Answers: MLPPP 2

This lab asks you to do a basic migration: move from using two parallel serial links as independent links to instead treating them as one layer 3 link using MLPPP. This lab requires new configuration as well as analysis of the existing configuration. Do the lab on your own first, and then check here for my answer and explanation.

Answers

Figure 1: MLPPP Topology – Before Migrating to MLPPP

Example 3: Router R1 Config

interface serial 0/0/0
 encapsulation ppp
 ppp multilink
 ppp multilink group 2
 no ip address
!
interface serial 0/0/1
 encapsulation ppp
 ppp multilink
 ppp multilink group 2
 no ip address
!
interface multilink 2
 encapsulation ppp
 ppp multilink
 ppp multilink group 2
 ip address 172.20.12.1 255.255.255.0
!
router ospf 1
 network 172.20.12.1 0.0.0.0 area 0
 no network 172.20.3.1 0.0.0.0 area 0
 no network 172.20.4.1 0.0.0.0 area 0

Example 4: Router R2 Config

interface serial 0/0/0
 encapsulation ppp
 ppp multilink
 ppp multilink group 2
 no ip address
!
interface serial 0/0/1
 encapsulation ppp
 ppp multilink
 ppp multilink group 2
 no ip address
!
interface multilink 2
 encapsulation ppp
 ppp multilink
 ppp multilink group 2
 ip address 172.20.12.2 255.255.255.0
!
router ospf 1
 network 172.20.12.2 0.0.0.0 area 0
 no network 172.20.3.2 0.0.0.0 area 0
 no network 172.20.4.2 0.0.0.0 area 0

Commentary

This lab has a rather long configuration answer. This commentary starts by working through the MLPPP configuration, and then discusses some of the migration choices, particularly to do with OSPF and IP addressing.

The requirements asked you to use MLPPP multilink group 2. As a result:

• Both routers create a multilink interface with the interface multilink 2 command
• Both routers use the ppp multilink group 2 subcommand on their physical serial interfaces and on the multilink interface itself

For the layer 3 details, note that the configuration on each router:

• Adds an IPv4 address to each multilink interface, for example, with the ip address 172.20.12.1 255.255.255.0 subcommand under multilink 2 on router R1.
• Both routers remove the IP address configuration from the physical interfaces, because they are no longer needed, with the no ip address interface subcommand.

Next, as normal, the physical serial interfaces plus the multilink interface, on each router, need the same three subcommands. These commands (respectively) enable PPP, enable the MLPPP feature of PPP, and reference the MLPPP multilink group:

encapsulation ppp

ppp multilink

ppp multilink group 2

Finally, note that the OSPF configuration needed to be updated. The initial configuration happened to show OSPF configuration that matched based on all 32 bits of each interface IP address. The MLPPP configuration in this case moved away from using the addresses in subnets 172.20.3.0/24 and 172.20.4.0/24, now using subnet 172.20.12.0/24. The end of Examples 3 and 4 show the addition of one OSPF network command on each router, matching each router’s new IP address in subnet 172.20.12.0/24. It also shows the removal of the two now unneeded network commands.