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ters can be applied on the other protocols to prevent loops.
203.250.15.0
255.255.255.192
RTC
S1
15.68
E0
OSPF
15.67
E0
203.250.15.0
255.255.255.192
RTA
E1
15.194
RIP
15.130
15.1
15.2
S0
To illustrate, suppose RTA, RTC, and RTE are running RIP. RTC and RTA are
also running OSPF. Both RTC and RTA are doing redistribution between RIP
and OSPF. Let us assume that you do not want the RIP coming from RTE to
be injected into the OSPF domain so you put a passive interface for RIP
on E0 of RTC. However, you have allowed the RIP coming from RTA to be
injected into OSPF. Here is the outcome (DO NOT USE THE FOLLOWING CONFIG-
URATION):
RTE#
interface Ethernet0
ip address 203.250.15.130 255.255.255.192
interface Serial0
ip address 203.250.15.2 255.255.255.192
router rip
network 203.250.15.0
RTC#
interface Ethernet0
ip address 203.250.15.67 255.255.255.192
interface Serial1
ip address 203.250.15.1 255.255.255.192
router ospf 10
redistribute rip metric 10 subnets
network 203.250.15.0 0.0.0.255 area 0
router rip
redistribute ospf 10 metric 2
passive-interface Ethernet0
OSPF DESIGN GUIDE-NSA group
April 25, 1996
41
RIP
E0
RTE
network 203.250.15.0
RTA#
interface Ethernet0
ip address 203.250.15.68 255.255.255.192
router ospf 10
redistribute rip metric 10 subnets
network 203.250.15.0 0.0.0.255 area 0
router rip
redistribute ospf 10 metric 1
network 203.250.15.0
RTC#sh ip rou
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
default
Gateway of last resort is not set
203.250.15.0 255.255.255.192 is subnetted, 4 subnets
C
203.250.15.0 is directly connected, Serial1
C
203.250.15.64 is directly connected, Ethernet0
R
203.250.15.128 [120/1] via 203.250.15.68, 00:01:08, Ethernet0
[120/1] via 203.250.15.2, 00:00:11, Serial1
O
203.250.15.192 [110/20] via 203.250.15.68, 00:21:41, Ethernet0
Note that RTC has two paths to reach 203.250.15.128 subnet: Serial 1 and
Ethernet 0 (E0 is obviously the wrong path). This happened because RTC
gave that entry to RTA via OSPF and RTA gave it back via RIP because RTA
did not learn it via RIP. This example is a very small scale of loops
that can occur because of an incorrect configuration. In large networks
this situation gets even more aggravated.
In order to fix the situation in our example, you could stop RIP from
being sent on RTA’s Ethernet 0 via a passive interface. This might not be
suitable in case some routers on the Ethernet are RIP only routers. In
this case, you could allow RTC to send RIP on the Ethernet; this way RTA
will not send it back on the wire because of split horizon (this might
not work on NBMA media if split horizon is off). Split horizon does not
OSPF DESIGN GUIDE-NSA group
April 25, 1996
42
allow updates to be sent back on the same interface they were learned
from (via the same protocol). Another good method is to apply distribute-
lists on RTA to deny subnets learned via OSPF from being put back into
RIP on the Ethernet. The latter is the one we will be using:
RTA#
interface Ethernet0
ip address 203.250.15.68 255.255.255.192
router ospf 10
redistribute rip metric 10 subnets
network 203.250.15.0 0.0.0.255 area 0
router rip
redistribute ospf 10 metric 1
network 203.250.15.0
distribute-list 1 out ospf 10
And the output of RTC’s routing table would be:
RTF#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate
default
Gateway of last resort is not set
203.250.15.0 255.255.255.192 is subnetted, 4 subnets
C
203.250.15.0 is directly connected, Serial1
C
203.250.15.64 is directly connected, Ethernet0
R
203.250.15.128 [120/1] via 203.250.15.2, 00:00:19, Serial1
O
203.250.15.192 [110/20] via 203.250.15.68, 00:21:41, Ethernet0
16.0 Injecting defaults into OSPF
An autonomous system boundary router (ASBR) can be forced to generate a
default route into the OSPF domain. As discussed earlier, a router
becomes an ASBR whenever routes are redistributed into an OSPF domain.
However, an ASBR does not, by default, generate a default route into the
OSPF routing domain.
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