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Cisco CCNP / BSCI Certification: Route Redistribution And The Seed Metric |
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In the first part of this free CCNP / BSCI tutorial, we looked
at how leaving one simple word out of our route redistribution
configuration - "subnets" - resulted in an incomplete
routing table when redistributing routes from RIP to OSPF.
(If you missed that part of the tutorial, visit my website's
"Free Tutorials" section.) Today, we'll look at
redistributing OSPF routes into RIP and identify another common
redistribution error.
We are using a three-router network. R5 is running RIP, R1
is serving as a hub between R5 and R3 and is running RIP and
OSPF, and R3 is running OSPF.
To begin this lab, we'll add three loopbacks to R3 and advertise
them to R1 via OSPF.
R3(config)#int loopback33
R3(config-if)#ip address 33.3.3.3 255.255.255.255
R3(config-if)#int loopback34
R3(config-if)#ip address 34.3.3.3 255.255.255.255
R3(config-if)#int loopback35
R3(config-if)#ip address 35.3.3.3 255.255.255.255
R3(config-if)#router ospf 1
R3(config-router)#network 33.3.3.3 0.0.0.0 area 1
R3(config-router)#network 34.3.3.3 0.0.0.0 area 1
R3(config-router)#network 35.3.3.3 0.0.0.0 area 1
R1 sees all three of these routes in its routing table.
R1#show ip route ospf
34.0.0.0/32 is subnetted, 1 subnets
O IA 34.3.3.3 [110/65] via 172.12.123.3, 00:00:55, Serial0
35.0.0.0/32 is subnetted, 1 subnets
O IA 35.3.3.3 [110/65] via 172.12.123.3, 00:00:45, Serial0
33.0.0.0/32 is subnetted, 1 subnets
O IA 33.3.3.3 [110/65] via 172.12.123.3, 00:00:55, Serial0
We'll now redistribute these routes into RIP on R1. Remember
the "subnets" option we talked about in the first
part of this tutorial? There is no such option when redistributing
OSPF routes into RIP, as IOS Help shows us.
R1(config)#router rip
R1(config-router)#redistribute ospf 1 ?
match Redistribution of OSPF routes
metric Metric for redistributed routes
route-map Route map reference
vrf VPN Routing/Forwarding Instance
R1(config-router)#redistribute ospf 1
The routes have been redistributed into RIP with the redistribute
ospf 1 command. (The "1" is the OSPF process number.)
Let's look at R5 and see the results.
R5#show ip route rip
R5#
The routes aren't there, but we didn't get a warning from
the router that we needed to do anything else. What is the
problem?
The problem is that RIP requires a seed metric to be specified
when redistributing routes into that protocol. A seed metric
is a "starter metric" that gives the RIP process
a metric it can work with. The OSPF metric of cost is incomprehensible
to RIP, since RIP's sole metric is hop count. We've got to
give RIP a metric it understands when redistributing routes
into that protocol, so let's go back to R1 and do so.
R1(config)#router rip
R1(config-router)#no redistribute ospf 1
R1(config-router)#redistribute ospf 1 metric 2
R5 now sees the routes. Note that the metric contained in
the brackets is the seed metric.
R5#show ip route rip
34.0.0.0/32 is subnetted, 1 subnets
R 34.3.3.3 [120/2] via 100.1.1.1, 00:00:24, Ethernet0
35.0.0.0/32 is subnetted, 1 subnets
R 35.3.3.3 [120/2] via 100.1.1.1, 00:00:24, Ethernet0
33.0.0.0/32 is subnetted, 1 subnets
R 33.3.3.3 [120/2] via 100.1.1.1, 00:00:24, Ethernet0
If you read the previous tutorial, you may have noticed that
we did not specify a seed metric for OSPF. OSPF does not require
a seed metric to be set during redistribution. You also noticed
that the router did tell us that there might be a problem
when we left the "subnets" option out of RIP>OSPF
redistribution, but the router didn't tell us anything about
a seed metric when we performed OSPF>RIP redistribution.
This is a detail you must know by heart in order to make your
route redistribution successful!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage
(/www.thebryantadvantage.com), home of free CCNA and CCNP
tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP
Study Packages. Video courses and training, binary and subnetting
help, and corporate training are also available. Pass the
CCNA exam and CCNP exam with Chris Bryant, CCIE #12933!
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