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Internet Routing Architectures, Second Edition

one sub-AS independently of IGPs in other sub-ASs, which would help control the instability of large IGPs.

In some situations, route reflectors can be used in conjunction with confederations. In particular, an AS can be divided into sub-ASs that each run route reflectors internally.

Whichever approach you use, you should always understand the restrictions and behavior of each method and design your network accordingly.

Controlling IGP Expansion

One of the ways in which administrators push their networks to the limit is by letting them grow in such a way that the IGP will be hard to manage. Whether the IGP is as outdated as RIP version 1 or as advanced as Open Shortest Path First (OSPF) and Intermediate System-to- Intermediate System (IS-IS), the issue of scalability will arise. So far, this chapter has discussed route reflectors and confederations as ways of managing IBGP growth. A scalable way of managing IGP expansion is to segment the AS into multiple regions, each running a single, distinct IGP. The individual regions, in turn, must be connected via BGP. With this design, the stability of one region would not affect the stability of another.

What criteria should network designers and architects follow in deciding whether their networks need segmentation? One thing is for sure: The Internet is one huge network that cannot be handled by running an IGP, and that is why it is segmented by BGP.

So what constitutes a large or small network? Is it the number of routers or the number of routes, and, if so, what number? You will hear different answers based on different administrators' experiences. The general answer to this question depends mainly on the robustness of the IGP, what tools it can offer to control the route explosion and instability, and whether BGP segmentation represents a more beneficial, less costly (in dollars and effort) method than relying on the IGP's tools.

Protocols such as OSPF and IS-IS offer certain hierarchical methods that can control route instabilities and provide means for route summarization. But even with these methods, the IGP can grow beyond control. A working guideline for today's networks is that IP routing tables that have 2,000 to 3,000 IGP interior routes might have reached a limit and need a closer look to make sure that they do not grow further. It is not the number of routes that causes problems, because BGP transit routers today carry more than 75,000 Internet routes with no problem. What causes problems is situations (such as hardware and access line instabilities) in which these routes end up bouncing and trying to converge, causing what is known as a network "meltdown."

Does this mean that networks with 3,000 IGP routes need to be segmented via BGP? Not necessarily. In most cases, a redesign of the IGP itself with more emphasis on using IGP segmentation and summarization techniques can bring down the number of routes to a manageable level.

To understand why the decision to control growth with BGP segmentation should be approached with caution, you need to understand what is compromised when ASs are segmented. The main strength of IGPs, especially IGPs based on link-state protocols, has

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always been convergence—their capability to quickly adapt to network changes. Another strength is their capability to develop a level of redundancy and load balancing.

BGP, on the other hand, was created to implement policies across AS boundaries, with no major emphasis on convergence. When segmenting routing domains with BGP, convergence is enhanced within the newly created smaller segments, but it might diminish when crossing locally administered AS boundaries because of the dependency of BGP on TCP sessions to carry routing updates.

Another drawback is the additional user intervention needed to control and manage the BGP policies that are imposed on this segmented, sub-AS design. As has been illustrated in this book, attribute manipulation is so far the only tool to manipulate the routing behavior of BGP. It should be obvious that controlling routing policies of multiple sub-ASs is more difficult than the routing policy of a single IGP. Understanding all these issues should help network architects use rational judgment when designing their networks. Although further discussion of these issues is beyond the scope of this book, Large-Scale IP Network Solutions[] provides some useful insight into how to identify and deal with these issues.

This section discusses two methods of segmenting the AS:

Multiple regions separated by IBGP

Multiple regions separated by EBGP

Segmenting the AS with Multiple Regions Separated by IBGP

The AS can be divided into multiple regions, each running different and independent IGPs. Regions are logically interconnected via a full IBGP mesh. For better redundancy, regions could also be physically interconnected in a fully meshed topology, as illustrated in Figure 9- 12.

Figure 9-12. Multiple Regions Via IBGP

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Each region will both inject its IGP routes into IBGP and announce a default route inside the region. As a result, each region will default to the BGP border router for destinations that do not belong to the region. Regional border routers will carry all routes from all regions (exchanged via IBGP) and will be able to direct the traffic accordingly. Each region is totally shielded from the instabilities of all the other regions because the internal non-BGP routers are exposed only to routes that comprise their respective region. You can use a separate IGP in case there is a need for a dynamic routing protocol to establish reachability between regional border routers participating in the full IBGP mesh.

This design is still missing one important piece—Internet connectivity. Connecting to the Internet in this scenario requires further planning. As illustrated in Figure 9-12, each region already follows a default route to reach other regions inside the AS. The problem occurs if the BGP border router (for the region) does not maintain continuous synchronization with the actual routes learned at the connection point to the Internet. In this case, internal non-BGP routers have to choose between the default to the Internet and the default to other regions, as illustrated in Figure 9-13.

Figure 9-13. Multiple Conflicting Defaults

To remedy this situation, all regions should always point to the regional BGP border router for the default, whether attempting to reach destinations on the Internet or in other intra-AS regions. This would require the Internet connections to be part of a central IBGP mesh, as illustrated in Figure 9-14.

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Figure 9-14. Multiple Regions with Internet Connectivity

Regions 1, 2, and 3 are interconnected via an IBGP mesh, which also provides Internet connectivity. Internal non-BGP routers in each region default to the BGP border router, which contains all routes. If the destination belongs to any other region, the traffic is directed to that region. Otherwise, the traffic will be sent to the Internet connections according to BGP policies.

This method does not have the flexibility to set policies for each region. Because all regions run under the same autonomous system, prefixes from each region cannot be easily differentiated from those in another region by BGP. Designs that are more complex could utilize the "community" attribute to differentiate prefixes between regions. This could be used in conjunction with a hierarchical route reflection configuration to create large virtual private networks, as you will see at the end of this chapter.

Segmenting the AS with Multiple Regions Separated by EBGP

If flexible policies are required between regions to direct traffic accordingly, for instance, each region can be represented as a separate autonomous system. EBGP operates between ASs, and IBGP is run within each AS. You can use a separate IGP if there is an additional requirement for a dynamic routing protocol to establish reachability between EBGP peers. Figure 9-15 illustrates this AS segmentation method.

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Figure 9-15. Multiple Regions Separated by EBGP

At first glance, this seems to be the most scalable solution for managing the growth of a large IGP. The AS is now divided into multiple ASs, with each region represented by a separate AS number and having a preferred connection to the Internet. Each AS injects its IGP routes into BGP so that they are propagated to all other regions and the Internet. Internal non-BGP routers in each region default to the regional BGP border routers, which contain all routes. BGP policies can set local preference of prefixes so that regional BGP border routers prefer interior company links, instead of exterior Internet connections, for communication between regions. With respect to external connectivity to the Internet, each region prefers its own external provider for Internet connectivity; if the link to its preferred provider fails, external providers from other regions can be used.

This design looks good on paper until you try to implement it. It's very difficult to justify all the AS numbers from American Registry for Internet Numbers (ARIN) or other Regional Internet Registries (RIRs) required for such a network. AS numbers are a finite resource that is quickly being depleted. ARIN and the other RIRs would likely require substantial justification before they would allocate multiple AS numbers to one network administrator. Unfortunately, using multiple ASs for better IGP stability is usually not a good-enough justification. You should consult your local RIR for information on precisely what "enough justification" entails.

Other alternatives include using private AS numbers or BGP confederations to control IGP expansion. We'll discuss these options in the following sections.

Using Private AS Numbers

Using private AS numbers is another way to divide a large AS into multiple regions, interconnected by EBGP. Regions will run IBGP internally and interconnect to each other via

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EBGP. In addition, each regional domain will inject its local prefixes into BGP to facilitate interregional connectivity. Internal non-BGP routers in each region will default to the regional BGP border routers, which contain all routes. Finally, you can run a separate IGP in case a dynamic protocol is needed to establish EBGP connectivity among regions.

This scenario works well without Internet connectivity. However, private AS numbers will have to be hidden from the outside world in order to connect to the Internet. Hiding private AS numbers requires a more involved design. Figure 9-16 illustrates a method that can be used to remedy the situation of private ASs with Internet connectivity.

Figure 9-16. Private ASs in a Multiprovider Environment

Figure 9-16 shows an AS that is broken into multiple private sub-ASs. Region 1 is now AS65001, region 2 is AS65002, and region 3 is AS65003. Different mutually exclusive IGPs are running in each private AS.

In order to facilitate multiple external Internet provider connectivity, AS100 is introduced as an interconnection point between all private ASs. It is important to note that this central backbone AS is a legal AS number. All private ASs will EBGP peer with backbone AS100 for interregional and Internet connectivity.

To prevent private AS numbers from leaking to external Internet providers, the network administrator uses the AS path-stripping technique discussed in Chapter 6. AS100 strips private AS numbers from each region before propagating those BGP updates to external Internet providers.

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