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Posts Tagged ‘exam’

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
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If there’s one CCNP / BSCI topic that looks so easy but can lead to a real headache, it’s route redistribution. I’m not even talking about the routing loops and suboptimal routing that can result when route redistribution is done without proper planning – I’m talking about the basic commands themselves. Leaving out one single command option, or forgetting what else needs to be redistributed when redistributing dynamically discovered routes, can leave you with a routing table that looks complete but does not result in full IP connectivity.

In this free CCNP / BSCI tutorial series, we’ll take a look at three common errors in route redistribution configurations, and how to fix them. We’ll use three routers, R1, R3, and R5. R1 and R5 are in a RIPv2 domain and R1 and R3 are in an OSPF domain. R1 will be performing two-way route redistribution.

R5 is advertising its loopback, 5.5.5.5/24, into the RIPv2 domain. R1 sees this route in its RIP routing table:

R1#show ip route rip

5.0.0.0/24 is subnetted, 1 subnets

R 5.5.5.0 [120/1] via 100.1.1.5, 00:00:01, Ethernet0

For R3 to see this route, route redistribution must be configured on R1. We’ll use the redistribute rip command to do so.

R1(config)#router ospf 1

R1(config-router)#redistribute rip

% Only classful networks will be redistributed

The router immediately gives us a message that “only classful networks will be redistributed”. What does this mean? Let’s go to R3 and see if that router is receiving this route.

R3#show ip route ospf

< no output >

When we get no result from a show command, that means there’s nothing to show. The only routes that will be successfully redistributed with the current configuration on R1 are classful networks, and 5.5.5.0/24 is a subnet.
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When studying for your BSCI exam for the CCNP, you get your first taste of BGP. One of the major differences between BGP and the other protocols you’ve studied to date is that BGP uses attributes to describe paths, and to influence the selection of one path over the other.

In this free tutorial, we’re going to take a look at the Local Preference attribute and compare it to the Cisco-proprietary BGP attribute “weight”.

The Local Preference (LOCAL_PREF) attribute is used to influence how traffic will flow from one Autonomous System (AS) to another when multiple paths exist. For example, if AS 100 has two different paths to a destination network in AS 200, the LOCAL_PREF attribute can be used to influence the path selection.

The major difference between the Weight and LOCAL_PREF attributes is that when the LOCAL_PREF attribute is changed, that change is reflected throughout the AS. The new LOCAL_PREF value will be advertised to all other routers in the AS, as compared to the Weight attribute, which is locally significant only. If you change the Weight for a path on one router in an AS, the other routers in the AS will not learn of the change.
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When you’re preparing to pass the BSCI exam and earn your CCNP certification, one of the biggest challenges is learning BGP. BGP is totally different from any protocol you learned to earn your CCNA certification, and one of the differences is that BGP uses path attributes to favor one path over another when multiple paths to or from a destination exist.

Notice I said “to or from”. In earlier free BGP tutorials, I discussed the BGP attributes “weight” and “local preference”. These attributes are used to favor one path to a destination over another; for example, if BGP AS 100 has two paths to a destination in AS 200, these two attributes can be set in AS 100 to favor one path over another. But what if AS 100 wants to inform the routers in AS 200 as to which path it should use to reach a given destination in AS 100?

That’s where the BGP attribute “Multi-Exit Discriminator”, or MED, comes in. The MED value can be set in AS 100 to tell AS 200 which path it should use to reach a given network in AS 100.

As with many BGP attributes, the MED can be set with a route-map. What you need to watch is that there is no “set med” value in route maps. To change the MED of a path, you need to change the metric of that path. Let’s say that there are two entry paths for AS 200 to use to reach destinations in AS 100. You want AS 200 to use the 100.1.1.0/24 path over the 100.2.2.0/24 path. First, identify the two paths with two separate ACLs.
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