When I first started studying for my CCNP, some of the concepts of OSPF really confused me. This was especially true for the ASBR, stub areas, and total stub areas. Sure, I could memorize the LSA types associated with these devices and area types, but there just weren't any illustrations that explained exactly what was going on.
Laptop Battery This CCNP tutorial shows an ASBR in operation, and also introduces you to a basic example of route redistribution. Don't worry, it gets more complicated. :)
Occasionally, during your CCNA and CCNP studies, you'll run into a term that just doesn't quite make sense to you. (Okay, more than occasionally!) One such term is "reverse telnet". As a Cisco certification candidate, you know that telnet is simply a protocol that allows you to remotely connect to a networking device such as a router or switch. But what is "reverse telnet", CCNP home lab setup
Thinkpad Here's the network we'll be working with in this tutorial:
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Microsoft R5
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R2 R3
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Laptop Computers Networks:
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Laptop Computer R1 - R5 Ethernet Segment: 10.1.1.0 /24
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Desktop Computer R1 - R2 - R3 Serial Segment: 172.16.123.0 /24 (Preconfigured with OSPF)
Notebooks R1 and R5 are running RIP over their common Ethernet segment, 10.1.1.0 /24. R5 has three loopbacks it will be advertising into the RIP domain.
Lenovo R1 is also running OSPF, with R2 and R3 as neighbors. Even though R1 knows about the loopbacks on R5, its OSPF neighbors do not. R1 has these routes in its RIP routing table, and for OSPF neighbors to learn of these routes, route redistribution must be manually configured.
Hard Drive R5#conf t
Travelstar R5(config)#router rip
Gateway R5(config-router)#version 2
Laptop Parts R5(config-router)#no auto-summary
Software R5(config-router)#network 5.0.0.0
Hard Drives R5(config-router)#network 10.0.0.0
Electronics R1#conf t
Canon R1(config)#router rip
Desktop Pc R1(config-router)#version 2
Desktop Computers R1(config-router)#no auto-summary
Think Pad R1(config-router)#network 10.0.0.0
Repair R1#show ip route rip
Data Recovery 5.0.0.0/24 is subnetted, 3 subnets
Cisco R 5.1.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0
Keyboard R 5.2.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0
Monitor R 5.3.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0
Desktop R1 has a route for all three of R5's loopback interfaces, learned via RIP. R1 is also running OSPF, but its neighbors R2 and R3 don't know about these RIP routes:
Infosys R2#show ip route ospf
Refurbished Laptops R2# < no output from show command means no routes! >
Wipro R3#show ip route ospf
Lap Top R3#
Refurbished Be careful when configuring redistribution use IOS Help to make sure you're not missing any vital options. IOS Help shows that there is a "subnets" option when redistributing RIP into OSPF. If that is left off, only classful networks are redistributed (as the router is kind enough to tell us). In this case, we have no classful networks, so there will be no redistribution. R2 will not see the RIP routes.
Memory R1(config)#router ospf 1
Intel R1(config-router)#redistribute rip?
As400 metric Metric for redistributed routes
Averatec metric-type OSPF/IS-IS exterior metric type for redistributed routes
Hardware route-map Route map reference
Dual Xeon subnets Consider subnets for redistribution into OSPF
Storage tag Set tag for routes redistributed into OSPF
Seagate
R1(config-router)#redistribute rip
Computer Sales % Only classful networks will be redistributed
Computer Hardware R2#show ip route ospf
Printers R2#clear ip route *
Technology R2#show ip route ospf
Mainframe R2#
Samsung On R1, we'll now use the "subnets" option, and the RIP subnets are successfully redistributed into OSPF.
Computer Repair R1(config)#router ospf 1
Used Computers R1(config-router)#redistribute rip subnets
Network R2 now sees the redistributed routes.
Digital Cameras R2#show ip route ospf
Desktops 5.0.0.0/24 is subnetted, 3 subnets
Cognos O E2 5.1.1.0 [110/20] via 172.12.123.1, 00:00:08, Serial0.123
Hosting O E2 5.2.1.0 [110/20] via 172.12.123.1, 00:00:08, Serial0.123
Netfinity O E2 5.3.1.0 [110/20] via 172.12.123.1, 00:00:08, Serial0.123
Internet 10.0.0.0/24 is subnetted, 1 subnets
Cheap Computer O E2 10.1.1.0 [110/20] via 172.12.123.1, 00:00:08, Serial0.123
Digital Camera Notice that the routes are marked as "E2". The E indicates that these are external routes these are routes that were learned by OSPF via redistribution.
Printer Naturally, you'll want to ping these networks to make sure you have two-way communication. Even though the routes to these loopbacks are in the routing table, pings fail:
Xseries R2#ping 5.1.1.1
Maxtor Type escape sequence to abort.
Data Storage Sending 5, 100-byte ICMP Echos to 5.1.1.1, timeout is 2 seconds:
Hitachi .....
Rational Success rate is 0 percent (0/5)
Websphere Keep this in mind when troubleshooting: It's not enough to have a route to a destination for pings to succeed there's got to be a path back. At this point, R5 doesn't know where the 172.12.123.0 network is, so there's no way the ping can get back. More route redistribution is necessary on R1.
Battery Your first instinct may be to redistribute OSPF routes into RIP to make this work, but remember that the 172.12.123.0 /24 network isn't known to R1 via OSPF it's a directly connected network. We can redistribute connected networks into RIP to give R5 a route to 172.12.123.0 /24, which will give the ICMP packets a path back to R2, which will allow pings to succeed.
It Support R1(config)#router rip
Western Digital R1(config-router)#redistribute connected
Music R5#show ip route rip
Networks 1.0.0.0/32 is subnetted, 1 subnets
Toner R 1.1.1.1 [120/1] via 10.1.1.1, 00:00:02, Ethernet0
Cheap Laptops 172.12.0.0/24 is subnetted, 1 subnets
Wholesale R 172.12.123.0 [120/1] via 10.1.1.1, 00:00:02, Ethernet0
Brother R2#ping 5.1.1.1
Netvista Type escape sequence to abort.
Camera Sending 5, 100-byte ICMP Echos to 5.1.1.1, timeout is 2 seconds:
Networking !!!!!
Sharp Success rate is 100 percent (5/5), round-trip min/avg/max = 68/69/76 ms
Cheap Performing this redistribution makes R1 an ASBR, as shown here with show ip ospf. This command even shows you what protocol(s) are being redistributed into OSPF.
Windows R1#show ip ospf
Monitors Routing Process "ospf 1" with ID 1.1.1.1
Linux Supports only single TOS(TOS0) routes
Computer Support Supports opaque LSA
Used Laptops It is an autonomous system boundary router
Cameras Redistributing External Routes from,
Scanners rip, includes subnets in redistribution
Panasonic Knowing exactly what the ASBR does and how to configure route redistribution are just a few of the many OSPF skills you'll need to earn your CCNP. Like any other Cisco skill, breaking a complex topic like OSPF down into smaller, manageable pieces is the best way to master these topics and pass the CCNP exams.
Workstation
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