Some thoughts below On Tue, 2002-12-03 at 13:26, p b wrote: > Comments inline: > > Howard C. Berkowitz wrote: > > > > At 5:00 PM +0000 12/3/02, p b wrote: > > >One of the cisco press books indicates one should use > > >type 1 externals when the route is being advertised by > > >>1 ASBR and type 2 externals when there's a single > > >ASBR. > > > > This is just plain wrong. The reason you have E1 and E2 is to > > have > > different routing policies. > > > > E1 enforces a closest-exit policy which gives a degree of load > > sharing. > > > > E2 enforces a best-exit policy. For example, you might have > > one fast > > link to an ISP and one dial backup link, or a primary and a > > backup > > provider. In both cases, you want an E2 because you always > > want to > > go to a specific exit UNLESS there is a failure. > > See ACRC (Chappel), page 217. Under E1 explanation "...Use > this packet type when you have multiple ASBRs advertising a > route to the same AS" > > Under E2 explanation "... use this packet type if only one router > is advertising a route to the AS..."
I'd go with Howard on this one ;-) E1 metrics simply let routers find the closest exit from the AS (so long as the external side of the metrics are relatively consistent) > > > > > > > > >Are there any issues if one uses type 1 external even > > >when the route is being advertised by a single ASBR? It > > >would seem useful, given the cost to the external is > > >compatible with the costing used in the OSPF network, to > > >use type 1 externals even if the route originates from a > > >single ASBR. The benefit being able to get a meaningful > > >cost value to the external. > > > > Why? If there's only one connection to the outside, does the > > internal > > cost really matter if you have to go there? > > Is there no benefit to knowing the cumulative cost? Is > there a benefit to knowing an E2 cost which has no cost > meaning within the OSPF AS? As mentioned, there is only > a single ASBR advertising this route, but there may be many > paths to this ABSR. So if there's no overhead with using a > type 1 here, why not use it and get the cost information? The path to the ASBR, or forwarding address if it isn't 0.0.0.0, comes out of the routing table. Hence, the router already knows the best path to ASBR. Having it represented in OSPF simply changes the outcome of the route selection process when there are mulitple entries for the same destination. In many cases, as Howard points out, you want all routers in the same AS to prefer ASBR1 over ASBR2 for the same destination. This is what routing policies are all about. In these cases, you simply set E2 metrics accordingly and accomplish your goal. Again, it's a matter of trying to figure out what you are trying to accomplish (what problem are you trying to solve) and picking the right tools to solve it. E1 and E2 are simply additional tools that can enable different routing strategies. > > > > > > > > > >Is there any unexpected issues which might arise when > > >doing this? Flooding of LSAs or SPF aren't imapcted > > >if a route is an E1 or E2, right? > > > > > >Thanks Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=58475&t=58454 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
