OSPF DR vs ISIS DIS, and Multiaccess Subnet Number


Like with DR (Designated Router) on OSPF, on broadcast multi-access, IS-IS elects a designated intermediate system (DIS) but keep in mind that unlike with OSPF where a Backup Designated Router is elected , on IS-IS there is no concept of a Backup DIS.
Then the pseudonode LSP is generated by the DIS. Like with the Network LSA (LSA Type 2) generated by a DR on OSPF which lists the attached routers, it represents the multi-access network itself and appears virtually as a pseudonode and the attached routers exchange the informations with the DR.
Also the DIS lists all attached routers and is see as the pseudonode LSP.Therefore the neighbor routers on the LAN send only advertisements to this pseudonode

Remember on OSPF ,in the Network LSA the metric is missing and it is is always equal to zero this prevents the pseudonode from adding a new cost to the SPF computing, the same thing is valid for the DIS,in the pseudonode LSP,the metric is zero.

But, what about the prefix of the multi-access network?

In OSPF, the DR is now responsible of advertising the Subnet Number for example 100.1.1.0/24 in our topology with the combination of two fields in the Type-2 LSA:

-Link State ID field which is the IP address of the DR in the multiaccess network (located in the LSA’s Header).
-Mask field located in the LSA’s Body.

This is to avoid non-DR to duplicate and carry the same infos (100.1.1.0/24) in their Type-1 LSA.

The result, other routers will compute that by ANDing the IP address of the DR with the network mask.

In ISIS, this is not the case, the Subnet Number of 100.1.1.0/24 is not carried in the Pseudonode LSP’ s DIS as we have seen previously with the Network LSA, rather each non-pseudonode isis router will carry this subnet in its own LSP L1 (similar to Router LSA).

The result is the duplication infos in LSPs of the non-pseudonodes routers.

Categories: OSPF Routing Protocol

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