Sorry. Wasn't trying to suggest Howard was wrong, just providing the source of where I read the information.
Everything Howard mentions makes sense. But when there's a single ASBR, it seems that there's no difference in E1 or E2 other than E1 give ya the cost to the external for free. I'm planning to use E1s in this situation but wanted to float this out to the list to see if there might be unexpected consequencies. From the feedback so far, there doesn't seem to be. Thanks Peter van Oene wrote: > > 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=58479&t=58454 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
