Scalable Routing Design Principles

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Copyright © The Internet Society (2000). All Rights Reserved.

Abstract

Routing is essential to a network. Routing scalability is essential to a large network. When routing does not scale, there is a direct impact on the stability and performance of a network. Therefore, routing scalability is an important issue, especially for a large network. This document identifies major factors affecting routing scalability as well as basic principles of designing scalable routing for large networks.

Table of Contents

   1           Introduction  ..................................      2
   2           Common Routing Design Goals  ...................      3
   3           Characteristics of Today's Large Networks  .....      3
   4           Routing Scaling Issues  ..........................    3
   4.1         Router Resource Consumption  .....................    4
   4.2         Routing Complexity  ..............................    5
   5           Routing Protocol Scalability .....................    6
   5.1         IS-IS and OSPF  ..................................    6
   5.2         BGP  .............................................    8
   6           Scalable Routing Design Principles  ..............    9
   6.1         Building Hierarchy  ..............................   10
   6.2         Compartmentalization  ............................   13
   6.3         Making Proper Trade-offs  ........................   13
   6.4         Reduce Burdens of Routing Information Process  ...   14
   6.4.1       Routing Intelligence Placement  ..................   14
   6.4.2       Reduce Routes and Routing Information  ...........   15
   6.4.2.1     CIDR and Route Aggregation  ......................   15
   6.4.2.2     Utilize Default Routing where it's Possible  .....   15
   6.4.2.3     Reduce Alternative Paths  ........................   16
   6.4.3       Use Static Route at Edge  .........................  16
   6.4.4       Minimize the Impact of Route Flapping  ............  16
   6.5         Scalable Routing Policy and Scalable Implementation  17
   6.6         Out-of-band Process  ..............................  19
   7           Conclusion and Discussion  ........................  19
   8           Security Considerations  ..........................  20
   9           Acknowledgement  ..................................  21
   10          References  .......................................  21
   Author's Address ..............................................  22
   Appendix A  Out-of-Band Routing Processes  ....................  23
   Full Copyright Statement  .....................................  26

1. Introduction

Routing is essential to a network. Without routing, packets cannot be delivered to desired destinations and the network would be non- functional. The challenge of designing the routing for a large network, such as a large ISP backbone network, is not only to make it work, but also to make it scale. Without a scalable routing system, a network may suffer from severe performance penalties, as unfortunately proven by disastrous events in large networks. This document attempts to analyze routing scalability issues and define a set of principles for designing scalable routing system for large networks.

The organization of this document is as follows: Section 2 describes routing functions and design goals. Sections 3 and 4 discuss the characteristics of today's large networks and the associated routing scaling issues. Section 5 explores routing protocol scalability, and Section 6 presents scalable routing design principles. Section 7 provides a conclusion to the document.

2. Common Routing Design Goals

The basic goals a routing system should achieve are as follows:

o Stability
o Redundancy and robustness
o Reasonable convergency time
o Routing information integrity
o Sensible and manageable routing policy

The challenge of designing routing in a large network is not only to achieve these basic goals but also to make the routing system scale.

3. Characteristics of Today's Large Networks

Today's large networks typically possess the following features:

• They are composed of a large number of nodes (routers and/or switches), typically in the hundreds. Some provider networks include customer CPE routers within their administrative domain, which increases the number of nodes to thousands.

• They have rich connectivity to meet redundancy and robustness requirements, and they consequently have complex topologies.

• They are default-free; that is, they carry all the routes known to the entire Internet. Currently, the total number is approximately 70,000.

• The customer aggregation routers inside the large networks connect sometimes hundreds of customer routers.

These characteristics impose a direct challenge to the routing scalability of the network.

4. Routing Scaling Issues

Today, the main issues surrounding routing scaling are: i) excessive router resource consumption, which can potentially increase routing convergency difficulties thus destabilize a network; and ii) routing complexity, resulting in poor management of network, producing low service quality.

4.1. Router Resource Consumption

The routing process puts bursty loads on routers, especially under unstable network conditions. In the extreme case, the routing process takes all available resources from the routers, which results in slow routing convergence or no convergence. A network is paralyzed when it cannot converge internal routing information.

It's worthy noting that routers with internal architectures that tightly couple forwarding and routing processes tend to handle the excessive routing load poorly. The emerging new generation of routers with the architecture of separating resource used for forwarding and routing could provide better routing scalability.

Today, a large network typically employs IS-IS [1,2] or OSPF [3] as an Interior Routing Protocol(IGP) and BGP [4] as an Exterior Routing Protocol(EGP), respectively. The IGP calculates paths across the interior of the network. BGP facilitates routing exchange between routing domains, or Autonomous Systems (AS). BGP also processes and propagates external routing information within the network. The presence of a large number of routers and adjacencies in a network, coupled with frequent topology changes due to link instability, will contribute to excessive resource consumption by the interior routing. In the case of exterior routing, a large quantity of routers in a BGP system plus frequent routing updates (route flapping) would put a heavy burden on the routers. Section 5 describes scaling issues with IS-IS, OSPF and BGP in detail.

In addition, having many destinations in a routing system, combined with multiple paths associated with these routes, impose the following scaling issues on BGP:

• A large number of routes combined with multiple paths for each increases the cost of routing processing for route selection, routing policy application and filtering.

• Too many routes combined with multiple paths requires large amounts of memory on routers for storage. The demand is even higher at InterExchange Points such as NAPs.

• The larger the number of routes, the greater the chance route flapping will occur and the more BGP routing updates will happen as a result. Based on statistics collected by [5], thousands of BGP updates in a measured 15 minute interval can occur on a typical default-free router at a NAP.

Route flapping refers to frequent routing updates occurring due to network instability, for example, when the state of a physical link in the network is fluctuating, or when a BGP session is torn down and re-established numerous time within a short period of time.

To facilitate fast convergence, topology change information must be propagated in a timely fashion. When a route becomes unavailable and is withdrawn, the information is typically sent immediately. If the affected routes have been announced to the global Internet, the update information is likely to be propagated to the entire Internet.

Route flapping has a profound impact on routers running BGP. The routers have to process routing information frequently and this consumes a tremendous amounts of the available resources. When a local route or link is oscillating, interior routing is affected as well by excessive topology information flooding and subsequent shortest path calculations. However, OSPF (or IS-IS) imposes rate limits on such activity to reduce the burden on the routers. For example, OSPF specifies that an individual SLA can be updated at most once every 5 seconds. This essentially dampens the flapping.

Moreover, large numbers of E-BGP sessions processed by a single router create another potential scaling issue. Large networks usually have huge customer subscriptions and connections. To scale the hardware and the number of nodes in the network, providers tend to dedicate a group of customer aggregation routers, each connecting as many customer CPE routers as possible. As a result, it's not uncommon for a customer aggregation router to handle hundreds of E-BGP sessions, which imposes potential problems, such as BGP session processing and maintenance, route processing, filtering and route storage.

4.2. Routing Complexity

Routing complexity can lead to network management difficulties, which will have an impact on trouble shooting and quick problem resolution. It can result in a less than desirable service quality across the network. Complicated routing policies and special cases or exceptions in a routing design can contribute to routing complexity in a large system.

Routing Policy refers to the administrative criteria for handling routing information, commonly in the form of routing path selection and route filtering. The way routing information is handled has a direct impact on traffic flow within a network and across domains. As a result, it affects business agreements among different networks. Therefore, the determination of routing policy is largely dominated by non-technical concerns, such as business considerations. Routing policy can be very complex, which would make management and configuration an unscalable task.

The keys to reducing routing complexity are systematic as well as consistent routing scheme and a routing policy that is simple but meets the requirement of administrative polices.

Another factor contributing to the complexity of routing management is prefix-based route filtering. As is well known, prefix-based filtering is necessary in order to protect the integrity of the routing system. This becomes a challenge when the number of routes known to the Internet is as large as it is today.

5. Routing Protocol Scalability

Today's commonly deployed routing protocols are IS-IS or OSPF for Interior routing (aka IGP) and BGP for exterior routing (aka EGP). In terms of scaling and other aspects, these protocols are already an improvement over the previous generation of protocols, such as RIP and EGP. However, scalability is still a major issue when a network is large, when a routing design is insensitive to scaling issues, or the protocol implementation is inefficient.

5.1. IS-IS and OSPF

As described earlier in the document, IS-IS and OSPF are Link State routing protocols. The basic components of a link state routing protocol are i) generation and maintenance of a Link-State-DataBase (LSDB) that describes the routing topology of a given routing area; and ii) route calculation based on the topology information in the database. Each node in a routing area is responsible for describing its local routing topology in a Link State Advertisement or LSA (LSP in the case of IS-IS.) Each individually generated LSA will be distributed or flooded to all the routers in the area. Each router receives LSAs from all the other routers, forming a link-state- database that reflects the routing topology of the entire routing area.

The main associated scaling issues are the complexity of the link state flooding and routing calculation, plus the size of the LSDB which contributes to the cost of routing calculation and router memory consumption.

Flooding is the process by which a router distributes its self- originated LSA to the rest of the routers in the area in case of any link state change. A router will send the LSA via all its interfaces. When receiving an LSA update, a router validates the information and updates its local LSDB before sending it out via all its own interfaces, except the one from which it received the original LSA update. Given the nature of IS-IS or OSPF flooding, a full-mesh network with N routers would have O(N^2) of LSAs flooded in the network when a single link failure occurs. A single router outage would cause LSA in the order of O(N^3) to be flooded in the system.

In the case of OSPF, the protocol will refresh or flood every 30 minutes even under stable network conditions, which could increase the problem for an already highly loaded router.

From the above discussion, one can easily observe that the more routers and adjacencies in a Link State IGP routing area, the more CPU burden there are for each router to bear. When a network is unstable, the load will be amplified.

A link-state protocol typically uses Dijkstra's Shortest Path First (SPF) algorithm for route calculation. The Dijkstra algorithm scales to the order of O(N^2), where N is the number of nodes. The algorithm could be improved to the order of O(l*logN) where l is the number of links in the network and N is the number of destinations or routers [6].

Consequently, link state routing protocols do not scale to a network topology with many routers and excessive adjacencies in an area. When the network topology is unstable, the computation, processing and bandwidth costs are magnified, which causes excessive consumption of router resources. When the instability prevents IS-IS or OSPF from maintaining adjacencies, a network routing meltdown occurs.

Node adjacencies are discovered and maintained through the exchange of HELLO messages sent periodically from each node. When a node fails to receive HELLO messages from its neighbor within a certain period of time (40 seconds for OSPF and less for IS-IS), it considers the neighbor down. When heavy flooding, re-calculation and other activities happen that make router CPU a scarce resource, a router may not be able to allocate CPU time to send or process HELLO packets. Routers in the network then lose adjacency, which magnifies the instability. As a result, an isolated instability can escalate to a routing failure across the entire network.

Link-state IGPs also do not scale well to carry a large number of routes such as the 70,000 routes known to the Internet today. Since external routes are included in the link-state-database and in LSA

(LSP for IS-IS) updates, the link bandwidth and router memory consumption will be tremendous. Moreover, due to the large size of LSA updates, it would aggravate router resource consumption in the process of LSA flooding, especially under unstable network condition.

To summarize, a scalable design should avoid inclusion of too many routers in an IGP routing area, a large external routes carried by IGP and, more important, excessive adjacencies in the area.

5.2. BGP

BGP is an inter-domain routing protocol allowing the exchange of routing or reachability information between different Autonomous- System networks. Functionally, BGP is composed of External BGP(E-BGP) and Internal BGP(I-BGP). E-BGP is used for exchanging external routes while I-BGP is typically used for distributing externally learned routes within an AS.

The general costs of BGP are as follows:

• CPU consumption in BGP session establishment, route selection, routing information processing, and handling of routing updates

• Router memory to install routes and multiple paths associated with the routes.

The major scaling issue associated with BGP lie in the full mesh I- BGP connections. Since it does not scale for an IGP to carry externally learned prefixes, as mentioned in the previous section, I-BGP assumes this duty. In order to prevent routing loops, prefixes learned via I-BGP are prohibited from being advertised to another I- BGP speaker. As a result, a full mesh of I-BGP sessions among the routers within an AS is required. In an AS with N routers, each router will have to establish I-BGP sessions with N-1 routers, and the system complexity is in the order of O(N^2). Therefore, BGP scales poorly when the number of routers involved in I-BGP mesh is large.

A large network normally learns all the routes known to the Internet, which is approximately 70,000. I-BGP will need to carry all these routes.

The large number of I-BGP sessions and routes consumes tremendous resources from each router, especially during BGP session establishment and during periods of heavy route flapping.

Frequent routing updates are another potential scaling problem in large networks. BGP uses incremental updates and sends out routing information about unreachable routes quickly for fast convergence. This is a great improvement from EGP, in which the whole routing table is updated at a fixed time interval. However, when a network is unstable the updates, especially those containing route withdrawals, are sent immediately, causing global BGP updates. As a result, network instability initiated anywhere in a network triggers updates all over the Internet. This effect is magnified when large amounts of routes are visible to the Internet, putting a heavy load on routers that participate in BGP.

The introduction of a routing hierarchy in BGP, through I-BGP Route Reflectors [7] and BGP Confederations [8], for example, will help alleviate the scaling problem caused by the requirement of full mesh I-BGP establishment.

Another potential solution is to avoid the requirement of full mesh pairwise I-BGP connections. This will change the way that BGP distributes routing information among the I-BGP peers. Mechanisms worth considering are using multicast to distribute information or adopting flooding mechanisms similar to those used in IS-IS or OSPF. Further investigation of the implication of using such mechanism for BGP route distribution is needed.

Route dampening [9] is one way to reduce excessive updates triggered by route flapping. The trade-off between fast convergence and stability of the network should be considered, as discussed in section 6.3.

6. Scalable Routing Design Principles

The routing design for a large-scale network should achieve the basic goals of accuracy, stability, redundancy and convergence as described in Section 2 and moreover should achieve it in a scalable fashion.

How routing scales is influenced by protocol design decisions, protocol implementation decisions, and network design decisions. A network engineer has direct control over network design decisions and can have substantial influence over protocol design and implementation. The focus of this document is network design decisions.

Following is a set of design principles for making a large network routing system more scalable:

o Building hierarchy
o Compartmentalization
o Making proper trade-offs
o Reducing route processing burdens
o Defining scalable routing policies and implementation o Utilizing out-of-band routing assistance

6.1. Building Hierarchy

As discussed in Section 5.1, OSPF and IS-IS scale poorly when a network has a large number of routers and in particular, a large quantity of adjacencies. This has unfortunately been proven by networks that deploy IP over ATM with full mesh adjacencies among the routers. The full mesh overlay design combined with the inefficient protocol implementation led to disastrous network outages. A lesson learned from this is to avoid full mesh overlay topology in a large network with a large, flat network routing structure.

Building hierarchical routing structures in the network is the key to achieving routing scalability in a large network. As discussed earlier in this document, large networks are usually composed of many routers with a complex topology, which results in a large number of adjacencies. As also discussed earlier, currently available routing protocols scale poorly for handling a large number of routers in a routing domain or many adjacencies among the routers. Therefore, it is sensible to build a routing hierarchy to reduce the number of routers as well as the number of adjacencies in a routing domain.

The current common practice is to build a two-tiered hierarchy in a network with a center component (or transit core network) to which a number of outskirt components (or access networks) attach. The transit core network covers the entire geographical area the network serves; each access network (aka regional network) covers one region. There are usually no direct link connections among the regional components. Traffic from one regional network to another traverses the transit core. Customer networks connect only to access or regional networks. There are a number of ways to build a routing hierarchy in the above described hierarchical network topology.

1. Completely Separate Routing Domains

This design treats the transit core network and each regional network as completely independent ASs with respect to routing, and each AS runs an independent IGP. Each regional network E-BGP with the transit core for exchanging routing knowledge. Full I-BGP connections need to be established only within each component network. With this design, the maximum number of routers in an IGP domain is the total number of routers in each component. As a result, the IGP processing load is reduced, and the number of routers in an I-BGP mesh in the network routing system is decreased dramatically.

Another advantage of this design is that it compartmentalizes the routing system so that instability in one such component has less impact on the entire system. See the discussion in section 6.2.

The main disadvantage of this scheme is that it inserts one extra AS in the routing path when routes are advertised to the Internet via BGP. This extra AS in the path may cause route selection difficulties for other providers.

2. One Domain with IGP and BGP Hierarchy

This method includes the transit core and each regional network into one AS domain. The routing hierarchy is realized by utilizing multi-level IS-IS or OSPF areas and either BGP Confederation or I-BGP Reflector or a combination of the two.

This mechanism avoids the introduction of an extra AS in the routing path, which is an advantage over the method described in Point 1). However, multi-area hierarchical IGP is rarely used now-a-days in large networks since most of them are using IS-IS for internal routing, which does not have sufficient multi-level support. Although IS-IS supports multi-area routing, it imposes a strict hierarchy between backbone and sub-areas and allows only the advertisement of a default route from the backbone area to the sub-areas instead of specific prefixes. This restriction may be suitable for a network with a simple sub-area topology. A sub-area in a large network, typically a regional or access network, itself has a complicated topology. Receiving highly abstract routing information, such as a default route, would affect the sub-area's ability to make route selections required for traffic engineering. It would also limit the information passed to external ASs, for example, IGP-derived BGP Multi-Exit-Discriminator (MED) information.

Efforts are being made to modify the IS-IS protocol to allow the distribution of specific route from backbone area to sub-areas. A mechanism facilitates such distribution is specified in [15]. When implementation of such mechanism become available, implementing multi-level IGP will be an attractive option for building routing hierarchy within a large network.

3. One IGP Area with BGP Hierarchy

In lieu of multi-area IS-IS, the routing hierarchy could be achieved by defining one IGP domain for the entire network while employing a BGP hierarchy. Fortunately, the hierarchical topology of the network in this case helps reduce adjacencies in the routing domain (recall there are no connections among the second- level network components). In addition, improvements could be made to further reduce the adjacency by carefully arranging the adjacencies to keep them at a minimum but still achieve good redundancy. However, this is less than ideal since the number of routers remains unchanged, which increases the load on the SPF calculation. Moreover, instability within any regional network would still affect the entire network (that is, there would be no fault isolation).

Even with one IGP domain, it is possible to build BGP hierarchy to make I-BGP more scalable in the network. BGP Reflectors and BGP Confederations are existing mechanisms to address the scaling problem of full-mesh I-BGP.

Further, a BGP reflector provides the ability to build more than two levels of hierarchy, as long as the interactions among the different levels of the hierarchy are carefully arranged to avoid the possibility of creating routing loops.

Questions worth asking are: "Are two levels of routing hierarchy sufficient for handling scaling issues?" "Is there really a need for more than two levels of hierarchy?"

When a second-tier sub-domain of a large network, such as a regional network, grows too big for routing protocols to handle, either another layer of hierarchy needs to be introduced or the sub-domain needs to be split into multiple second-tiered sub-domains.

Keeping two levels of hierarchy and adding more sub-domains appears to be more manageable than adding another level to the hierarchy. However, one concern is to avoid adding more nodes to the top-level or transit core network to make it less scalable. Connecting the split sub-areas to the same core router would eliminate the need to add more nodes in the core area than is recommended.

Having more than two levels of hierarchy would exceed the capability of IGPs as they are defined today. In OSPF, for example, all the areas must be connected via the backbone area, which eliminates the possibility of having more than two levels of hierarchy. IS-IS has the same limitation. Therefore, the protocols need to be redefined should more than two hierarchical layers in IGP be desirable.

The complexity of protocols and management will increase with the number of levels added to the hierarchy. According to [6], most of the OSPF protocol bugs found over the years are related to routing area support. Because the interaction among the multiple levels increases management and debugging complexity, it is desirable to keep the levels within a hierarchy to a minimum.

6.2. Compartmentalization

A scalable routing design of a large network should be able to localize problems or failures, thus preventing them from spreading to the entire network, consuming resources of network routers, and causing network wide instability. This is compartmentalization. Network compartmentalization makes fault isolation possible which contributes the stability of a large network.

To achieve compartmentalization in routing design for a large network, one needs to avoid a design where the whole large network is one flat routing system or routing domain. This is the reason for the architecture of dividing interior and exterior routing in the global routing system. Within a network, it is best to divide the network into multiple routing domains or multiple routing areas. For example, in OSPF, only summary route SLAs, rather than individual area routes, are flooded beyond the area. When an area border router aggregates the routes in its sub-area, instability of any route included in the summary route would not cause flooding of SLAs to other areas. As a result, router resources in other areas would not be consumed for handling flooding and the SPF recalculation. In other words, instability within each individual area would be prevented from spreading to the entire routing domain.

Since building a routing hierarchy essentially divides a big routing area into smaller areas or domains, it help achieve the goal of compartmentalization.

6.3. Making Proper Trade-offs

When designing routing for a large network, the overall goal should be set with considerations of routing scalability and stability. The trade-offs between conflicting goals should be taken into account. Examples of such trade-offs are redundancy vs. scalability and convergence vs. stability.

Redundancy introduces complexity and increased adjacencies to the network topology. Redundancy also imposes the need for as many alternative paths as possible for each route, which increases route processing and storage burdens. Because of these problems, it may be necessary to sacrifice absolute redundancy in favor of a reasonable level that scales better for the routing system.

Fast convergence requires that changes in network topology be propagated to the network as quickly as possible. Such action increases routing updates and, consequently, the route processing burden. The burden is aggravated when a network carries full Internet routing information, as large networks usually do, and topology changes happen frequently. Route dampening may be necessary to achieve stability at the expense of absolute fast convergence.

6.4. Reduce Burdens of Routing Information Processing

The tasks of reducing routing processing burdens includes: i) strategically place the routing intelligence within the network, ii) avoid carrying unnecessary routing information and iii) reduce the impact of route flapping.

6.4.1. Routing Intelligence Placement

A router that executes routing policies, performs route filtering and dampening is said to posses routing intelligence. Routing intelligence is needed for a network i) to enforce the business agreement between network entities in the form of routing policies; ii) to protect the integrity of the routing information within the network and sometimes iii) to shield a network from instability happening elsewhere in the Internet.

The more routing intelligence a router has, the more resources of the router are needed to perform those tasks. It is logical, then, to place as little routing intelligence as possible on routers that already are heavily burdened with other tasks.

Usually, traffic is heavily concentrated in the core of the network. Because traffic aggregates from the edge of the network toward the core, traffic is less concentrated near the edge of the network. Consequently, to build a scalable routing system, it is wise to place routing intelligence at the edge of the network, especially in the networks deployed with routers that do not sufficiently decouple forwarding and routing. In addition, pushing routing intelligency as close to the edge of the network as possible also serves the purpose of distributing computational and configuration burdens across all routers.

It is also desirable to move the heavy burden of processing routes to out-of-band processors, freeing more resources in network routers for packet forwarding and handling.

6.4.2. Reduce Routes and Routing Information

As discussed in Section 4.1, a large number of routes in the system is one of the major culprits in route scaling problems. Therefore, it is best to reduce the number of routes in the system without losing necessary routing information.

6.4.2.1. CIDR and Route Aggregation

CIDR as specified in [10] provides a mechanism to aggregate routes for efficiently utilizing IP address space as well as reducing the number of routes in the global routing table. CIDR offers a way to summarize routing information, which is one of the keys for routing scalability in today's Internet.

Route aggregation would not only help global Internet scalability but would also contribute to scalability in local networks. The overall goal is to keep the routes in the backbone to a minimum.

To achieve better aggregation within the network; that is, to reduce the number of routes in the network, a block of consecutive IP addresses should be allocated to each access or regional network so that when a regional network announces its routes to the transit core network, they can be aggregated. This way, the core and other regional networks would not need to know the specific prefixes of any particular access network. Although assignment of customer addresses from a provider block would have to be planned to support aggregation, the effort would be worthwhile.

6.4.2.2. Utilize Default Routing When Possible

The use of a default route achieves ultimate route summarization, which reduces routing information to minimum. Route summarization also masks the instability associated with an individual route, for example, in the case of route flapping. It's beneficial for a network to utilize default routing when appropriate. For example, if a second-tiered regional network is a stub and there is no connected customer requesting full Internet routing information, the regional network can simply point default to its connected core network. However, over-summarization of routing information has the danger of losing routing granularity and as a result, management of network such as traffic engineering would be adversely affected. Therefore, caution needs to be exercised when using default routing.

6.4.2.3. Reduce Alternative Paths

Due to the requirement of reliability, the connectivity in the Internet is rich, resulting in many paths toward a particular destination. In other words, there are many alternate paths in the BGP routing table towards the same destination, which consumes router memory and adds to the routing processing burden.

To make routing scale, it is desirable to reduce alternate paths while preserving reasonable redundancy. For example, on a given border router (such as a NAP router), one primary path plus an alternate path should provide reasonable redundancy. In this case, a third or a fourth alternate route could be discarded for the sake of scaling. This is a trade-off decision every network administrator needs to make based on the particular needs of her network.

6.4.3. Use Static Route at Edges

As mentioned earlier, one of the scaling issues in large networks is that a single router may fan out to hundreds of customer routers. As a result, resource consumption will be very intensive if all the customer routers communicate via BGP with the edge router. Is it necessary for the edge router to BGP with all of its attached customer routers?

At first glance, it seems necessary for a customer network in a different Autonomous System(AS) to exchange routing information with the provider network via BGP. However, this is not necessarily the case. When a customer network is single-homed (that is, if the sole network connection for a customer is via its provider network), BGP is not necessary and static routing can work. Since the customer network is single-homed, static routing will not have any negative impact on services. The advantages are that the customer aggregation router will have fewer E-BGP sessions to handle, and no route flapping can result from the statically configured customer routes.

Configuration of the customer's static routes on the provider's aggregation router may add management overhead, especially if a customer advertises a large number of routes. On the other hand, the set of routes a customer announces to the provider usually changes infrequently; thus it requires low maintenance once it is configured.

6.4.4. Minimize the Impact of Route Flapping

As discussed earlier, route flapping is largely caused by link instability and/or BGP session instability that results in excessive routing updates across the Internet. Route flapping can originate anywhere in the global Internet and affect every network in the

Internet routing mesh (BGP mesh). Given that there are over 70,000 routes known to the Internet and there is little isolation for route flapping, handling route flapping could be overwhelming to routers in any network.

One way to reduce the effect of route flapping is to turn on route dampening as specified in [10]. Essentially, dampening suppresses an unstable route until it becomes stable. The current practice is for each ISP to enable route dampening on its border routers. This way, excessive routing updates can be stopped at the border.

An ideal model is to suppress the announcement of a flapping route right at the source. One way to implement this is to have a router recognize instability associated with its directly connected links and suppress the announcement of the route. So far, there is no such implementation. This approach should be explored.

Route aggregation often masks route flapping since components of an aggregated route (more specific routes) would not cause the aggregated route to flap. Therefore using CIDR can also help to alleviate route flapping.

6.5. Scalable Routing Policy and Scalable Implementation

Routing policy involves routing decisions about acceptance and advertisement of certain routes to or from other networks and about routing preference when more than one route becomes available. Routing policy enforces business agreements between network entities and is largely governed by non-technical criteria. In essence, routing policy involves defining criteria for route filtering and route selection.

One aspect of route filtering has to do with traffic control between routing domains or between different provider networks. Making policy based on individual prefixes should be avoided in this case because, with the large number of prefixes in the Internet, it does not scale. Making policy based on ASs that administratively represent a set of prefixes scales better.

Another purpose of route filtering is to protect the integrity of routing information by preventing the acceptance of falsely advertised routing information that would lead traffic to 'black holes'. In this case, only prefix-based filtering will sufficiently achieve the goal. Prefix-based filtering needs to occur at the borders between a network and its direct customers or peer networks. The filtering is harder to manage at the boundary of the peer networks since a peer network usually advertises a large amount of prefixes. As mentioned earlier, there are about 70,000 routes known to the Internet. This means a large default-free network would need to filter on the order of hundred of thousands of prefixes or even more since a route could be advertised by more than one sources. The sheer amount of the prefixes to be filtered imposes challenges for router configuration memory and configuration management. To make it scale, one would need to rely on the help from an out-of-band process to sort out which prefixes should be accepted or denied from which source. IRR [11] and DNS [12] are among the current proposed mechanisms for implementing prefix-based filtering.

Route selection policy determines which path should be used to send traffic toward a certain destination. This is important, for example, when a network has two connections to another network and learns routes from both connections. The decision involves which path to select to send traffic to the customers behind the other network. The choices are typically:

• Directing traffic to the closest interconnection point for traffic to exit the network. This policy is also known as Hot- Potato-Routing

• Directing traffic to the optimal network exit point. The optimal exit point is determined based on certain criteria by the network administrator and is not necessary the closest exit point

• Always preferring routes advertised by directly connected customers

• Allowing other network or customer to determine the path

When a policy is defined, its implications for scalable implementation need to be considered. For example, if the policy allows customers to determine which paths traffic follows, customers, not the provider, should be required to set routing parameters to make the routing favor their preferred path. Customers can use the BGP community or mechanisms such as MED to set routing preferences in a much more scalable way. This avoids putting such routing management burdens solely on the provider. Distributing the routing management burden makes the policy implementation more scalable.

Another scaling measure is to avoid making complex policy. When routing policy is complex, management, such as configuration of the router and debugging, would be a problem. The ultimate goal is to make the network manageable.

The following basic principles would help scale the routing policy management.

• Making policies as simple as possible but meet the requirements

• Automating as much as possible to avoid error-prone manual work

• Avoiding policy based on individual prefixes as much as possible with the exception of prefix-based route filtering for protecting routing integrity

• Avoiding making exceptions

• Using out-of-band routing policy processing where possible

6.6. Out-of-Band Process

A typical router assumes both routing and forwarding functions. However, conceptually, routing and forwarding are two separate processes. A router's ultimate task is to forward packets based on its forwarding table, which is derived from routing information. One of the main causes of route scaling problems is that routers run out of processing power because routing requires too much processing. While a router has to forward packets, it does not necessarily have to exchange and process routing information or execute routing policy; these tasks can be performed elsewhere. Thus the question should be: Would it be possible to remove the routing process from a router to reduce its burden? Moving the routing process from the routers to other systems is referred to as out-of-band route processing.

Out-of-band route processes would, in short, perform the heavy-duty routing tasks. They would build a forwarding table for the router, select routes based on pre-defined policy, filter routes, and shield the router from route flapping attacks.

The shortcomings of out-of-band route processing are the possible introduction of delays in routing changes; the de-coupling of routing and forwarding paths, which could introduce inaccurate routing information; and the cost of extra equipment.

Appendix A presents a current example of out-of-band route processing. It also suggests other possible solutions.

7. Conclusion and Discussion

How routing scales has a direct impact on network stability and performance. With the fast growth of the Internet and consequent expansion of providers' networks, routing scaling become increasingly an important issue to address. This document identifies the major factors that affect route scalability and establishes basic principles for designing scalable routing in large networks.

The major routing scaling issues we are facing today are excessive router resource consumption due to routing processing burdens causing routing convergency difficulties thus introducing network instability; and routing complexity resulting in difficulties of management and trouble shooting causing degradation of service.

The outlined principles for designing a scalable routing system are building routing hierarchy; introducing fault isolation; reducing routing processing burden where possible; defining manageable routing policies and using the assistance of available out-of-band routing process.

The use of out-of-band resources to assist routing processing is a concept only been used in the Internet Exchange Points (IXPs). However, it could potentially be used to advantage within a network to help addressing routing scaling issues. This is a topic worthy of further exploration.

Routing protocols and/or their implementations can still be improved or enhanced for better handling of the scaling issues. For example, the IS-IS multiple level mechanism is needed in order to scale the IGP in large network. Also, using multicast or a reliable flooding mechanism for I-BGP updates instead of pairwise full mesh peering is something worth investigating.

It is our belief that even with the deployment of new technologies such as DWDM, MPLS and others in the future, the fundamental routing scheme will remain the current IGP/BGP paradigm. Therefore, the scalable routing design principles outlined in this document should still apply with the deployment of new technologies.

8. Security Considerations

This document deals with routing scaling issues and thus is unlikely to have a direct impact on security.

However, certain routing scaling improvement mechanisms suggested in the document, such as network compartmentalization, will possibly alleviate network outages caused by denial-of-service attacks since it would help prevent such outages from spreading to the entire network.

Although the mechanisms described in this document do not enhance or weaken the security aspect of routing protocols, it is worth indicating here that security enhancement of routing protocols or routing mechanisms may impact routing scalability. Therefore, when applying security enhancement in routing, one has to be aware of the implications on scalability.

For example, TCP MD5 signature option is proposed to be a mechanism to protect BGP sessions from being spoofed [13]. It is done on a per-session basis and the overhead of MD-5 extensions are minimal thus has no direct impact on scalability. There have been concerns about doing per-prefix AS path verification as any one ISP along a path could have forged or modified information (maliciously or not). One extreme solution is to have a signature for each prefix which gives very strong security but presents enormous scaling issues in terms of processing, memory and administrative overhead.

9. Acknowledgement

Special thanks to Curtis Villamizar and Dave Katz for the extensive review of the document and many helpful comments. Many thanks to Yakov Rekhter, Noel Chiappa and Rob Coltun for their insightful comments. The author also like to thank Susan R. Harris for the much needed polishing of English language in the document.

The author was made aware after the publication of this document that there is a relevant and independent presentation made by Enke Chen on the subject. The presentation is thus referenced in [14].

10. References

   [1]  "Intermediate System to Intermediate System Intra-Domain
        Routeing Exchange Protocol for use in Conjunction with the
        Protocol for Providing the Connectionless-mode Network Service
        (ISO 8473)", ISO DP 10589, February 1990.
   
   [2]  Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual
        Environments", RFC 1195, December 1990.
   
   [3]  Moy, J., "OSPF Version 2", RFC 2328, April 1998.
   
   [4]  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
        RFC 1771, March 1995.
   
   [5]  C. Labovitz, R. Malan, F. Jahanian, "Origins of Internet Routing
        Instability," in the Proceedings of INFOCOM99, New York, NY,
        June, 1999
   
   [6]  J. Moy, "OSPF-Anatomy of an Internet Routing Protocol",
        Addison-Wesley, January 1998.
   
   [7]  Bates, T., Chandra, R. and E. Chen, "BGP Route Reflection - An
        alternative to full mesh IBGP", RFC 2796, April 2000.
   
   [8]  Traina, P., "Autonomous System Confederation Approach to Solving
        the I-BGP Scaling Problem", RFC 1965, June 1996.
   
   [9]  Curtis, V., Chandra, R. and R. Govindan, "BGP Route Flap
        Damping", RFC 2439, November 1998.
   
   [10] Fuller, V., Li, T., Yu, J. and K. Varadhan "Classless Inter-
        Domain Routing (CIDR): an Address Assignment and Aggregation
        Strategy", RFC 1519, September 1993.
   
   [11] Villamizar, C., Alaettinoglu, C., Govindan, R. and D. Meyer,
        "Routing Policy System Replication", RFC 2769, February 2000.
   
   [12] Bates, T., Bush, R., Li, T. and Y. Rekhter, "DNS-based NLRI
        origin AS verification in BGP", Work in Progress.
   
   [13] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
        Signature Option", RFC 2385, August 1998.
   
   [14] E. Chen, "Routing Scalability in Backbone Networks." Nanog
        Presentation: http://www.nanog.org/mtg-9901/ppt/enke/index.htm
   
   [15] T. Li, T. Przygienda, H. Smit,  "Domain-wide Prefix Distribution
        with Two-Level IS-IS", Work in Progress.

Author's Address

   Jieyun (Jessica) Yu
   CoSine Communications
   1200 Bridge Parkway
   Redwood City, CA  94065

EMail:

          jyy@cosinecom.com

Appendix A. Out-of-Band Routing Processes

The use of a Route Server(RS) at NAPs is an example of achieving routing scalability through an out-of-band routing process. A NAP is a public inter-connection point where ISP networks exchange traffic. ISP routers at a NAP establish BGP peer sessions with each other. The result is full mesh E-BGP peering with a complexity of O(N^2) system wide. When the RS is in place, each router peers only with the RS (and its backup) to obtain necessary routing information (or more precisely, the necessary forwarding information). In addition, the RS also filters routes and executes policy for each provider's router, which further reduces the burden on all routers involved.

The concept of the Route Server can also be used to help address routing scalability in a large network.

1) RS Assisted Peering between Customer Aggregation Router and Customer Routers

Currently, in a typical large provider network, it's not unusual that a customer aggregation router connects up to hundreds of customer routers. That means the router has to handle hundreds of E-BGP sessions and filter a large number of prefixes. These tasks impose a heavy burden on the aggregation router. Reducing the number of customer routers per aggregation router is not an optimal option, since this would introduce more routers in the routing system of the whole network, which is neither scalable for backbone routing, nor cost efficient. Using an RS between customers and the providers' customer aggregation router become an attractive option to reduce the burden on the router.

Figure 1 shows one way of incorporating an RS router between a provider's customer aggregation router and customer routers.

                ---------------------------  LAN Media in a POP
                        |           |
                      -----        -----
                      |CR |        |RS |
                      -----        -----
                      / | \
                     /  |  \
                    C1  C2..Cn

Figure 1:

RS serving customer aggregation router connecting customer routers

In a scenario without an RS, the customer aggregation router(CR) has to peer with customer routers C1, C2 ... Cn (where n could be in the hundreds). When an RS router is introduced, CR, C1, C2 ... Cn peer with the RS router instead, and the RS passes the processed routing information (or forwarding information) to all of them, according to policy and filters.

The advantages are obvious:

• The customer aggregation router peers only with the RS router instead of with hundreds of customer routers.

• The customer aggregation router does not need to filter prefixes or process routing policies, which frees resources for packet forwarding and handling.

One general concern with the use of an RS router is the possibility of a mismatch of routing connectivity and the physical connectivity. For example, if the link between the CR and C1 is down and if the RS router is not aware of the outage, it will continue to pass routes from C1 to the CR, and the traffic following these routes will be black holed. However, this is not a problem in the specific application described here. This is because the RS router has to go through the CR to peer with C1, C2 ... Cn. When the link is down, C1 is inaccessible from the RS router, and no routing information can be exchanged between the two. Consequently, the RS will announce no routes related to C1.

Another concern is the creation of single point of failure. If the RS router is down, no routing information can be exchanged between the customer aggregation router and C1, C2 ... Cn, and no traffic will flow between them. This problem could be addressed by adding a second RS router as a backup.

In this scenario, since RS peers with C1 ... Cn via CR, it requires that when the RS router passes routing information to C1...Cn, it designates the IP address of the CR as the next hop. Likewise, when the RS router passes routes from each customer router to the customer aggregation router, it needs to place the correct next hop on the route. Modifications need to be made to the RS code to include this function.

2. Private RS Router at InterExchange Point

A large provider network often has many BGP peers at the Interexchange Point, NAP or private interconnection. This means a border router has to handle many E-BGP sessions. Since an

Interconnect points is usually the administrative boundary between ISPs, policy and route filtering are very demanding. This imposes a scaling problem on the border router.

Deploying many routers to distribute the load among them is an expensive solution: extra hardware and extra ports cost money. Shifting the routing burden to an RS router is a promising alternative solution. In the case of using RS for multiple peers at a private interexchange point, the scenario is similar to RS used between customer aggregation router and customer routers as described in 1) above. In the case of such peering at a NAP, the private RS could be placed either on the same NAP media or a private media between the ISP's NAP router and the RS.

3. RS Routers at Each POP in a Large Network

Even in a network with a hierarchical routing structure, a sub-area may become too large, and I-BGP full meshing may impose a scaling problem. One way to address this would be to split the sub-area or add yet another tier of I-BGP reflector structure. Another possible solution would be to use an RS router as an I-BGP Server. Depending on the topology of a POP, this solution may or may not be suitable. The use of RS routers at network POPs need to be investigated further.

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