Configuring Redistribution, E1 Metric, and Route Tags in OSPF

In the tutorial “Understanding OSPF Redistribution” , we discussed different concepts in redistribution. In this one, we’ll go through the various OSPF commands related to redistribution. We’ll walk through a step-by-step process to configure redistribution in OSPF, focusing on scenarios like redistributing from Static routes and EIGRP. We’ll also cover how to adjust the metric by using External Type 1 routes and the use of Route Tags when OSPF redistributes networks from other routing protocols.

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Let’s understand the OSPF redistribution commands using the following topology.

Let’s begin by looking at how to redistribute static routes.

Redistribution of Static Routes: Topology and Configuration

In the topology above, Router 1 and Router 2 are using OSPF. There is a network 30.0.0.0/8 on Router 3’s loopback interface. To reach this network from Router 2, we add a static route. Now, Router 2 and Router 3 can reach the 30.0.0.0 network, but Router 1 cannot. This is where redistribution comes into play. On Router 2, we can configure the “redistribute static subnets” command under OSPF configuration mode. This command injects the static route into OSPF, allowing Router 2 to advertise this network to Router 1 using an LSA type 5.

Note: Adding the keyword “subnets” at the end of the command is optional. If you omit it, only the major networks will be redistributed, and subnets will not be included. Therefore, if your network includes subnets that need to be redistributed, you must include the “subnets” keyword at the end of the command.

Now, let’s go through the configuration commands for redistributing static routes in the above given topology.

Configuration Commands:

Configuring the IP address on FastEthernet 0/0: 1.1.1.1/8 and enabling OSPF on network 1.0.0.0 with a wildcard mask of 0.255.255.255 in Area 1 on Router 1:

#Configuring IP address on the FE0/0:

R1(config)#
R1(config)#interface FastEthernet0/0
R1(config-if)#ip address 1.1.1.1 255.0.0.0
R1(config-if)#exit
R1(config)#

#Enabling OSPF and advertising the network 1.0.0.0/8 :

R1(config)#
R1(config)#router ospf 1
R1(config-router)#network 1.0.0.0 0.255.255.255 area 0
R1(config-router)#exit
R1(config)#

R2 Configuration: On Router 2, we need to configure IP addresses on the interfaces, add a static route, and redistribute the static route into OSPF. The following commands will configure the IP 1.1.1.2/8 on FastEthernet 0/0, 2.2.2.2/8 on FastEthernet 0/1, add a static route to 30.0.0.0/8, and finally redistribute that static route into OSPF.

#1.Configuring IP address on the FE0/0 (Connected to R1)

R2(config)#
R2(config)#interface FastEthernet0/0
R2(config-if)#ip address 1.1.1.2 255.0.0.0
R2(config-if)#exit
R2(config)#

#2.Configuring IP address on the FE0/1 (Connected to R3)

R2(config)#
R2(config)#interface FastEthernet0/1
R2(config-if)#ip address 2.2.2.2 255.0.0.0
R2(config-if)#exit
R2(config)#

#3.Configuring the static route to 30.0.0.0/8 via 2.2.2.3 (R3's IP)

R2(config)#ip route 30.0.0.0 255.0.0.0 2.2.2.3

#4.Enabling OSPF and advertising the network 1.0.0.0/8

R2(config)#router ospf 1
R2(config-router)#network 1.0.0.0 0.255.255.255 area 0

#5. Configuring redistribution of static routes in to OSPF

R2(config-router)#redistribute static subnets
R2(config-router)#exit
R2(config)#

R3 Configuration: Note: You don’t need to configure Router 3 if you only want to check whether the static route is being redistributed into OSPF. By configuring the commands on R1 and R2, the static route to 30.0.0.0/8 will be redistributed into OSPF, and you can verify this route in R1’s routing table. However, if you want to test whether data packets can be sent to this redistributed network using Ping, you’ll need to configure R3 as well. Additionally, R3 must have a static route to the 1.0.0.0 network for the ping packets to return successfully. Following commands will configure IP address 2.2.2.3/8 on Fastethernet 0/1 , 30.1.1.1/8 on Loopback interface and will add static route to 1.0.0.0/8

#1.Configuring IP address on the FE0/1 (Connected to R2)

R3(config)#
R3(config)#interface FastEthernet0/1
R3(config-if)#ip address 2.2.2.3 255.0.0.0
R3(config-if)#exit
R3(config)#

#2. Configuring IP address on Loopback interface

R3(config)#
R3(config)#interface loopback 1
R3(config-if)#ip address 30.1.1.1 255.0.0.0
R3(config-if)#exit
R3(config)#

#3. Add a static route to 1.0.0.0/8 network via 2.2.2.2
R3(config)#ip route 1.0.0.0 255.0.0.0 2.2.2.2

Verifying the OSPF Redistribution Configuration

Now, let’s check the results of this configuration. Ideally, Router 1 should receive the 30.0.0.0/8 network from Router 2. This route will appear in Router 1’s routing table with the code ‘O E2,’ indicating that it is a redistributed route. ‘O E2’ stands for OSPF External Type 2 route. As we discussed in the other chapter about redistribution, O E2 routes do not factor in the internal cost to reach the Autonomous System Boundary Router (ASBR). Therefore, the cost or metric to reach this network will be 20, which is the default cost that OSPF assigns when redistributing external routes.

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Understanding and Configuring Redistribution from EIGRP to OSPF

Next, let’s explore how to perform redistribution from another protocol. In this example, we’ll use EIGRP, though you could use any other protocol like BGP, ISIS, or RIP. To redistribute from EIGRP, we’ll add one more loopback interface on R3 with the network address 31.0.0.0/8. R3 will advertise this network to R2 using EIGRP, and R2 will then redistribute this network into OSPF.

Below are the commands you need to add on R2 and R3. Assuming that you have already configured the IP addresses from the previous exercise, we’ll only show the commands required to configure EIGRP and redistribute the networks learned from EIGRP into OSPF. When configuring EIGRP, you need to specify the Autonomous System (AS) number. .

R2 Configuration: In the following configuration, we’re using AS number 100. When issuing the redistribution command, this AS number must be included. So, in this case, the command to redistribute EIGRP networks of AS 100 into OSPF will be “redistribute eigrp 100 subnets“

#1. Configuring EIGRP with AS number 100 and advertising the network 2.0.0.0

R2(config)#
R2(config)#router eigrp 100
R2(config-router)#network 2.0.0.0
R2(config-router)#exit
R2(config)#

#2. Redistributing EIGRP networks in to OSPF

R2(config)#
R2(config)#router ospf 1
R2(config-router)#redistribute eigrp 100 subnets
R2(config-router)#exit
R2(config)#

R3 Configuration: On Router 3, we are only configuring the loopback interface and enabling EIGRP to advertise that network. Assuming the IP address for the physical interface is already configured, the following commands will configure the IP 31.1.1.1/8 on Loopback 2 and enable EIGRP with AS number 100 to advertise networks 2.0.0.0 and 31.0.0.0.

#1. Configuring IP address on second Loopback interface

R3(config)#
R3(config)#interface loopback 2
R3(config-if)#ip address 31.1.1.1 255.0.0.0
R3(config-if)#exit
R3(config)#

#2. Configuring EIGRP with AS number 100 and advertising the networks 2.0.0.0 and 31.0.0.0

R3(config)#
R3(config)#router eigrp 100
R3(config-router)#network 2.0.0.0
R3(config-router)#network 31.0.0.0
R3(config-router)#exit
R3(config)#

Verifying the results:

In the output of “show ip route” on R1 shown above, you can see two newly redistributed routes on R1: 2.0.0.0 and 31.0.0.0, both marked with the code ‘O E2.’ These are the networks that OSPF has redistributed from EIGRP on R2. Note that, similar to the previously redistributed static route, these networks also have a metric of 20.

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Configuring redistribution with Metric Type 1 : O E1 routes

In the previous examples, we saw OSPF advertising the redistributed routes with the code ‘O E2,’ which stands for External Type 2 routes. In these routes, the metric assigned to the networks remains constant across the OSPF network, meaning routers inside the OSPF domain do not consider the internal cost to reach the ASBR. So, the metric that the ASBR announces is the one used by these routers. In other words, with ‘O E2’ routes, the metric shown in the routing table is not the actual path cost – it’s just the metric set by the ASBR for that network.

If you want OSPF to calculate the actual cost, including the internal cost to reach the ASBR, you can use the ‘Metric Type 1’ option. This requires configuring your ASBR to announce the redistributed networks with ‘Metric Type 1.’ You can do this in OSPF configuration mode. For example, if you want to use Metric Type 1 in the scenarios we discussed earlier, you can use “redistribute static subnets metric-type 1” or “redistribute eigrp 100 subnets metric-type 1“.

Configuring redistribution with metric type 1 on R2 (ASBR) :

R2(config)#
R2(config)#router ospf 1
R2(config-router)#redistribute static subnets metric-type 1
R2(config-router)#redistribute eigrp 100 subnets metric-type 1
R2(config-router)#exit
R2(config)#

Now, let’s check the impact of this configuration on Router 1’s routing table using “show ip route”.

In the output of “show ip route” on R1 shown above, you can see that the code has changed to ‘O E1,’ and the metric is now 21. In this setup, there’s a fast Ethernet link between Router 1 and Router 2, which has a cost of 1. Therefore, the total cost is calculated as the advertised cost of 20 plus the internal cost of 1, resulting in a metric of 21

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Using the Route Tag Field in OSPF

When configuring OSPF redistribution, the tag is an optional field that can be used to mark redistributed routes with a 32-bit value known as the ‘External Route Tag.’ This tag appears in Type 5 and Type 7 LSAs, which advertise external routes in OSPF. The tag is a number that admins can set when redistributing networks into OSPF. It can be used for various purposes, such as filtering routes, preventing loops when multiple ASBRs are involved, applying route policies, and tracking routes.

Configuring Route Tags while configuring static routes to OSPF :

R2(config)#
R2(config)#router ospf 1
R2(config-router)#redistribute static subnets tag 150
R2(config-router)#exit
R2(config)#

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OSPF Commands to Configure Redistribution from Other Protocols:

You can use the same method to configure redistribution from other routing protocols. Below are the commands you need to use to redistribute from BGP, ISIS, RIP, and directly connected routes:

1. redistribute bgp 10 subnets (where 10 is the BGP AS number)
2. redistribute isis subnets
3. redistribute rip subnets
4. redistribute connected subnets

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