When getting ready to tackle the Cisco Certified Network Professional (CCNP) certification, there are a number of different technologies that you must be familiar with. Of these topics, a number of them are tested as part of the Cisco ROUTE (642-902) exam.

The purpose of this article is to give you some direction when studying some of the hardest topics tested on the CCNP ROUTE exam. Of course, the selection of the five most difficult topics is very subjective; the topics selected here are based on an observation of the most discussed topics.

5. EIGRP Advertised Distance and Feasible Distance

When preparing for the ROUTE exam, an understanding of EIGRP theory is essential. One of the concepts that seems to be the most misunderstood is the difference between feasible distance and advertised distance.

To explain the difference between the feasible distance (for best route(s) to the destination) and the advertised (sometimes called reported distance) is easy, but many times it is overthought and ends up confusing people. The feasible distance is the EIGRP metric from a specific device; the advertised distance is the EIGRP metric from the neighboring device (the one with the best path) to the same destination. For a specific route to make it into the routing table it must meet the specifications of the feasibility condition; the feasibility condition states that. The advertised distance for a specific route to a destination must be less than (<) the feasible distance for that same network. When there is only one path to a destination, the advertised distance will always be less than the feasible distance, but when there are multiple paths this condition must be considered to avoid loops.

For additional clarification, take a look at the example below.  Figure 1 shows a simple lab that includes two routers that are connected and have an EIGRP neighborship between them.

Figure 1 - Simple Lab

Figures 2 and 3 shown below display the status of these router's EIGRP topology tables.

Figure 2 - R1's EIGRP Topology Table

Figure 3 - R2's EIGRP Topology Table

The display from Figure 3 shows that from R2 the feasible distance to the 20.20.20.0/24 network is 28160, when advertising this network to the neighboring R1 router this same number is used. When R1 receives this advertisement, it compares it against any existing known routes to the same destination to ensure it meets the feasibility condition.  If it does meet this condition, R1 will add the additional cost of its interface to the neighboring router (R2) and enter it into the local EIGRP topology table; this is shown as 30720 (feasible distance)/28160 (advertised distance).

4. OSPF LSAs

Another difficult topic on the CCNP ROUTE exam is OSPF LSAs (Open Shortest Path First Link-State Advertisements); this topic is not only very helpful when it comes to the ROUTE exam but is vital when trying to understand the concepts of stub areas. There are a number of different Link State Advertisement types that exist within OSPF; for the purposes of this article, we'll take a look at the most common: LSA type 1, 2, 3, 4 and 5.

LSA Type 1

The LSA Type 1 or Router LSA is sent by every router within an area to describe the state of the each interface connected to the area.

LSA Type 2

The LSA Type 2 or Network LSA is sent by the Designated Router (DR) for a specific multi-access network and describes the set of routers attached to the network.

LSA Type 3 & 4

Both LSA Type 3 and 4 are considered Summary LSAs which are sent by the Area Border Router (ABR). An LSA Type 3 is used to describe the routes to the area's networks and an LSA Type 4 is used to describe the routes to Autonomous System Boundary Routers (ASBR). An LSA Type 3 is used when summarizing routes from one OSPF area to another.

LSA Type 5

The LSA Type 5 or Autonomous System External LSA is used by the ASBR to advertise routes that are external to the OSPF network. Unlike the other LSA's, this type is sent everywhere within the OSPF network regardless of area with the exception of stub networks.

3.OSPF Stub Areas

OSPF stub areas limit the parts of the network where specific LSAs are allowed. The idea being that if an OSPF router receives an LSA it must process it, which takes a certain amount of processor and memory resources. By limiting the types of LSAs that can reach specific networks, the devices within these stub areas do not have to be as powerful but still retain reachability to the rest of the OSPF network.

There are three main types of OSPF stub areas:

• Stub Areas
• Totally Stubby Areas
• Not So Stubby Areas

Stub Areas

An area that is configured as a stub is able to receive all types (as discussed above) of LSA except an LSA Type 5. Any routes that are destined for external networks are forwarded using a default route that is injected into the network in place of the LSA Type 5.

Totally Stubby Areas

Like a stub area, a totally stubby area is unable to receive LSA Type 5 packets. Along with this, the area is also unable to receive LSA Type 3 packets that include network advertisements (Not External) from other areas. Again, like a stub area, all traffic that is destined for these networks (both internal and external networks outside the area) is destined for a default router that is injected in place of both the LSA Type 3 and Type 5.

Not So Stubby Areas (NSSA)

A NSSA is almost exactly the same as a normal stub area but allows an ASBR (Autonomous System Boundary Router) to exist within the area. With a typical stub area, it is not possible to locate an ASBR inside the area as LSA type 5 packets are not allowed. A NSSA gets around this by using an LSA Type 7 packet in place of the LSA Type 5 packet within the NSSA; once this traffic from the ASBR exits the NSSA it is converted to an LSA Type 5 for transmission to the rest of the OSPF network.

2. Redistribution

At its simplest, the idea of redistribution is easy. Simply take networks that exist within one routing protocol and place them in another. For example, it is common to see OSPF networks redistributed into EIGRP or vice versa. It is also possible to redistribute networks from one like network to another; for example, from one OSPF network to another separate OSPF network.

There are two issues that typically can become large issues when dealing with redistribution. The first being what happens when using two-way redistribution and the other is related to metrics. When performing redistribution, it is easy to simply configure both networks to redistribute routes to the other; however a problem that can occur is when one network advertises routes that it just learned back into the initial network with a different metric. For example, what if an OSPF and RIP network are being redistributed into each other using multiple routers; it is possible for a RIP route to be redistributed into OSPF and then have that same route be redistributed back into RIP with a different metric on the other redistributing router, now the redistribution configuration has created a loop. While this issue is covered in the ROUTE exam it is covered in much more detail with the expert level certifications and should be well tested and understood before being configured in a production network.

1. BGP Best Path Selection

Unlike other Internal Gateway Protocols (IGP), BGP (Border Gateway Protocol) requires that a single best path be selected for insertion into the routing table. To find this one best path, BGP has what can be a confusing stepped decision process. The following table shows the steps that are used in this process and how they are considered.

Step	Consideration	Description
1	Next Hop	This step checks to see if a route exists to the next hop indicated in the BGP table, if no route exists the path is not usable.
2	Weight	The weight attribute is Cisco proprietary and allows the person configuring the router to weight a specific preferred path manually. (Higher is better). The weight attribute is only locally significant to the device; i.e. to prefer a specific interface exit point
3	Local Preference	There can be many routers that provide routes out of a specific autonomous system (AS); the one with the highest Local Preference is considered the preferred exit point for a specific prefix.
4	Locally Injected Routes	Routes that are advertised from the local router (injected into BGP from the local router) are preferred over routes advertised from other routers within the AS.
5	AS_PATH length	Since BGP routing tables contain paths that use AS's as hops (i.e. from AS 101 to AS 201 to AS 301…), the one with the lowest AS_PATH length is preferred. For example, if router1 has a path to a specific prefix through only one other AS it would be preferred over a second path that has a path through two other AS's.
6	ORIGIN	The ORIGIN consideration bases its decision on whether the path was injected from an Internal Gateway Protocol (IGP) or External Gateway Protocol (EGP) or unknown (?). Internal is preferred over External and External is preferred over unknown (I > E > ?)
7	MED	The MED or metric is a path attribute that is communicated from one AS to another; it is used to tell a neighboring AS that a specific entrance point is preferred. The lower the MED the higher that specific path is considered. Networks are not required to pay attention to this attribute (optional).
8	Neighbor Type	Routes from eBGP peers are considered over iBGP peers.
9	IGP metric to Next Hop	The shortest IGP metric to the next hop device listed in the advertisements is preferred.
10	Oldest eBGP route	The oldest known route for a given prefix is preferred
11	Neighbor BGP Router ID	Routes coming from the lowest BGP Router ID (RID) are preferred.
12	Neighbor IP Address	Routes coming from a neighbor with lowest IP address are preferred.

Summary

There are certainly a large number of topics that are integrated into the Cisco ROUTE exam, choosing which ones specifically are the "hardest" is a bit of a challenge and very subjective. Hopefully the content covered in this article will at least make these specific topics a little clearer and make passing the ROUTE exam a little easier.

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