One quick point below. Trimmed rest. Question from Jenny
> >One thing I'm not clear on, though, is why the problem (reportedly) > >happened before we upgraded to IOS 12.1 - so before a route to null0 was > >used for the summarised networks (we didn't add one manually). Any >ideas? > > I can understand why it's happening now, so this is more for my >curiosity > >and understanding. Correct me if I'm wrong, the post 12.1, IOS adds the null on ABR's when area-ranges are used? In any event, adding a null route for a summary address is usually a good thing. Although these null routes do nothing to enhance reachability, they do prevent traffic from looping when reachability is lost. In your case, if your non backbone area was partitioned and traffic arrived at the ABR which had no specific routes for the destination in question, this traffic would be forwarded toward default (or another less specific summary) assuming the null route didn't exist. Hence, for 192.168.1.1 as an example, your core routers might like the 192.168/16 route from ABR1 who might like your core routers 0.0.0.0/0 in which case you'll generate lots of useless forwarding. Adding the null route here would simply discard the traffic gracefully. Interestingly, if your ABR1 happened to default toward ABR2 for some reason, not having the null route would actually enable you to route around the problem. However, this would only help half the area and implementing a default on ABR2 toward ABR1 at the same time to fix the other half would be a case of not looking at the whole picture :) Some topologies might actually benefit from a design of this nature which might explain why the null route wasn't automatically added before. However, more topologies would benefit from having it which likely explains why Cisco changed their default behavior. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=40593&t=40269 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
