Designated Router (or DIS) concepts ..

The DR concept is used by both IS-IS and OSPF on the broadcast media to limit the amount of link state information exchanged between the routers on such media. It helps to reduce the number of adjacencies formed on broadcast media to O(n) instead of O(n^2), where n is the number of nodes.

First things first. There are no routers in IS-IS; there are only Intermediate Systems. So the leader and the evangelist for a LAN in IS-IS is called a DIS – Designated Intermediate System.

IS-IS

– DIS election is deterministic.
– No concept of backup DIS.
– A new DIS is elected when the current goes down.

OSPF

– DR election is non-deterministic.
– Elects DR and BDR to conduct flooding on a LAN.
– All routers on the LAN are only synchronized with the DR and BDR.
– DRship is sticky.

(i) What is meant by a deterministic/non-deterministic election?

In IS-IS, deterministic DIS election makes the possibility of predicting the router that will be elected as DIS from the same set of routers. The router advertising the numerically highest priority wins, with numerically highest MAC address breaking the tie. In IS-IS, DIS can be pre-empted at any time by a different IS-IS node (or a router in plain-speak) with higher priority coming alive.

In OSPF, the DR election is sticky meaning that after a router has been elected, no other router can take over the position unless the original DR goes down. When a router comes up, it accepts the DR regardless of its own priority if a DR is already there. Otherwise the router itself becomes DR if it has the highest priority on the network (a LAN to be precise).

The above scheme makes it harder to predict the identity of the DR, but ensures that DR changes less often. The rationale behind this sticky nature of DRship in OSPF is that it is disruptive to have DR changes as DR keeps track of which routers have acknowledged which link state information and it would require a lot of time and protocol messages for another router to take over in case the DR went down.

Both the sticky and deterministic mechanisms of DR/DIS elections in OSPF and IS-IS can be modified to provide the functionality of the other with some simple modifications in the implementations.

(ii) Do we need a backup Designated router or an Intermediate System?

A backup DIS is redundant in IS-IS because all the routers are synchronized with each other and also because the shorter Hello interval used by the DIS allows for faster detection of failures and subsequent replacement of the DIS.

The presence of BDR in OSPF makes the replacement of the DR transparent in case the DR goes down. All routers on the LAN are only adjacent and synchronized with DR and BDR; and backup DR is fully synchronized with the DR. Forming adjacencies with only the DR/BDR is done to reduce the complexity of data exchange and minimize flooding.