From: Chua, Parry (Parry.Chua@xxxxxxxxxx)
Date: Tue Apr 02 2002 - 23:19:32 GMT-3
When due with multiple points redistribution, don't use filter to block the rou
te. Change the AD value to control which one should install in routing table. U
se higer metric(cost, hop etc) for redistributed route. This will prevent loop
and sub-optimal route. Redistribute a route learn from lower AD to a higer AD i
s not much of issues but not the other way.
Take your topology as reference:
Router R5 and R6 are the routers(RIP/OSPF) that we are concern with.
In OSPF, when redistribute RIP route into OSPF, it AD value now become 110 and
this advertise to all other OSPF domain, in this case, redistributed RIP route(
150.50.30.0) at R5 will be in R6, in R6, the orginal RIP learn route(via R2) wi
ll give way to OSPF due to AD value and prefer the path from R7 or R5.
To address this problem, at R5 and R6, in OSPF process, set the AD value of rou
te learn from RIP to 121(higher than RIP). In fact, route redistributed into OS
PF is type Ex, it is easy to change the AD value w/o route-map and access-list.
It is good practice to use both.
Next is the metric use when do redistribution, you should use higher(less prefe
r) metric as compare to the orginal route. Take RIP for example, there are 3 ho
p across the OSPF domain,
so we can set metric to 4 so it is less prefence and still reachable.
RIP is a classful routing protocol, the whole 150.50.0.0 will be in RIP and you
may want to filter network that not belong to RIP when redistribute RIP into O
SPF. The other issues is the FLSM and VLSM that need to take care off.
Parry Chua
-----Original Message-----
From: Gregg Malcolm [mailto:greggm@sbcglobal.net]
Sent: Wednesday, April 03, 2002 2:49 AM
To: ccielab@groupstudy.com
Subject: Mutual Redistribution Problem
Folks,
I'm having trouble with the following scenario. I set it up with the purpose
of playing with routing loops.
______ E0 - 150.50.30.2/27
|
R2
/ \
/ \
/ \
R5 R6
| |
| |
---------------------
|
R7
R2, R5 and R6 are on multipoint frame cloud. They run RIP in the address
range 150.50.100.0/27.
R5, R6 and R7 are on Ethernet. They run ospf in the address range
150.50.7.0/25
R5 and R6 do mutual redist of RIP/OSPF.
I have added a distribute-list on R5 and R6 that denies the 150.50.30 route
from re-entering OSPF via the Ethernet. Prior to that (as you can imagine),
OSPF's lower admin dist caused R5 and R6 to see the subnet as OSPF. I've
also prevented RIP from re-advertising the .30 subnet from OSPF. (I think :))
Problem is, when I do a trace from R7 to the .30 subnet, I get sub-optimal
routing where it bounces from R6 to R5 back to R6 and then to .30. Both R5
and R6 show the .30 route as RIP with a next hop of R2. I've played around
with DR selection thinking that might be the problem, but no change.
FLSM/VLSM issues have been addressed. R2 has 2 routes to the R5,R6,R7
Ethernet and R7 has 2 routes to the .30 subnet.
I'm fairly certain that I could remedy this with policy routing, but I'd
rather not. I'd like to know why. Any ideas ? This one is kickin my butt.
Sorry if the ASCII art format is screwed up.
Thanks, Gregg
This archive was generated by hypermail 2.1.4 : Thu Jun 13 2002 - 10:57:52 GMT-3