Posts Tagged ‘free’
Cisco CCNA / CCNP Certification: OSPF E2 vs. E1 Routes
OSPF is a major topic on both the CCNA and CCNP exams, and it’s also the topic that requires the most attention to detail. Where dynamic routing protocols such as RIP and IGRP have only one router type, a look at a Cisco routing table shows several different OSPF route types.
R1#show 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
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2, E – EGP
In this tutorial, we’ll take a look at the difference between two of these route types, E1 and E2.
Route redistribution is the process of taking routes learned via one routing protocol and injecting those routes into another routing domain. (Static and connected routes can also be redistributed.) When a router running OSPF takes routes learned by another routing protocol and makes them available to the other OSPF-enabled routers it’s communicating with, that router becomes an Autonomous System Border Router (ASBR).
Let’s work with an example where R1 is running both OSPF and RIP. R4 is in the same OSPF domain as R1, and we want R4 to learn the routes that R1 is learning via RIP. This means we have to perform route redistribution on the ASBR. The routes that are being redistributed from RIP into OSPF will appear as E2 routes on R4:
R4#show ip route ospf
O E2 5.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
6.0.0.0/32 is subnetted, 1 subnets
O E2 6.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
O E2 172.12.21.0/30 [110/20] via 172.34.34.3, 00:33:32,
Ethernet0
O E2 7.1.1.1 [110/20] via 172.34.34.3, 00:33:21, Ethernet0
15.0.0.0/24 is subnetted, 1 subnets
O E2 15.1.1.0 [110/20] via 172.34.34.3, 00:33:32, Ethernet0
E2 is the default route type for routes learned via redistribution. The key with E2 routes is that the cost of these routes reflects only the cost of the path from the ASBR to the final destination; the cost of the path from R4 to R1 is not reflected in this cost. (Remember that OSPF’s metric for a path is referred to as “cost”.)
In this example, we want the cost of the routes to reflect the entire path, not just the path between the ASBR and the destination network. To do so, the routes must be redistributed into OSPF as E1 routes on the ASBR, as shown here.
R1#conf t
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Cisco CCNA / CCNP Certification: Introduction To BGP Attributes
BGP is one of the most complex topics you’ll study when pursuing your CCNP, if not the most complex. I know from personal experience that when I was earning my CCNP, BGP is the topic that gave me the most trouble at first. One thing I keep reminding today’s CCNP candidates about, though, is that no Cisco technology is impossible to understand if you just break it down and understand the basics before you start trying to understand the more complex configurations.
BGP attributes are one such topic. You’ve got well-known mandatory, well-known discretionary, transitive, and non-transitive. Then you’ve got each individual BGP attribute to remember, and the order in which BGP considers attributes, and what attributes even are… and a lot more! As with any other Cisco topic, we have to walk before we can run. Let’s take a look at what attributes are and what they do in BGP.
BGP attributes are much like what metrics are to OSPF, RIP, IGRP, and EIGRP. You won’t see them listed in a routing table, but attributes are what BGP considers when choosing the best path to a destination when multiple valid (loop-free) paths exist.
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Cisco CCNA / CCNP Certification Exam Tutorial: Floating Static Routes
To pass the Cisco CCNA and CCNP certification exams, as well as becoming a world-class networker, you’ve got to know how and when to use floating static routes. And if you’re wondering what makes them “float” — read on!
In this example, R1 and R2 are running OSPF over a Frame Relay network, 172.12.123.0 /24. They’re also connected by a BRI ISDN link, 172.12.12.0 /24. R1 is advertising a loopback network, 1.1.1.1 /32, via OSPF. We want R2 to have a route to that loopback even if the frame goes down – and here, we’ll use a floating static route to make that happen.
R2 sees the route to the loopback interface via OSPF, and can ping that interface successfully.
R2#show ip route ospf
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:02, Serial0
R2#ping 1.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 68/68/68 ms
This is when it’s important to know your administrative distances…. or at least know where to look to see them! The AD of OSPF is 110, which means we can configure a static route to 1.1.1.1 /32, and as long as the AD of the static route is higher than 110, it won’t be used unless the OSPF route leaves the routing table. That’s why this kind of route is called a “floating” static route – the route “floats” in the routing table and isn’t seen unless the primary route leaves the table.
You learned how to write a static route in your CCNA studies, but you also remember that the default AD of a static route is either 1 or 0… and both of those values are less than 110! To change the AD of a static route, configure the desired distance at the end of the ip route command.
R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 ?
<1-255> Distance metric for this route
A.B.C.D Forwarding router’s address
name Specify name of the next hop
permanent permanent route
tag Set tag for this route
R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 111
The static route has an AD that’s only one higher than that of the OSPF route, but that’s enough to make the route “float” and not yet be seen in the routing table.
R2#show ip route
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:06:44, Serial0
172.12.0.0/24 is subnetted, 2 subnets
C 172.12.12.0 is directly connected, BRI0
C 172.12.123.0 is directly connected, Serial0
Let’s see the effect on the routing table when the Serial0 interface is closed.
R2(config)#int s0
R2(config-if)#shutdown
12:04:53: %OSPF-5-ADJCHG: Process 1, Nbr 172.12.123.1 on Serial0 from FULL to DOWN, Neighbor Down: Interface down or detached
Cisco / MCSE Exam Study: Creating A Road Map To Success
Planning for success on the CCNA, CCNP, and other Cisco exams is much like taking a trip in your car. You’ve got to plan ahead, accept the occasional detour, and just keep on going until you get there. But what do you do before you get started?
Create a road map – for success.
If you were driving from one side of the country to another, you certainly wouldn’t just get in your car and start driving, would you? No. You would plan the trip out ahead of time. What would happen if you just got in the car and started driving in the hope that you would someday arrive at your final destination? You would never get there, and you’d spend a lot of time wandering aimlessly.
Don’t spend your study time and slow your progress by studying for a Cisco exam without planning the trip. Schedule your study time as you would an appointment with a client, and keep that appointment. Make sure that your study time is quality study – turn your TV, iPod, and cell off. If you hit a bump in the road and don’t get your certification the first time you take the exam, regroup and create another plan. Study until you get to the point that on exam day, you know that you are already a CCNA or CCNP and you’re just there at the testing center to make it official.
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