Multilink PPP (MLPPP) makes multiple parallel links act like one link from a layer 3 perspective. In this next lab, the lab begins with a working configuration that does not use MLPPP, with two parallel serial links between two routers. Your job: reconfigure the two routers to use MLPPP, so that instead of balancing traffic using layer 3 balancing, MLPPP then load balances the traffic. This lab requires you to both configure MLPPP and consider any changes to the existing configuration that might be required as a result of the changes.
Requirements
Configure MLPPP on both router R1 and R2 in the figure, so that the routers have a single subnet (172.20.12.0/24) used between the two routers. Note that the figure shows two serial links between the routers, with two different subnets in use. As you will see when you look at the initial configurations, these routers begin in a state where they are using both links independently, so that each router would learn layer 3 routes that use each link as the outgoing interface. Your job: convert the configuration to treat the two links as a single layer 3 link using MLPPP, so that the routing protocol (OSPF) learns one route to remote subnet, with an outgoing interface that refers to the MLPPP multilink interface.
Follow these requirements:
- Configure the two serial links to use MLPPP:
- Use MLPPP interface 2
- Use subnet 172.20.12.0/24
- Give R1 an IPv4 address that ends with .1, and R2 an address that ends with .2
- Do not use any PPP authentication
- As for the transition from the existing configuration, examine the initial configurations shown in this lab exercise, and decide whether or not any existing configuration needs to be updated as a result:
- Check the OSPF configuration, and make changes as needed, so that both routers still become OSPF neighbors and still exchange routes for their LAN subnets
- Check the IP addressing configuration to remove any IP address overlaps or remove any unnecessary IP addresses.
- As for the topology:
- Assume all interfaces shown in the figure are up and working
Figure 1: MLPPP Topology – Initial State Before You Make Your Configuration Changes
Initial Configuration
Examples 1 and 2 show the initial configurations on routers R1 and R2, respectively.
Example 1: Router R1 Initial Configuration
hostname R1 ! interface GigabitEthernet0/1 no shutdown ip address 172.20.1.1 255.255.255.0 ! interface serial 0/0/0 no shutdown clock rate 1536000 ip address 172.20.3.1 255.255.255.0 ! interface serial 0/0/1 no shutdown clock rate 1536000 ip address 172.20.4.1 255.255.255.0 ! router ospf 1 network 172.20.1.1 0.0.0.0 area 0 network 172.20.3.1 0.0.0.0 area 0 network 172.20.4.1 0.0.0.0 area 0
Example 2: Router R2 Initial Configuration
hostname R2 ! interface GigabitEthernet0/1 no shutdown ip address 172.20.2.2 255.255.255.0 ! interface serial 0/0/0 no shutdown ip address 172.20.4.2 255.255.255.0 ! interface serial 0/0/1 no shutdown ip address 172.20.3.2 255.255.255.0 ! router ospf 1 network 172.20.2.2 0.0.0.0 area 0 network 172.20.3.2 0.0.0.0 area 0 network 172.20.4.2 0.0.0.0 area 0
Answer on Paper, or Maybe Test in Lab
To answer on paper, or in a text editor, just write your answers. The next post will list my suggested solution. And with this particular lab, as worded, there should be only a single correct answer.
To test in lab, you cannot use VIRL, because this lab uses serial interfaces. However, you can use Packet Tracer or real hardware. Feel free to do the lab in either environment. You can use any serial interface numbers that you want to use.
If you do implement this lab, try these show commands to get some insights into your solution:
show ppp multilink
show interfaces multilink 2
show ip interface brief
show ip ospf interface