From: Jongsoo.Kim@Intelsat.com
Date: Tue Mar 22 2005 - 02:34:27 GMT-3
This is just my summary note but I post here maybe some of you can share
some thoughts.
Any comment is welcomed.
First, most ISDN task is asked for back-up link to create a full
reachability to stub router in case of F/R link failure.
This keyword is to restore full reachability. It is only possible by 1) two
floating static routes ( default route from stub router to the rest of
network and a specific static route from the other router to the stub
router), 2) ip ospf demand circuit or Rip tigger w/ more metric , 3) Dynamic
routing protocol w/ dialer watch group and supressing routing protocol over
ISDN, 4)back-up link 5) Rip w/ Snapshot routing.
1)Two floating static route is very straight forward and interesting traffic
could be all ip packet.
All IP traffic can be interesting traffic
2-1) ip ospf demand circuit is also straight forward and "no peer
neighbor.." is important to prevent link flapping.
The dynamic link cost needs to be something high to play a similar role as
floting route with higher AD.
I don't think ip OSPF cost need any value . I think OSPF Demand link has a
high cost by default.
All IP traffic can be interesting traffic.
2-2) Rip trigger will require higher metric ( off-set). Otherwise, traffic
may continue to flow over ISDN after F/R is restored.
All IP traffic can be interesting traffic. To function as back-up link, Rip
need to run over ISDN as trigger mode and F/R as any mode.
3) If Dialer watch group is configured to bring up ISDN link, an IGP
protocol should run across ISDN link to reslove full reachability.
It is because dialer watch group doesn't create any route itself in to route
look-up table. But what is important is to make this IGP protocol hello or
update not interesting traffic. This will make normall case, igp peering
down, but when dialer watch group trigger, igp will be able to send hello so
that igp will be able to update routing table, which will create full
reachability. If IGP is OSPD, then ip ospf demand circuit will cover most
case of back-up. But dialer watch group can be used to meet a specific task
such as dial ISDN if Vlan X dispear from routing table even if frame-relay
is up, or ISDN should be up until F/R is up or until VLAN X show up again on
ISDN. Ip OSPF demand will can make ISDN up when there is topological change
but as long as topology is stable even with F/R down and if there is no
interesting traffic, the ISDN link will eventually go down. Also some
other advantage of dialer watch is that it doesn't care of topologic change
while IP ospf demand or rip tigger will dial isdn when there is topological
changes that have nothing to do with F/R link.
4) Back-up link is much simpler than dialer watch. It doesn't have to
supress IGP hello. It doesn't need high IGP link cost. For this reason,
most task will ask not to use back-up link, which is the problem.
5) Snapshot routing is basically used to supress rip route update over ISDN.
With higher rip metric over ISDN, it can function as back-up link if rip
runs over f/r link and ISDN.
Thanks
Jongsoo
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