Designing Implementing an OSPF Network | OSPF Network Design Solutions

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Sales Office LAN Addressing

Given the host requirements of 50-150 nodes per remote LAN and the existing subnetting scheme of /24 (class C mask) on the corporate network, it makes sense to assign /24 subnets of the 172.17.64.0/19 space to every one of the 25 spoke LANs. Every LAN will have addressing space for up to 254 nodes with this subnetting scheme that will facilitate future growth requirements at each site. This fulfills the earlier requirement concerning network growth and planning. This masking scheme will be easily understood by the desktop support staff and will work with existing routers running the IGRP routing protocol.

Notes:
IGRP does not carry subnet information in routing updates and routers require a uniform masking scheme enterprise-wide.

The spoke router LAN subnets on this network will be assigned out of the range 172.17.64.0-172.17.88.255.

The hub router will attach to an existing corporate backbone Ethernet segment, and will be assigned an IP address from that subnet, which does not fall within the 172.17.64.0/19 range. You are given 172.17.10.240/24 for the Hub router Ethernet IP address.

WAN Addressing

Before the WAN IP address plan can be devised for the routers on this network, the decision must be made as to whether to treat the frame relay PVCs as a single multipoint subnet or a collection of point-to-point links on the Cisco routers.

Notes:
Multipoint mode models the Frame Relay cloud as a LAN subnet, whereas the point-to-point mode models each PVC as a separate WAN point-to-point link in terms of addressing and routing

Given the additional requirements to support the IPX protocol in an any-to-any fashion, the point-to-point model is the only option.

TIPS:
IPX RIP is a distance-vector routing protocol with limitations of the split-horizon behavior of not sending routing updates out an interface on which they were received. The multipoint model would not facilitate IPX any-to-any as the router would not send IPX routing updates out to any of the remote routers.

To support the point-to-point model, you must define individual router serial port logical interfaces or subinterfaces, each of which will represent a discrete IP subnet and IPX network. TCP/IP addressing can accommodate this model most efficiently by assigning each of the subinterfaces with a /30 subnet. IP address space for these WAN links will be derived from further subnetting of a single /24 bit subnet (172.17.95.0) The example Hub router configuration (TENN) that follows provides more details:

    TENN#    interface serial 0         encapsulation frame-relay ietf         frame-relay lmi-type ansi         no ip address         interface serial 0.1 point-to-point         description PVC to Cumberland router         ip address 172.17.95.1 255.255.255.252         ipx network 179500         frame-relay interface-dlci 401 broadcast         interface serial 0.2 point-to-point         description PVC to west LA router         ip address 172.17.95.5 255.255.255.252         ipx network 179504         frame-relay interface-dlci 402 broadcast

OSPF Area Organization

Given the relatively small size of this network (less than 50 routers), it will be practical to include all routers into one single OSPF area. This will create a “portable” OSPF network that can be easily integrated into the enterprise corporate OSPF network once converted from IGRP. Because you do not know the future location of the OSPF backbone, you decide to be safe and put all routers in this network into a non-zero area. Putting this network into a non-zero area will allow you to avoid a future mass router reconfiguration after the corporate network is converted to OSPF. You assign this non-zero OSPF area an identifier of 64, because this number is the base number of the /19 CIDR block which is a logical representation of the addressing. You decide to use the company’s registered BGP AS# of 5775 as the OSPF process ID# for this network:

    router ospf 5775

The hub router in Tennessee will be the sole ASBR in this network, as it must run OSPF and IGRP to support mutual redistribution of routes between the Campus and WAN networks.

Because all routers in the frame relay network will be in area 64, no backbone area (area 0) will be created, and subsequently no routers will be configured as ABRs or backbone routers at this point.

Figure 7-29 shows the OSPF area architecture established for Terrapin.

Figure 7-29 OSPF Frame Relay network area architecture.

Specifying the OSPF Network Type

Use the default OSPF network type of point-to-point because you are modeling the router frame-relay cloud as individual point-to-point subinterfaces. The initial step of DR/BDR election is not required because only two routers exist on point-to-point networks, resulting in quick adjacency formation upon startup.

Implementing Authentication

The IS security manager insists that you use OSPF authentication to provide a low level of security on this network. You implement simple password authentication by assigning a key of “watchtower” to your OSPF area 64. All OSPF routers added to this frame relay network need to have this key configured in order to form an OSPF adjacency with the Hub router. This authentication will need to be entered under the OSPF process ID and on each serial interface as follows:

    interface serial 0.1 point-to-point      description PVC to Cumberland router      ip address 172.17.95.1 255.255.255.252      ip ospf authentication-key watchtower      ipx network 179500      frame-relay interface-dlci 401 broadcast    !    router ospf 5775      area 64 authentication

Notes:
If you are planning on implementing OSPF authentication, you should also enable the Cisco Password encryption option through the use of the service password- encryption command.

Configuring Link Cost

Because all spoke routers will only have one PVC provisioned to the hub router, there is no need to configure specific OSPF costs to links to engineer traffic patterns in a particular matter. Use the defaults by not assigning costs in router configurations.

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