Re: [Idr] New Draft Notification draft-dickson-idr-last-resort

Brian,
> 
> In all cases, a router will only announce a prefix if it is locally 
> selected as "best". LAST_RESORT affects local selection, but does not 
> change what happens once local selection is complete.
> 
> If after this explanation, you still believe a condition can exist where 
> a permanent routing loop would occur, please walk me through a specific 
> example, as I think that would be the best way to identify what leads 
> the the loop.

The problem occurs inside iBGP. For example, consider the following AS

NU1 ----1-----U1----1----NU2------100-----U2

NU1 and U2 are border routers that receive an eBGP path with same AS 
Path length, same MED, ... but NU1 receives the path with LAST_RESORT 
while U2's path does not contain LAST_RESORT. The number on the links 
indicate the IGP cost

NU1 and NU2 do not understand LAST_RESORT while U1 and U2 understand it.
There is an iBGP full mesh.

NU1 accepts the path and places it inside its FIB. Same for U2.

U1 understands LAST_RESORT and thus selects U2 as its BGP nexthop.
NU2 does not understand LAST_RESORT and thus selects NU1 as its BGP 
nexthop. There is a forwarding loop between U1 and NU2. This loop did 
not exist before the introduction of LAST_RESORT

Olivier

-- 
http://inl.info.ucl.ac.be , UCLouvain, Belgium
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