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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
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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.

When BGP has to decide between such paths, there is an order in which BGP considers the path attributes. For success on the CCNP exams, you need to know this order. BGP looks at path attributes in this order:

Highest weight (Cisco-proprietary BGP value)

Highest local preference (LOCAL_PREF)

Prefer locally originated route.

Shortest AS_PATH is preferred.
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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.
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I was reading The Big Moo: Stop Trying To Be Perfect and Start Being Remarkable this morning, and I’d recommend a copy of this to anyone who wants to improve their career and their future. And that’s all of us, right?

There was one particular line that really stood out to me: Betting on change is always the safest bet available. That describes life perfectly, but it also describes a career in Information Technology perfectly as well. There is no field in the world that has the constant and never-ending changes that IT does. And every single one of us can look at this as a massive opportunity for personal and professional growth.

Is that how you’re looking at it? I remember when I passed my first certification exam, the Novell CAN, back in 1997. Man, I thought I knew it all then! But I quickly learned that you’ve got to keep learning in IT. I also learned that if you’re willing to put in the work and make the sacrifices, there’s no other field with the limitless potential for growth and excellence.

Like everyone else, my career has had its ups and downs, but I always kept learning and growing. Today, I’ve got my dream job, working with studI was reading The Big Moo: Stop Trying To Be Perfect and Start Being Remarkable this morning, and I’d recommend a copy of this to anyone who wants to improve their career and their future. And that’s all of us, right?

There was one particular line that really stood out to me: Betting on change is always the safest bet available. That describes life perfectly, but it also describes a career in Information Technology perfectly as well. There is no field in the world that has the constant and never-ending changes that IT does. And every single one of us can look at this as a massive opportunity for personal and professional growth.
Read more…