Internet Engineering Task Force Jieyun (Jessica) Yu
INTERNET DRAFT UUNET
Expires in December, 1999 June, 1999
draft-yu-routing-scaling-01.txt
Scalable Routing Design Principles
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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.
Contents
1 Introduction .................................. 2
2 Common Routing Design Goals ................... 2
3 Characteristics of Today's Large Networks ..... 3
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4 Routing Scaling Issues .......................... 3
4.1 Router Resource Consumption ..................... 3
4.2 Routing Complexity .............................. 5
5 Routing Protocol Scalability ..................... 6
5.1 IS-IS and OSPF .................................. 6
5.2 BGP ............................................. 7
6 Scalable Routing Design Principles .............. 9
6.1 Building Hierarchy .............................. 9
6.2 Compartmentalization ............................ 12
6.3 Making Proper Trade-offs ........................ 13
6.4 Reduce Burdens of Routing Information Process ... 13
6.4.1 Routing Intelligence Placement .................. 13
6.4.2 Reduce Routes and Routing Information ........... 14
6.4.2.1 CIDR and Route Aggregation ...................... 14
6.4.2.2 Utilize Default Routing where it's Possible ..... 15
6.4.2.3 Reduce Alternative Paths ........................ 15
6.4.2.4 Use New Technologies ............................ 15
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 .............................. 18
7 Conclusion and Discussion ........................ 19
8 Security Considerations .......................... 20
9 Acknowledgement .................................. 20
10 References ....................................... 20
Appendix A Out-of-Band Routing Processes .................... 21
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 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 penalty 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
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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:
o 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.
o They have rich connectivity to meet redundancy and robustness
requirements, and they consequently have complex topologies.
o They are default-free; that is, they carry all the routes known
to the entire Internet. Currently, the total number is
approximately 60,000.
o The customer aggregation routers sometimes connect hundreds of
customer routers.
These characteristics impose direct challenge to the routing
scalability of the network.
4. Routing Scaling Issues
Today, the main issues surrounding routing scaling are excessive
router resource consumption which can potentially destabilize a
network; and routing complexity resulting poor management of network
thus 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.
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It's worthy noting that routers with architecture that tightly couple
forwarding and routing process tend to handle the excessive routing
load poorly. Emerging new generation of routers separate resource
used for forwarding and routing are in the hope of scaling better.
Today, a large network typically employs IS-IS[1,2] or OSPF[3] as
Interior Routing Protocol(IGP) and BGP[4] as Exterior Routing
Protocol (EGP). IGP is responsible for routing information with
regarding to routers within the network. BGP facilitates routing
exchange between routing domains, or Autonomous Systems(AS). BGP
processes and propagates external routing information within the
network. Large number of routers and adjacencies in a network coupled
with frequent topology changes due to link instability would
contribute to excessive router resource consumption in the case of
interior routing. In the case of exterior routing, large quantity of
routers in a BGP system plus frequent routing updates (route
flapping) would put heavy burden on the routers. Section 5 describes
scaling issues with IS-IS, OSPF and BGP in detail.
In addition, many routes in a routing system combined with multiple
paths associated with these routes impose the following scaling
issues on BGP:
o Large number of routes combined with multiple paths for each
increase the cost of routing processing for route selection,
routing policy application and filtering.
o Too many routes combined with multiple paths require large
amounts of memory on routers for storage. The demand is even
higher at InterExchange Points such as NAPs.
o The large 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 high 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.
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
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to the entire Internet.
Route flapping has profound impact on routers running BGP. The
routers have to processing routing information frequently and this
consumes tremendous resources. When a local route or link is
oscillating, interior routing is affected as well by excessive
topology information flooding and 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
that a customer aggregation router handles 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 impact on trouble shooting and quick problem resolution. It
can result in 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
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 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
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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 about 60,000 today.
5. Routing Protocol Scalability
Today's commonly deployed routing protocols are IS-IS or OSPF for
Interior routing (aks IGP) and BGP for exterior routing (aka EGP). In
terms of scaling and other aspects, these protocols are already an
improvement over previous generation 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 generation and maintenance of a Link-State-DataBase
(LSDB) that describes routing topology of a given routing area and
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 link state
flooding and routing calculation. In addition, the size of LSDB
which contributes to the cost of routing calculation and router
memory consumption.
Flooding is a process for a router to distribute 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 occur. A single router outage
would cause LSA in the order of O(N^3) flooded in the system.
In the case of OSPF, the protocol will refresh or flood every 30
minutes even under stable conditions, it could magnify an already
highly loaded routers.
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From the above discussion, one can easily observe the more routers
and adjacencies in a Link State IGP routing area and adjacency, the
more CPU burden each router needs 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 the 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 cost are magnified, which causes excessive consumption of
route resources. When the instability prevents IS-IS or OSPF from
maintaining adjacencies, 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
routing failure across the entire network.
Link-state IGP also does not scale well to carry large number of
routes such as the 60,000 routes known to the Internet today. Since
external routes are included in 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'd 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, too many external routes carried by
IGP and, more important, excessive adjacency 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
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routes while I-BGP is typically used for distributing externally
learned routes within an AS.
The general cost of BGP are as follows:
o CPU consumption in BGP session establishment, route selection,
routing information processing, and handling of routing updates
o Router memory to install routes and multiple paths associated
with the routes.
The major scaling issue associated with BGP lies in the full mesh I-
BGP connections. Since it does not scale for 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, 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 60,000. 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 update is another potential scaling problem in large
networks. BGP uses incremental updates and sends out routing
information about unreachable routes quickly to reduce convergence
time. 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, for example, under route flapping, or BGP
session oscillation, 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
amount 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
Reflector [7] and BGP Confederations [8], for example, will help
alleviate the scaling problem caused by the requirement of full mesh
I-BGP establishment.
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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.
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
Routing design for a large-scale network should achieve the basic
goals of accuracy, stability, redundancy and convergence as described
in Section 2 and more over achieving 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 routing process 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 scales poorly when a
network has large number of routers and especially 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 had 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 is usually composed of many
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routers with a complexed topology, which result in large amount of
adjacencies. As also discussed earlier, the 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) attached by
a number of outlying components (or access networks). The transit
core network covers the entire geographical area the network serves;
each access network (aks 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 to 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
connection needs 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 of such components has
less impact to 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 using
multi-area IS-IS or OSPF and either BGP Confederation or I-BGP
Reflector or a combination of the two.
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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 simple sub-area topology. A sub-area
in a large network, typically a regional or access network, itself
has 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.
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 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 minimum but still achieve good
redundancy. However, this is less than ideal since the number of
routers remains unchanged, which increases the load on 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 Reflector and BGP
Confederation are existing mechanisms to address the scaling
problem of full-mesh I-BGP.
Further, BGP reflector provides the ability to build more than two
levels of hierarchy, as long as the interaction 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
enough for addressing scaling issues?" "Is there really a need for
more than two levels of hierarchy?"
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When a second-tiered sub-domain, 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 precaution is to avoid adding more nodes to top-level or
the 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 specific area of the protocols
needs to be redefined should more than two hierarchical layers in IGP
be desirable. There is work in progress to build multi-layer IS-IS
for more than two levels of hierarchy.
The complexity of protocol 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 the 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 resource of network routers and causing
network wide instability. This is compartmentalization.
To achieve compartmentalization in routing design for a large
network, one needs to avoid the design of involving the whole large
network as 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 routers
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 SPF recalculation. In other
words, instability within an area would be prevented from spreading
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to the entire routing domain.
Since building routing hierarchy essentially divides a big routing
area into smaller areas or domains, it helps achieving 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 shall 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 to the routing system.
Fast convergence requires that changes in network topology be
propagated to the network as quickly as possible. This 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 Process
The tasks of reducing routing process burdens includes strategically
place the routing intelligence within the network, avoid carrying
unnecessary routing information and reduce the impact of route
flapping.
6.4.1. Routing Intelligence Placement
A router that executes routing policies, performs route filtering and
dampening posses routing intelligence. Routing intelligence is needed
for a network 1) to enforce the business agreement between network
entities in the form of routing policies; 2) to protect integrity of
the routing information within the network and sometimes and 3) 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 for related tasks. It is logical, then, to place as
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little routing intelligence as possible on routers that already are
heavily burdened with forwarding and switching 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 routers
that are not sufficiently decouple forwarding and routing are
deployed in the network. 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 on 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. It is best to
reduce the 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 to
efficiently utilize IP address space as well as reduce 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.
Aggregating routes 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
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.
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6.4.2.2. Utilize Default Routing where it's Possible
Using of default route achieves ultimate route summerization which
reduces routing information to minimum. Route summerization also
masks the instability associated with individual route such as route
flapping. It's beneficial for a network to utilize default routing
where it fits. 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 summerization of routing
information has the danger of loosing routing granularity and as a
result, management of network such as traffic engineering would be
tempered. Therefore, Cautions need 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 result in many paths toward a particular
destination. In another word, there are many alternative paths in the
BGP routing table towards the same destination which consumes router
memory and 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.
6.4.2.4. Utilizing New Technologies
New technologies such as MPLS [11,12] would alleviate IGP scaling
issues for large networks. A network with MPLS over SONET technology
would have much less IGP adjacencies than that with IP over ATM
overly technology. The reduction is due to the fact that IGP only
route on physical links rather virtual links, therefore, IGP does not
need to work with full-mesh adjacency topology. In addition, the
deployment of MPLS could also free routers in the core network from
carrying full external routes, which would significantly reduce the
total amount of routes carried and processed by these routers. This
is possible because, in an MPLS domain, Label Switched Path (LSP) can
be established within the core network. An ingress edge router that
functions as a Label Switch Router (LSR) could assign a packet with a
label. Each LSR in the core along the LSP would forward the packet
based on the label, eliminating the need for full routing knowledge
in the core routers.
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6.4.3. Use Static Route at Edge
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 BGP with the edge router. At first glance, it seems
necessary for a customer network in a different Administrative
System(AS) to exchange routing information with the provider network
via BGP. However, it is not necessary the case. When a customer
network is single-homed; that is, if the sole network connection for
a customer is via its provider network, static routing can work in
this situation. 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 announcing to provider network 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 there are about 60,000 routes
known to the Internet and there is little isolation for route
flapping, handling of 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,
the 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
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aggregated route to flap. Therefore using CIDR would 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 60,000 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 accepting 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 60,000 routes known to the Internet. This
means a large default-free network would need to filter in the order
of hundred thousands 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 out-of-band process to sort out which prefix should be
accepted or denied from which source. IRR [13] and DNS [14] are among
the current proposed mechanisms for implementing prefix-based
filtering.
Route selection policy determines which path would 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 would be involving which
path to use to send traffic to the customers behind the other
network. The choices are typically:
o Direct traffic to the closest path for traffic to exit the
network. This policy is also known as Hot-Potato-Routing.
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o Direct 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.
o Always prefers routes advertised by directly connected customers
o Allow other network or customer to determine the path
When a policy is defined, its implications for scalable
implementation needs to be considered. For example, if the policy
allows customer 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, the 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 scaling the routing policy
management.
o Make policies as simple as possible but meet the requirements
o Automate as much as possible to avoid error-prone manual work
o Avoid policy based on individual prefix as much as possible with
the exception of prefix-based route filtering for protecting
routing integrity
o Avoid making exceptions
o Use out-of-band routing policy process 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 steam 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
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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 direct impact on network stability and
performance. With the fast growth of the Internet and consequently
fast expanding of providers' network, routing scaling become more and
more 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
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; reducing routing processing burden where
possible; defining manageable routing policies and using assistant of
out-of-band routing process.
Using out-of-band resource to assist routing processing is a concept
only been used in the IXPs, there are a lot of potential to use it
within a network to help addressing routing scaling issues. This is a
topic worthy further exploring.
Routing protocols and/or its implementations can still be improved or
enhanced for better handling of the scaling issues. For example, the
IS-IS multiple area mechanism is needed in order to scale the IGP in
large network. Also, using multicast or reliable flooding mechanism
for I-BGP updates instead of pairwise full mesh peering is something
worth investigating.
It is our believe that even with the deployment of new technologies
such as DWDM, MPLS and others in the future, the fundermental 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.
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8. Security Considerations
Security considerations are out of scope of this document.
9. Acknowledgement
Special thanks to Curtis Villamizar for the extensive review of the
document and many helpful comments. Many thanks to Dave Katz and
Yakov Rekhter for their insightful comments.
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] R. Callon. "Use of OSI IS-IS for Routing in TCP/IP and Dual
Environments.", RFC 1195, December 1990.
[3] J. Moy. "OSPF Version 2." RFC 2328, April 1998
[4] Y. Rekhter, 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] T. Bates, R. Chandra, E. Chen, "An alternative to full mesh IBGP"
Working in Progress
[8] P. Traina, "Autonomous System Confederation Approach to Solving
the I-BGP Scaling Problem." RFC1965, June 1996
[9] V. Curtis, R. Chandra, R. Govindan, "BGP Route Flap Damping." RFC
2439, November 1998.
[10] V. Fuller, T. Li, J. Yu, K. Varadhan "Classless Inter-Domain
Routing (CIDR): an Address Assignment and Aggregation Strategy." RFC
1519, September 1993.
[11] E. Rosen, A. Viswanathan, R. Callon, "A Proposed Architecture
for MPLS", Work in Progress.
Yu Scalable Routing Design Principles [Page 20]
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[12] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow, A.
Viswanathan, "A Framework for Multiprotocol Label Switching." Work in
Progress.
[13] V. Curtis, C. Alaettinoglu, R. Govindan, D. Meyer, "Distributed
Routing Policy System." Internet Draft, February, 1999.
[14] T. Bates, R. Bush, T. Li, Y. Rekhter, "DNS-based NLRI origin AS
verification in BGP." Working in Progress.
[15] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, W. Weiss,
"An Architecture for Differentiated Services." RFC 2475, December
1998.
[16] Y.Bernet, et al "A Framework for Differentiated Services."
Internet Draft, February 1999.
[17] D. Awduche, J. Malcolm, J. Agogbua, M. O'Dell, J. McManus,
"Requirements for Traffic Engineering Over MPLS. " Internet-Draft,
October 1998.
Author's Address
Jieyun (Jessica) Yu
UUNET Technology
880 Technology Dr.
Ann Arbor, MI 48108
Phone: (734) 214-7314
EMail: jyy@uu.net
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 execute policy for each provider's router,
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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 connect up to hundreds of customer
routers. That means the router has to handle hundreds of E-BGP
sessions and numerous number of prefix filtering. These tasks would
impose a heavy burden on the aggregation router. Reducing the number
of customer router 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 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 to incorporate 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 in
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 each
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:
o The customer aggregation router peers only with the RS router
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instead of with hundreds of customer routers.
o 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 case 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 boundaries 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 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
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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 should be investigated further.
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