From: Joe Rinehart (jjrinehart@hotmail.com)
Date: Tue Jul 06 2004 - 15:42:52 GMT-3
I have a question that has been bugging me for a little while and would like
to see what the overall opinion is on the subject.
When redistributing routing protocols (I am speaking here mainly of IGP
redistribution), I know that route feedback can be an issue, especially
where mutual redistribution is concerned. Something I have been doing in
those instances is putting in filters and route maps to prevent that.
Here is the basic methodology, per protocol:
1. Create an access list with the routes to be filtered (i.e., those
already in that IGP that shouldnt be coming back in from another source).
This is a series of deny statements followed by a permit any statement.
2. Create a route map calling that access list.
3. Configure the redistribute statement under the target IGP calling that
route map.
Here is an example below, you may recognize it from CPSV2:
router eigrp 2003
redistribute ospf 1 route-map filter-eigrp
redistribute rip route-map filter-eigrp
passive-interface default
no passive-interface Serial0/0.103105
network 175.10.16.0 0.0.0.255
network 175.10.120.0 0.0.0.255
default-metric 1544 100 254 1 1500
no auto-summary
no eigrp log-neighbor-changes
!
router ospf 1
log-adjacency-changes
area 0 authentication message-digest
area 500 stub
redistribute connected route-map CONNECTED
redistribute eigrp 2003 subnets route-map filter-ospf
redistribute rip subnets route-map filter-ospf
network 10.1.1.0 0.0.0.255 area 500
network 192.168.1.0 0.0.0.255 area 100
network 192.168.2.0 0.0.0.255 area 0
!
router rip
version 2
redistribute eigrp 2003 route-map filter-eigrp
redistribute ospf 1 route-map filter-eigrp
passive-interface default
no passive-interface Serial0/0.100
network 175.10.0.0
default-metric 5
no auto-summary
access-list 1 deny 175.10.1.0 0.0.0.255
access-list 1 deny 175.10.16.0 0.0.0.255
access-list 1 permit any
access-list 2 deny 175.10.0.0 0.0.63.255
access-list 2 permit any
access-list 3 deny 10.10.1.0 0.0.0.255
access-list 3 deny 10.10.2.0 0.0.0.255
access-list 3 deny 192.200.0.0 0.0.255.255
access-list 3 deny 192.168.0.0 0.0.31.255
access-list 3 permit any
dialer-list 1 protocol ip permit
route-map filter-ospf permit 10
match ip address 3
!
route-map filter-eigrp permit 10
match ip address 1
!
route-map filter-rip permit 10
match ip address 2
Here is my question. I KNOW that this approach works rather splendidly and
can take pretty much everything into account as a template overall, but is
this the kind of thing that can cost you points on the exam? I know that
the rules of thumb are if they dont forbid something you are allowed to do
it, and ultimately the other test is if it works, but I also know there is a
subjective aspect as well.
Thoughts?
Joe Rinehart
AT&T
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