I’m currently doing a CCNA course. I’m looking at how routers exchange information. One thing that has come up in passing is the subject of bounded updates, which means that, “the propagation of partial updates are automatically bounded so that only those routers that need the information are updated.”
The definition given above is all I need to know at this level. But out of curiosity, what does this mean, exactly. How does a router determine which other routers “need to know” and on what basis?
Even the instructor doesn’t know. He told the class that he spent several hours trying to find that out, without success. But then he didn’t know about The Straight Dope.
My guess is that they mean that updates to the table are only sent to the same AS (Autonomous System). Partial updates would only be sent to routers that need the updates (i.e. border routers and the like).
It’s all part of the endless struggle to manage routing tables and speed up lookups and modifications to the table.
Cisco really hates being clear on that sort of stuff, don’t they? I poked around a bit, and as far as I can tell, they only list bounded updates when describing EIGRP.
An EIGRP router only sends updates to routers that it has formed a neighbor relationship with. (Neighbor relationships are formed for routers that pass authentication, have the same AS number, receive/send updates from the primary connected subnet on the interface and use the same K numbers - i.e., any messing with the metric factorization has to match. )
And it will not announce a network to a neighbor that it uses for traffic to that same network - EIGRP’s version of split horizon.
These are the only two parts of the algorithm that bound the update scope that I can locate.
(Not really relevant to updates per se, but routing protocol traffic can be further reduced by manually configuring a router as a stub router - much like a stubby area in OSPF, stub routers will never be used for transit traffic. The neighbors learn of the stub status when forming the relationship and won’t send route queries to the stub router if they need to locate a feasible route to a given subnet.)
Good luck on the CCNA - remember, if any of the options make Cisco look good, that’s the correct answer. (I’m currently boning up for the written CCIE, admittedly mostly to refresh my CCNP and CCDA.)