.00
Metric: 0 IP 10.3.3.3 255.255.255.255
Metric: 10 IP 10.23.0.0 255.255.0.0
Metric: 10 IP 10.34.34.0 255.255.255.0
R4.00-00 0×00000002 0xF840 342 0/0/0
Area Address: 49.0000
NLPID: 0xCC
Hostname: R4
IP Address: 10.4.4.4
Metric: 10 IS R3.00
Metric: 0 IP 10.4.4.4 255.255.255.255
Metric: 10 IP 10.34.34.0 255.255.255.0

Note this doesn’t affect the way packets are forwarded, only the logical topology of the neighbors used for sending updates!

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