Categories
OSPF

Comparing internal routing protocols (IGPs)

The following table lists the differences between the three most popular interior routing protocols:

 

Feature RIP EIGRP OSPF
Type distance vector hybrid distance vector
Metric hop count bandwidth and delay cost
Speed of convergence slow fast fast
Routing classful (RIPv1), classless(RIPv2) classless classless
Updates periodical broadcast (RIPv1), multicast(RIPv2) multicast multicast
Manual summarization no (RIPv1), yes (RIPv2) yes yes
Supported on non-Cisco routers yes no yes
Configuration complexity easy medium hard

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

Full Version 200-301 Dumps

Try 200-301 Dumps Demo

Categories
OSPF

Differences between OSPF and EIGRP

The following table lists the differences between OSPF and EIGRP:

differences between ospf and eigrp

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

Full Version 200-301 Dumps

Try 200-301 Dumps Demo

Categories
OSPF

OSPF summary

Here is a list of the most important OSPF features:

  • link state routing protocol
  • classless routing protocol
  • supports VLSM (Variable Length Subnet Mask)
  • converges fast
  • uses the concept of areas
  • uses multicast address for routing updates (224.0.0.5 and 224.0.0.6)
  • sends partial routing updates
  • supports only equal cost load balancing
  • manual summarization can be done only on ABRs and ASBRs
  • uses interface cost as a metric
  • uses SPF algorithm to calculate the best paths
  • open standard
  • each router inside an area has the same topology table
  • supports clear-text and MD5 authentication

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

Full Version 200-301 Dumps

Try 200-301 Dumps Demo

Categories
OSPF

OSPF authentication

OSPF can be configured to authenticate every OSPF message. This is usually done to prevent a rogue router from injecting false routing information and therefore causing a Denial-of-Service attack.

Two types of authentication can be used:
1. clear text authentication – clear text passwords are used
2.  MD5 authentication – MD5 authentication is used. This type of authentication is more secure because the password doesn’t go in clear-text over the network.

NOTE
With OSPF authentication turned on, routers must pass the authentication process before becoming OSPF neighbors.

 

To configure clear text authentication, the following steps are required:

  1. configure the OSPF password on the interface by using the ip ospf authentication-key PASSWORD interface command
  2. configure the interface to use OSPF clear-text authentication by using the ip ospf authentication interface command

 

In the following example, we will configure OSPF clear-text authentication.

ospf authentication topology

Both routers are running OSPF. On R1, we need to enter the following commands:

ospf authentication configuration 1

The same commands have to be entered on R2:

ospf authentication configuration 2
To verify that clear-text authentication is indeed turned on, we can use the show ip ospf interface INTERFACE_NUMBER/INTERFACE_TYPE command on either router:

show ip ospf interface simple password

Configuring OSPF MD5 authentication is very similar to configuring clear-text authentication. Two commands are also used:

  1. First you need to configure the MD5 value on an interface by using the ip ospf message-digest-key 1 md5 VALUE interface command
  2. Next, you need to configure the interface to use MD5 authentication by using the ip ospf authentication message-digest interface command

 

Here is an example configuration on R1:

ospf authentication configuration md5

You can verify that R1 is using OSPF MD5 authentication by typing the show ip ospf INTERFACE/INTERFACE_TYPE command:

show ip ospf interface md5

NOTE
OSPF authentication type can also be enabled on an area basis, instead of configuring OSPF authentication type per interface basis. This is done by using the area AREA_ID authentication [message-digest] command under the OSPF configuration mode. If you omit the message-digest keyword, a clear-text authentication will be used for that area. All interfaces inside the area will use OSPF authentication.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

Full Version 200-301 Dumps

Try 200-301 Dumps Demo

Categories
OSPF

OSPF summarization

Route summarization helps reduce OSPF traffic and route computation. OSPF, unlike EIGRP, doesn’t support automatic summarization. Also, unlike EIGRP, where you can summarize routes on every router in an EIGRP network, OSFP can summarize routes only on ABRs and ASBRs.

The following command is used for OSPF summarization:

(config-router) area AREA_ID range IP_ADDRESS MASK

To better understand OSPF summarization, consider the following example network:

ospf summarization topology

All three routers are running OSPF and exchanging routers. Before OSPF summarization is configured, the router R1 inside the backbone area has two entries for the networks 11.0.0.0/24 and 11.0.1.0/24 in its routing table.

show ip route ospf without summarization

We could summarize these two subnets on R2, so that R1 receive only one routing update for both subnets. To do that, the following command can be used on R2:

OSPF summarization command

Now, R1 has only one entry in its routing table for R3’s directly connected subnets:

show ip route ospf with summarization

NOTE
Be careful with summarization. In this case, router R1 thinks that R2 has routes for all subnets in the range 11.0.0.0 – 11.0.255.255. When summarizing, try to be as specific as possible.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

Full Version 200-301 Dumps

Try 200-301 Dumps Demo

Categories
OSPF

Designated & Backup Designated Router

Based on the network type, OSPF router can elect one router to be a Designated Ruter (DR) and one router to be a Backup Designated Router (BDR). For example, on multiaccess broadcast networks (such as LANs) routers defaults to elect a DR and BDR. DR and BDR serve as the central point for exchanging OSPF routing information. Each non-DR or non-BDR router will exchange routing information only with the DR and BDR, instead of exchanging updates with every router on the network segment. DR will then distribute topology information to every other router inside the same area, which greatly reduces OSPF traffic.

To send routing information to a DR or BDR the multicast address of 224.0.0.6 is used. DR sends routing updates to the multicast address of 224.0.0.5. If DR fails, BDR takes over its role of redistributing routing information.

Every router on a network segment will establish a full neighbor relationship with the DR and BDR. Non-DR and non-BDR routers will establish a two way neighbor relationship between themselves.

NOTE
On point-to-point links, a DR and BDR are not elected since only two routers are directly connected.

 

On LANs, DR and BDR have to be elected. Two rules are used to elect a DR and BDR:

  1. router with the highest OSPF priority will become a DR. By default, all routers have a priority of 1.
  2. if there is a tie, a router with the highest router ID wins the election. The router with the second highest OSPF priority or router ID will become a BDR.

To better understand the concept, consider the following example.

ospf dr bdr topology

All routers depicted above are in the same area (area 0). All routers are running OSPF. Routers R1 and R2 have been elected as DR and BDR because they have the highest and the second highest router ID (100.0.0.0 and 90.0.0.0 respectively). If, for example, R3’s directly connected subnet fails, R3 informs R1 and R2 (the DR and BDR for the segment) of the network change (step 1). R1 then informs all other non-DR and non-BDR routers of the change in topology (step 2).

We can verify that R1 and R2 are indeed the DR and BDR of the segment by typing the show ip ospf neighbors command on R3:

R3#show ip ospf neighbor 


Neighbor ID     Pri   State           Dead Time   Address         Interface
60.0.0.0          1   FULL/DROTHER    00:00:33    10.0.0.5        FastEthernet0/0
100.0.0.0         1   FULL/DR         00:00:33    10.0.0.1        FastEthernet0/0
70.0.0.0          1   FULL/DROTHER    00:00:33    10.0.0.4        FastEthernet0/0
90.0.0.0          1   FULL/BDR        00:00:33    10.0.0.2        FastEthernet0/0

NOTE
You can influence the DR and BDR election process by manually configuring the OSPF priority. This is done by using the ip ospf priority VALUE command interface command.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

Full Version 200-301 Dumps

Try 200-301 Dumps Demo

Categories
OSPF

OSPF configuration

Configuring OSPF 1

OSPF basic configuration is very simple. Just like with other routing protocols covered so far (RIP, EIGRP) first you need to enable OSPF on a router. This is done by using the router ospf PROCESS-ID global configuration command. Next, you need to define on which interfaces OSPF will run and what networks will be advertised. This is done by using the network IP_ADDRESS WILDCARD_MASK AREA_ID command from the ospf configuration mode.

NOTE
The OSPF process number doesn’t have to be the same on all routers in order to establish a neighbor relationship, but the Area ID has to be the same on all neighboring routers in order for routers to become neighbors.

 

Let’s get started with some basic OSPF configuration. We will use the following network topology:

ospf sample topology

First, we need to enable OSPF on both routers. Then we need to define what network will be advertised into OSPF. This can be done by using the following sequence of commands on both routers:

ospf configuration 1

ospf configuration 2

The network commands entered on both routers include subnets directly connected to both routers. We can verify that the routers have become neighbors by typing the show ip ospf neighbors command on either router:

show ip ospf neighbor

To verify if the routing updated were exchanged, we can use the show ip route command. All routes marked with the character O are OSPF routes. For example, here is the output of the command on R1:

show ip route ospf route

You can see that R1 has learned about the network 192.168.0.0/24 through OSPF.

Configuring OSPF 2

Although basic OSPF configuration can be very simple, OSPF provides many extra features that can get really complex. In this example, we will configure multiarea OSPF network and some other OSPF features.

Consider the following multiarea OSPF network:

ospf sample topology areas

In this example we have two OSPF areas – area 0 and area 1. As you can see from the network topology depicted above, routers R1 and R3 are in the area 0 and area 1, respectively. Router 2 connects to both areas, which makes him an ABR (Area Border Router). Our goal is to advertise the subnets directly connected to R1 and R3. To do that, the following configuration on R1 will be used:

ospf configuration areas 1

 

NOTE
We have used the router-id 1.1.1.1 command to manually specify the router ID of this router. OSPF process will use that RID (router-id) when communicating with other OSPF neighbors.

 

Because R1 connects only to R2, we only need to establish a neighbor relationship with R2 and advertise directly connected subnet into OSPF.

Configuration of R3 looks similar, but with one difference, namely area number. R3 is in the area 1.

ospf configuration areas 3

What about R2? Well, because R2 is an ABR, we need to establish neighbor relationship with both R1 and R3. To do that, we need to specify different area ID for each neighbor relationship, 0 for R1 and 1 for R2. We can do that using the following sequence of commands:

ospf configuration areas 2

Now R2 should have neighbor relationship with both R1 and R3. We can verify that by using the show ip ospf neighbor command:

show ip ospf neighbor areas

To verify if directly connected subnets are really advertised into the different area, we can use the show ip route ospf command on both R1 and R3:

show ip route ospf areas 1

show ip route ospf areas 3
Characters IA in front of the routes indicate that these routes reside in different areas.

NOTE
Since they reside in different areas, R1 and R3 will never establish a neighbor relationship.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

Full Version 200-301 Dumps

Try 200-301 Dumps Demo

Categories
OSPF

OSPF overview

OSPF (Open Shortest Path First) is a link state routing protocol. Because it is an open standard, it is implemented by a variety of network vendors. OSPF will run on most routers that doesn’t necessarily have to be Cisco routers (unlike EIGRP which can be run only on Cisco routers).

Here are the most important features of OSPF:

  • a classless routing protocol
  • supports VLSM, CIDR, manual route summarization, equal cost load balancing
  • incremental updates are supported
  • uses only one parameter as the metric – the interface cost.
  • the administrative distance of OSPF routes is, by default, 110.
  • uses multicast addresses 224.0.0.5 and 224.0.0.6 for routing updates.

 

Routers running OSPF have to establish neighbor relationships before exchanging routes. Because OSPF is a link state routing protocol, neighbors don’t exchange routing tables. Instead, they exchange information about network topology. Each OSFP router then runs SFP algorithm to calculate the best routes and adds those to the routing table. Because each router knows the entire topology of a network, the chance for a routing loop to occur is minimal.

Each OSPF router stores routing and topology information in three tables:

  • Neighbor table – stores information about OSPF neighbors
  • Topology table – stores the topology structure of a network
  • Routing table –  stores the best routes

OSPF neighbors

OSPF routers need to establish a neighbor relationship before exchanging routing updates. OSPF neighbors are dynamically discovered by sending Hello packets out each OSPF-enabled interface on a router. Hello packets are sent to the multicast IP address of 224.0.0.5.

The process is explained in the following figure:

ospf hellos

Routers R1 and R2 are directly connected. After OSFP is enabled both routers send Hellos to each other to establish a neighbor relationship. You can verify that the neighbor relationship has indeed been established by typing the show ip ospf neighbors command.

show ip ospf neighbor

In the example above, you can see that the router-id of R2 is  2.2.2.2. Each OSPF router is assigned a router ID. A router ID is determined by using one of the following:

1.    using the router-id command under the OSPF process.
2.    using the highest IP address of the router’s loopback interfaces.
3.    using the highest IP address of the router’s physical interfaces.

The following fields in the Hello packets must be the same on both routers in order for routers to become neighbors:

  • subnet
  • area id
  • hello and dead interval timers
  • authentication
  • area stub flag
  • MTU

 

By default, OSPF sends hello packets every 10 second on an Ethernet network (Hello interval). A dead timer is four times the value of the hello interval, so if a routers on an Ethernet network doesn’t receive at least one Hello packet from an OSFP neighbor for 40 seconds, the routers declares that neighbor to be down.

OSPF neighbor states

Before establishing a neighbor relationship, OSPF routers need to go through several state changes. These states are explained below.

1. Init state – a router has received a Hello message from the other OSFP router
2. 2-way state – the neighbor has received the Hello message and replied with a Hello message of his own
3. Exstart state – beginning of the LSDB exchange between both routers. Routers are starting to exchange link state information.
4. Exchange state – DBD (Database Descriptor) packets are exchanged. DBDs contain LSAs headers. Routers will use this information to see what LSAs need to be exchanged.
5. Loading state – one neighbor sends LSRs (Link State Requests) for every network it doesn’t know about. The other neighbor replies with the LSUs (Link State Updates) which contain information about requested networks. After all the requested information have been received, other neighbor goes through the same process
6. Full state – both routers have the synchronized database and are fully adjacent with each other.

OSPF areas

OSPF uses the concept of areas. An area is a logical grouping of contiguous networks and routers. All routers in the same area have the same topology table, but they don’t know about routers in the other areas. The main benefits of creating areas is that the size of the topology and the routing table on a router is reduced, less time is required to run the SFP algorithm and routing updates are also reduced.

Each area in the OSPF network has to connect to the backbone area (area 0). All router inside an area must have the same area ID to become OSPF neighbors. A router that has interfaces in more than one area (area 0 and area 1, for example) is called Area Border Router (ABR). A router that connects an OSPF network to other routing domains (EIGRP network, for example) is called Autonomous System Border Router (ASBR).

NOTE
In OSPF, manual route summarization is possible only on ABRs and ASBRs.

 

To better understand the concept of areas, consider the following example.

ospf areas

All routers are running OSPF. Routers R1 and R2 are inside the backbone area (area 0). Router R3 is an ABR, because it has interfaces in two areas, namely area 0 and area 1. Router R4 and R5 are inside area 1. Router R6 is an ASBR, because it connects OSFP network to another routing domain (an EIGRP domain in this case). If the R1’s directly connected subnet fails, router R1 sends the routing update only to R2 and R3, because all routing updates all localized inside the area.

NOTE
The role of an ABR is to advertise address summaries to neighboring areas. The role of an ASBR is to connect an OSPF routing domain to another external network (e.g. Internet, EIGRP network…).

LSA, LSU and LSR

The LSAs (Link-State Advertisements) are used by OSPF routers to exchange topology information. Each LSA contains  routing and toplogy information to describe a part of an OSPF network. When two neighbors decide to exchange routes, they send each other a list of all LSAa in their respective topology database. Each router then checks its topology database and  sends a Link State Request (LSR) message requesting all LSAs not found in its topology table. Other router responds with the Link State Update (LSU) that contains all LSAs requested by the other neighbor.

The concept is explained in the following example:

ospf lsa lsr lsu

After configuring OSPF on both routers, routers exchange LSAs to describe their respective topology database. Router R1 sends an LSA header for its directly connected network 10.0.1.0/24. Router R2 check its topology database and determines that it doesn’t have information about that network. Router R2 then sends Link State Request message requesting further information about that network. Router R1 responds with Link State Update which contains information about subnet 10.0.1.0/24 (next hop address, cost…).

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

Full Version 200-301 Dumps

Try 200-301 Dumps Demo