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Designing Advanced Routing

This section discusses elements of advanced routing solution design using route summarization and default routing. It also discusses utilizing route filtering and redistribution in advanced routing designs. The discussion in this section

  • Describes why route summarization and default routing should be used in a routing design
  • Describes why route filtering should be used in a routing design
  • Describes why redistribution should be used in a routing design

Route Summarization and Default Routing

Route summarization procedures condense routing information. Without summarization, each router in a network must retain a route to every subnet in the network. With summarization, routers can reduce some sets of routes to a single advertisement, reducing both the load on the router and the perceived complexity of the network. The importance of route summarization increases with network size, as shown in Figure 3-1.

Figure 3-1

Figure 3-1 Route Summarization

Medium-to-large networks often require the use of more routing protocol features than a small network. The larger the network, the more important it is to have a careful design with attention to properly scaling the routing protocol. Stability, control, predictability, and security of routing are also important. Converged networks are increasingly used to support voice, IP telephony, storage, and other drop-sensitive traffic, and so networks must be designed for fast routing convergence.

Route summarization is one key network design element for supporting manageable and fast-converging routing. The Implementing Cisco IP Routing (ROUTE) course covers the configuration of route summarization and its benefits for routing and troubleshooting.

The design recommendations for summarizations are straightforward and include

  • Using route summarization to scale routing designs.
  • Designing addressing by using address blocks that can be summarized.
  • Using default routing whenever possible. Route summarization is the ultimate route summarization, where all other routes are summarized in the default.

Originating Default Routes

The concept of originating default routes is useful for summarization in routing. Most networks use some form of default routing. It is wise to have the default route ( /0) advertised dynamically into the rest of the network by the router or routers that connect to Internet service providers (ISP). This route advertises the path to any route not found; more specifically in the routing table, as shown in Figure 3-2.

Figure 3-2

Figure 3-2 Originating Default Routes

It is generally a bad idea to configure a static default route on every router, even if recursive routing is used. In recursive routing, for any route in the routing table whose next-hop IP address is not a directly connected interface of the router, the routing algorithm looks recursively into the routing table until it finds a directly connected interface to which it can forward the packets. If you configure a static default route on every router to the ISP router, the next hop is the ISP-connected router rather than a directly connected peer router. This approach can lead to black holes in the network if there is not a path to the ISP-connected router. This approach also needs to be reconfigured on every router if the exit point changes or if a second ISP connection is added.

If manually configured next hops are used, more configuration commands are needed. This approach can also lead to routing loops and is hard to change. If there are alternative paths, this static approach might fail to take advantage of them.

The recommended alternative is to configure each ISP-connected router with a static default route and redistribute it into the dynamic routing protocol. Static default route configuration needs to be done only at the network edge devices. All other routers pick up the route dynamically, and traffic out of the enterprise uses the closest exit. If the ISP-connected router loses connectivity to the ISP or fails, the default route is no longer advertised in the organization.

You might need to use the default-information originate command, with options, to redistribute the default route into the dynamic routing protocol.

Stub Areas and Default Route

Explicit route summarization is not the only way to achieve the benefits of summarization. The various kinds of OSPF stub areas can be thought of as a simpler form of summarization. The point of using OSPF stub areas, totally stubby areas, and not-so-stubby areas (NSSA) is to reduce the amount of routing information advertised into an area. The information that is suppressed is replaced by the default route (IPv4) or ::/0 (IPv6)

OSPF cannot filter prefixes within an area. It only filters routes as they are passed between areas at an Area Border Router (ABR).

OSPF stub areas do not work to IP Security (IPsec) virtual private network (VPN) sites such as with generic routing encapsulation (GRE) over IPsec tunnels. For IPsec VPN remote sites, the 0/0 route must point to the ISP, so stub areas cannot be used. An alternative to the default route is to advertise a summary route for the organization as a "corporate default" route and filter unnecessary prefixes at the ABR. Because OSPF cannot filter routes within an area, there still will be within-area flooding of link-state advertisements (LSA).

You can use this approach with the EIGRP, too. The ip default-network network-number command is used to configure the last-resort gateway or default route. A router configured with this command considers the network listed in the command as the candidate route for computing the gateway of last resort. This network must be in the routing table either as a static route or an IGP route before the router will announce the network as a candidate default route to other EIGRP routers. The network must be an EIGRP-derived network in the routing table or be generated by a static route that has been redistributed into EIGRP.

EIGRP networks typically configure the default route at ISP connection points. Filters can then be used so that only the default and any other critical prefixes are sent to remote sites. In many WAN designs with central Internet access, HQ just needs to advertise default to branch offices, in effect "this way to the rest of the network and to the Internet." If the offices have direct Internet access, a corporate summary can work similarly, "this way to the rest of the company."

In a site-to-site IPsec VPN network, it can be useful to also advertise a corporate summary route or corporate default route (which might be /8) to remote sites. The advantage of doing so is that all other corporate prefixes need not be advertised to the IPsec VPN site. Even if the IPsec network uses two or three hub sites, dynamic failover occurs based on the corporate default. For the corporate default advertisement to work properly under failure conditions, all the site-specific prefixes need to be advertised between the hub sites.

Filtering the unnecessary routes out can save on the bandwidth and router CPU that is expended to provide routing information to remote sites. This increases the stability and efficiency of the network. Removing the clutter from routing tables also makes troubleshooting more effective and speeds convergence.

Route Filtering in the Network Design

This section discusses the appropriate use of route filtering in network design. Route filtering can be used to manage traffic flows in the network, avoid inappropriate transit traffic through remote nodes, and provide a defense against inaccurate or inappropriate routing updates. You can use different techniques to apply route filtering in various routing protocols.

Inappropriate Transit Traffic

Transit traffic is external traffic passing through a network or site, as shown in Figure 3-3.

Figure 3-3

Figure 3-3 Avoid Inappropriate Transit Traffic

With poorly configured topology, poorly configured filtering, or poorly configured summarization, a part of the network can be used suboptimally for transit traffic.

Remote sites generally are connected with lower bandwidth than is present in the network core. Remote sites are rarely desirable as transit networks to forward network from one place to another. Remote sites typically cannot handle the traffic volume needed to be a viable routing alternative to the core network. In general, when core connectivity fails, routing should not detour via a remote site.

In OSPF, there is little control over intra-area traffic. LSAs cannot be filtered within an area. OSPF does not allow traffic to arbitrarily route into and then out of an area. The exception is area 0, which can be used for transit when another area becomes discontiguous.

With EIGRP, it can be desirable to configure EIGRP stub networks. This informs central routers that they should not use a remote site as a transit network. In addition, the use of stub networks damps unnecessary EIGRP queries, speeding network convergence. Filtering can help manage which parts of the network are available for transit in an EIGRP network.

With BGP, the most common concern about transit traffic is when a site has two Internet connections. If there is no filtering, the connections advertise routes. This advertisement can put the site at risk of becoming a transit network. This should not be a problem with two connections to the same ISP, because the autonomous system number is present in the BGP autonomous system path. Based on the autonomous system path, the ISP router ignores any routes advertised from the ISP to the site and then back to the ISP.

When two ISPs are involved, the site might inadvertently become a transit site. The best approach is to filter routes advertised outbound to the ISPs and ensure that only the company or site prefixes are advertised outward. Tagging routes with a BGP community is an easy way to do this. All inbound routes received from the ISP should be filtered, too, so that you accept only the routes the ISP should be sending you.

Defensive Filtering

Route filtering can also be used defensively against inaccurate or inappropriate routing traffic. This is illustrated in Figure 3-4.

Figure 3-4

Figure 3-4 Defensive Filtering

One common problem some organizations experience is that they inherit inappropriate routes from another organization, such as a business partner. Your business partner should not be advertising your routing prefixes back to your network. Those destinations are not reached through the partner, unless you have a very odd network design. The default route should not be reached via the partner, unless the partner is providing your network with Internet connectivity.

Inappropriate partner advertisements can disrupt routing without filtering. For example, a partner may define a static route to your data center. If this route leaks into your routing process, a portion of your network might think that the data center has moved to a location behind the router of the partner.

Defensive filtering protects the network from disruptions due to incorrect advertisements of others. You configure which routing updates your routers should accept from the partner and which routing updates should be ignored. For example, you would not accept routing updates about how to get to your own prefixes or about default routing.

For security reasons, you should advertise to a partner only the prefixes that you want them to be able to reach. This provides the partner with minimum information about your network and is part of a layered security approach. It also ensures that if there is an accidental leak of another partner's routes or static routes into the dynamic routing process, the inappropriate information does not also leak to others.

The approach of blocking route advertisements is also called route hiding or route starvation. Traffic cannot get to the hidden subnets from the partner unless a summary route is also present. Packet-filtering ACLs should also be used to supplement security by route starvation.

Designing Redistribution

Redistribution is a powerful tool for manipulating and managing routing updates, particularly when two routing protocols are present in a network. This is shown in Figure 3-5.

Figure 3-5

Figure 3-5 Designing Redistribution

In some situations, routing redistribution is useful and even necessary. These include migration between routing protocols, corporate mergers, reorganization, and support for devices that speak only RIP or OSPF.

However, redistribution should be used with planning and some degree of caution. It is easy to create routing loops with redistribution. This is particularly true when there are multiple redistribution points, sometimes coupled with static routes, inconsistent routing summaries, or route filters.

Experience teaches that it is much better to have distinct pockets of routing protocols and redistribute than to have a random mix of routers and routing protocols with ad hoc redistribution. Therefore, running corporate EIGRP with redistribution into RIP or OSPF for a region that has routers from other vendors is viable, with due care. On the other hand, freely intermixing OSPF-speaking routers with EIGRP routers in ad hoc fashion is just asking for major problems.

When more than one interconnection point exists between two regions using different routing protocols, bidirectional redistribution is commonly considered. When running OSPF and EIGRP in two regions, it is attractive to redistribute OSPF into EIGRP, and EIGRP into OSPF.

Filtered Redistribution

When you use bidirectional redistribution, you should prevent re-advertising information back into the routing protocol region or autonomous system that it originally came from. This is illustrated in Figure 3-6.

Figure 3-6

Figure 3-6 Filtered Redistribution

For example, filters should be used so that OSPF information that was redistributed into EIGRP does not get re-advertised into OSPF. You also need to prevent information that came from EIGRP into OSPF from being re-advertised back into the EIGRP part of the network. This is sometimes called a manual split horizon. Split horizon is a routing protocol feature. The idea behind it is that it is counterproductive to advertise information back to the source of that information, because the information may be out of date or incorrect, and because the source of the information is presumed to be better informed.

If you do not do this filtering or use a manual split horizon, you will probably see strange convergence after an outage, you will probably see routing loops, and in general, you will experience routing problems and instability.

Both EIGRP and OSPF support the tagging of routes. A route map can be used to add the numeric tag to specific prefixes. The tag information is then passed along in routing updates. Another router may then filter out routes that match, or do not match, the tag. This is done using a route map in a distribution list.

One typical use of tags is with redistribution. In Figure 3-6, routers A and B can apply tags to routes from IGP X when they are advertised outbound into IGP Y. This in effect marks them as routes from IGP X. When routers A and B receive routes from Y, they would then filter out routes marked as from X when received from IGP Y, because both routers learn such routes directly from IGP X. The process of filtering also applies in the opposite direction.

The point is to get routes in the most direct way, not via an indirect information path that might be passing along old information.

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