OSPF ELECTION

[lauriusndipwashimwe]

Name a condition where OSPFrouters, although properly configured, would
choose not to conduct such election. Also briefly explain the rationale of this
behaviour.

[Joseph W. Doherty]

You mean a DR/BDR election?

For a p2p connection or if election priority set to zero. For the former, as connection is not multi-access a DR/BDR isn't needed. For the latter, you've manually configured the device not to participate in the DR/BDR election, which might be used in the situation where you have a very "low powered" router on the same multi-access segment as much more powerful routers.

[Leo Laohoo]

To all concerned,
It is very clear this thread is school work.
Help the students learn: Post the links to documents and (configuration guides).
Please refrain from handing out answers.

[Richard Burts]

As I read the original post my first thought was like @Joseph W. Doherty to suggest situations where OSPF priority was set to zero. But then my (perhaps a bit picky) tendency to think carefully about terminology kicked in and I focused on this phrase "choose not to conduct such election". With priority set to zero the router will still participate in the election - it just will not be elected. So I believe that the answers to the question will be about situations where OSPF network type is non broadcast/point to point.

HTH

Rick

[Joseph W. Doherty]

Rick, I wasn't sure whether setting the priority to zero kept the router from just being elected or actually excluded the router from being elected, so I did a quick search on that question, before I posted, and what I read said a zero priority router actual doesn't participate in the election. Or course, it's possible the information I read is mistaken, and I cannot, for myself, say for sure whether it does or doesn't participate. Are you sure?

[Richard Burts]

@Joseph W. Doherty I think I remember seeing this documented somewhere. But I am not able to find it today. So I fall back on this logic for the answer to the question. The election of BDR/DR (the BDR is elected first and DR is elected second) is based on sending OSPF hello messages which contain fields identifying the DR and BDR. The router with OSPF priority of zero will still send the hello messages with fields for DR and BDR. It just will not suggest itself for either function. Since it is sending and receiving messages with the fields used in the election I would say that it is participating in the election. A second point is that a router needs to know who is DR and who is BDR in its network so that it knows which neighbors it should form a full adjacency with. If the router with zero priority were not participating in the election, would it know which router won the election?

HTH

Rick

[Joseph W. Doherty]

Rick, that's good logic, but when in doubt, read the RFC. ;)

RFC2328, section 9.4 "Electing the Designated Router" describes the process. The election is done on each router as:

"The Designated Router election algorithm proceeds as follows:
Call the router doing the calculation Router X. The list of
neighbors attached to the network and having established
bidirectional communication with Router X is examined. This
list is precisely the collection of Router X's neighbors (on
this network) whose state is greater than or equal to 2-Way (see
Section 10.1). Router X itself is also considered to be on the
list. Discard all routers from the list that are ineligible to
become Designated Router. (Routers having Router Priority of 0
are ineligible to become Designated Router.) The following
steps are then executed, considering only those routers that
remain on the list:"

The above's "following steps" (NB: there are seven steps) refer to the "election logic", but as noted, before that's done, routers with a priority of zero are excluded from "election logic" analysis.

Laugh, I think we're down to a gray area of semantics. As zero priority routers are not considered by the "election algorithm", they might be considered excluded. But, supporting your logic, if you consider their initial exclusion, as part of the overall election process, then they are included.

That said, if you consider the zero priority routers don't "get a vote", even though they "show up at the polls", I think they should be considered as not being included in the election.

[Richard Burts]

@Joseph W. Doherty I agree this has become a matter of semantics. What does it mean to "participate" in the election? If participate means that you suggest yourself for election then clearly a router with priority of zero does not participate. But perhaps participate might mean having a vote? In that context I find this part of the RFC to be significant "The list of
neighbors attached to the network and having established bidirectional communication with Router X is examined." It seems to me that these are the "voters" in the election and would clearly include routers whose priority was zero.

So what does "participate" mean to the original poster?

HTH

Rick