Advanced OSPF Design Concepts

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Notice that Router A has learned about as an external route with metric of 10. The gateway of last resort is set to as expected. Thus, its default route is the E0 interface of Router C that has a default route in Router E.

Designing OSPF On-Demand Circuits

On-demand circuits can come in many different forms, from ISDN to SVCs. They tend to be implemented in one of two ways. First, they are put in place as a backup for the dedicated circuit, or second, they are for sites that require connectivity, just not all the time.

The OSPF on-demand circuits is an enhancement to the original OSPF protocol that allows efficient operation over on-demand circuits like ISDN, X.25 SVCs, and dial-up lines. This feature was first introduced in RFC 1793, “Extending OSPF to Support Demand Circuits.” It is fully supported by Cisco in all releases of their IOS.

This feature is useful when you want to connect telecommuters or branch offices to an OSPF backbone at a central site. As the pricing of demand circuits has gone down and the criticality of networks has increased, many network designers are turning to demand circuits as a means of back up.

Prior to this feature, OSPF periodic hello and LSAs updates would be exchanged between routers that connected via the on-demand link, even when no changes occurred in the hello or LSA information. This is, of course, normal operation for the OSPF protocol, but it has the unwanted side effect of causing the demand circuit to remain active because there was always interesting traffic to route across it.

However, with this new RFC, periodic hellos are suppressed and the periodic refreshes of LSAs are not flooded over the demand circuit. These packets bring up the link only when they are exchanged for the first time, or when a critical change occurs in the information they contain.

This suppression allows the demand circuit to be released. This is extremely important because most of the demand circuits have usage fees relating to them in either the length of use or amount of use and sometimes even both.

In this case, OSPF for on-demand circuits allows the benefits of OSPF over the entire domain, without excess connection costs. Periodic refreshes of hello updates, LSA updates, and other protocol overhead traffic is prevented from enabling the on-demand circuit when there is no “real” data to transmit.

Golden Rules for Designing Demand Circuits

As with every configuration of OSPF, there are a series of golden rules that you must be aware of before proceeding on. This is a list of them for demand circuits:

  Because LSAs that include topology changes are flooded over an on-demand circuit, it is advised to put demand circuits within OSPF stub areas or within NSSAs to isolate the demand circuits from as many topology changes as possible.
  To take advantage of the on-demand circuit functionality within a stub area or NSSA, every router in the area must have this feature loaded. If this feature is deployed within a regular area, all other regular areas must also support this feature before the demand circuit functionality can take effect. This is because Type 5 external LSAs are flooded throughout all areas.
  You do not want to implement this on a broadcast-based network topology because the overhead protocols (such as hellos and LSAs) cannot be successfully suppressed, which means the link will remain up.

Dial On-Demand Design Scenarios

There are a number of common scenarios that will be encountered if you are planning on using this feature of OSPF. The first two are ways (NOT the best ways!) to implement OSPF. In the following scenarios, Site Router A is the router equipped with on-demand dialing.

Design Scenario #1: Site Router Is in Two Areas (Neither Is Area 0)

This approach does not work as the LAN interface cannot be in more than one area, as shown in Figure 6-10. There is no exchange of link-state information between areas 1 and 2.

Figure 6-10  Site Router is in two areas (neither area is area 0).

As shown in Figure 6-10, the site router is located in two different OSPF areas with neither of them being area 0. However, if the site LAN is not included in the OSPF routing, and its routing information is injected with a static route either at the site router or at the distribution router, this could be made to work, although it is not the most optimal OSPF network design.

Design Scenario #2: Site Router Is in Two Areas (One Is Area 0)

This approach makes the Site Router (Router A) an area border router (ABR) under failure. It does work; however, it is not considered an acceptable design because it would make the Site Router part of area 0. This design would require more resources than would be cost-effective in all but the smallest networks (see Figure 6-11).

Figure 6-11  Site Router is in two areas (one is area 0).

Design Scenario #3: Site Router Is in One Area

This approach is the most suitable and works even if the backup server (“C” in Figure 6-12) is located elsewhere. The secret is that “C” does not summarize for its attached areas; thus, more specific prefixes are originated by “C” for those sites in failure. The disadvantage is that dedicated backup interfaces are required for each area.

Figure 6-12  Site Router is in one area.

The following are some sample configurations for this design scenario:

Site Router A Configuration

    Interface ethernet 0      ip address    interface serial 0      ip address    backup interface serial 1      backup delay 0 5    interface serial 1      ip address    router ospf 1      network area 1 

Site Router B Configuration

    interface fddi 0      ip address    interface serial 0      ip address    router ospf 1      network area 1      area 1 range      network area 0 

Site Router C Configuration

    interface fddi 0      ip address    interface serial 0      ip address    router ospf 1      network area 1      network area 0 

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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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