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Interexchange Carrier MPLS Network Design Study

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jul 22, 2005.

Chapter Description

USCom is a fictitious nationwide data and long-distance voice service provider in the U.S. This chapter discusses the current USCom MPLS network design, its evolution, and how USCom characteristics and objectives influenced the corresponding design decisions that were made in order to illustrate how design decisions should stem from the characteristics of your company.

Layer 3 MPLS VPN Service Design Overview

USCom's Layer 3 MPLS VPN service is designed to target the growing number of organizations that are outsourcing their information technology to a third-party service provider. It also targets organizations that want to move away from a traditional overlay Layer 2 VPN model (such as Frame Relay or ATM). This trend is primarily driven by cost reductions for the end customer and the ability to receive additional services in a scalable manner, such as QoS and multicast. In many cases a hub-and-spoke topology, which is common if the environment is Frame Relay-based, is no longer sufficient to meet the end users' application requirements. The ability to use an infrastructure that inherently provides any-to-any connectivity is very attractive from an availability, scale, and service deployment perspective. The initial service offering addresses only the "unmanaged" market, where USCom provides network connectivity for the end user but the end user maintains control over their own routing. However, the design positions USCom to offer "managed" service, where it manages the end-user equipment, in the future.

USCom uses all 100 POPs to provide its Layer 3 MPLS VPN service. Customer access is via Frame Relay, ATM, leased line, and PoS. Access speeds range from * DS0 (64 kbps) to OC-3 (low- to medium-speed) and from OC-3 to OC-48 (high-speed).

The current network deployment has 255 PE routers; this may be considered a dense deployment in the U.S. These are spread around all 100 POPs, with an average of two in each Level 3 POP, three in each Level 2 POP, and six in each Level 1 POP. However, PE routers are deployed based on customer demand at each location; therefore, the average numbers do not necessarily correspond to the actual deployed topology. For example, 30 of the Level 3 POPs currently have only one PE router deployed rather than two. On the other hand, the New York POP, which is a Level 1 facility, has ten PE routers, which is more than the average.

USCom initially defined two different types of customers who may access the national Layer 3 MPLS VPN service, as described in the following list. Note that Internet connectivity is considered a separate service and therefore is not bundled with the VPN service:

  • VPN intranet—This customer requires connectivity between internal sites for the creation of an intranet. No extranet connectivity is provided. However, if the evolution of the USCom network introduces any central services (such as web hosting, firewalls, and so on), the customer is eligible for connectivity to these services.
  • VPN extranet—This customer requires connectivity between internal and external partner sites for the creation of an extranet.

USCom breaks its VPNs into three categories—small, medium, and large—as described in Table 3-3. These categories are based on the customer's size as measured by the number of sites in the VPN. Current statistics show that 500 VPNs are deployed, with a combined total of 12,500 VPN sites, representing ten large VPNs, 200 medium VPNs, and 290 small VPNs. The VPN sites represent 62,500 total VPNv4 routes in the network.

Table 3-3 IP VPN Categories

VPN Category

Number of Sites

Percentage of Total Sites

Number of Prefixes in VPN

Percentage of Total Customers

Small VPN

2 to 10

15 %

Ones to tens


Medium VPN

11 to 200


Tens to hundreds


Large VPN

201 to thousands


Hundreds to thousands


As you can see, although the majority of VPN customers fall within the small VPN category, they represent only 15 percent of the total number of sites. Only 2 percent of customers fall within the large VPN category, but they represent 40 percent of the total number of sites.

PE Router Basic Engineering Guidelines

Configuring a new Layer 3 MPLS VPN customer requires a set of engineering guidelines that is flexible and easy to implement from a centralized management system. A number of common attributes need to be configured for each new customer. These are outlined in Chapter 1 and can be summarized as follows:

  • Definition and configuration of the Virtual Routing/Forwarding instance (VRF)
  • Definition and configuration of the Route Distinguisher (RD)
  • Routing protocol context and/or static routing configuration
  • Import/export policies
  • Interaction between the backbone control plane and the VRF
  • Configuration and association of customer-facing router interfaces with previously defined VRFs
  • Quality of service (QoS) policies

The values chosen for each VRF attribute, as well as specifics of the routing protocol context, vary from VPN customer to customer. However, because a number of default attributes can be assumed, the provisioning system needs only to provide a template that can accept different values on a per-customer basis. Most commercially available provisioning systems today have default templates for configuring these attributes.

In most cases, the PE-CE links used for the Layer 3 MPLS VPN service have their IP addresses allocated from the USCom IP address space. However, on an exception basis, customer address space is sometimes used. The decision to use customer address space is primarily driven by the customer's size and topology and whether the customer's address space can be summarized into convenient blocks. If customer address space is used, USCom requires that it be a globally assigned block and not from the private range so as to avoid any potential address range clash with other VPN customers.

In all cases, USCom uses the Interface Group MIB (see [IF-MIB]) to monitor the status of the physical PE-CE links. This is achieved by polling the IfOperStatus.ifIndex object, the details of which are specified in [IF-MIB].

USCom will not provision more than 15 percent of the total access links of any given customer onto a single PE router. This will help prevent a large percentage of the VPN from losing connectivity in the event of a PE router hardware failure or planned maintenance. Although USCom has not reached the scaling limits on any of its PE routers, it has decided to apply an upper boundary to the total number of Layer 3 MPLS VPN customer accesses per PE router. This is driven by a number of factors, including the type of access device (such as the router's size and capability), traffic throughput requirements, additional service requirements (such as QoS), and so on. Table 3-4 provides an overview of USCom's engineering rules in this space for two of its router platforms. USCom will stop adding customers on each platform if any of the hard limits is reached.

Table 3-4 PE Router Sizing Rules

Engineering Parameter Limits

Platform 1 Limits

Platform 2 Limits

USCom IGP routes



Number of iBGP/eBGP peers



Number of VRFs



Total VRF routes



Average number of sites per VRF



Average number of routes per VPN



Total CE connections per PE



Table 3-4 also shows that because the typical average number of sites per VRF per platform is 3, this translates into a total number of CE connections per PE of 600 and 3000.

VRF Naming Convention

When choosing a name for a given VPN VRF, it is important to remember that the network operations staff will use the name to troubleshoot connectivity problems for the VPN. Several naming conventions might be adopted. USCom chose to use a representation of the name followed by an abbreviation of the customer name, starting with a VRF name of V101 and incrementing it by 1 for each new VPN deployed. This allocation scheme is shown in Table 3-5.

Table 3-5 VPN Name Allocation Scheme

Customer Name

VRF Name

U.S. Post Office


SoccerOnline International


BigBank of Massachusetts


<Next customer>

V104 and so on

Route Distinguisher Allocation

The route distinguisher (RD), as described in [2547bis], is an 8-byte entity that lets the MPLS VPN architecture uniquely identify a particular route within the operator's backbone network. The structure of the RD depends on the type specified in the first 2 bytes of the attribute. USCom chose to use its autonomous system number plus a uniquely defined number specific to a given VPN customer. Figure 3-9 shows the format of the RD chosen by USCom.

Figure 9

Figure 3-9 Route Distinguisher Format

In theory, several schemes are available when choosing an RD allocation method. The main ones can be summarized as follows:

  • Use a unique RD for every VPN—A unique RD for each VPN is the easiest option to deploy because every PE router uses the same value for a given VPN customer. However, deploying this scheme prevents the operator, which has VPNv4 route RRs in its topology (which is typically the case; USCom has such a topology), from offering load-balancing services to customers who are dual-homed to the Layer 3 MPLS VPN service. This is because the VPNv4 routes cannot be guaranteed to be unique. Therefore, certain paths may be unavailable to the PE routers that will perform the load balancing, because the RRs will advertise only the "best" path.
  • Use a unique RD for each VPN on a per-PE basis—A unique RD may be allocated to each VPN at each PE router, although the value may be different between PE routers for a given VPN. In this case the operator can provide load-balancing services when RRs are deployed. This is because the VPNv4 routes can be guaranteed to be unique within the MPLS backbone. Note that such a scheme requires a little more memory space to store the additional VPNv4 routes.
  • Use a unique RD for each VPN on a per-interface/per-PE router basis—A unique RD may be allocated for each VRF on a per-interface basis. The advantage is that a particular site within a VPN can be identified based on the RD value of any route originated by that site. However, other methods are available to achieve the same aim, such as use of the site of origin (SoO) attribute, which is much less resource-consuming. The format of this attribute can be found in [EXTCOM].

USCom chose to use a unique RD per VRF (the second option), because it required load-balancing services for a number of VPN customers with dual-homed CE routers. Although this scheme requires additional memory at the PE routers, the ability to provide load balancing when RRs are deployed was necessary to address USCom customer requirements. The range of RDs available is 32765:1 through 32765:4,294,967,295, which is way beyond what USCom will ever require.

Route Target Allocation for Import/Export Policy

Selecting a route target for each VPN is necessary to specify that VPN's specific import/export policies. [EXTCOM] specifies three main formats that may be used for the route target extended-community attribute.

USCom chose to use the two-octet AS format with its own AS number, 32765, as the ASN portion of the community. Use of any customer AS numbers was rejected in the design because the possibility of conflicting numbers was apparent if any VPN customers were using private AS numbers from the [64512–65535] range.

Route target values 32765:[1–100] were reserved for future use, so values 32765:101 through 32765:65535 are available for VPN customer allocation. This fits nicely with USCom's VRF naming convention, in which it maps the VRF name to the number in the route target. For example, BigBank of Massachusetts, whose VRF name is v103:BigBank, uses a default route target value of 32765:103.

Basic PE Router Configuration Template

Example 3-2 provides the basic PE router configuration template used by USCom.

Example 3-2 PE Router Configuration Template

hostname USCom.cityname.PErouter-number
ip vrf vpn-name
 rd 32765:1-4294967295
 route-target export 32765:101-65535
 route-target import 32765:101-65535
interface Loopback0
 description ** interface used for BGP peering **
 ip address range address and network mask

PE Router Control-Plane Requirements

One of the most significant challenges for any Layer 3 network-based VPN service is distributing customer-specific routing information between edge routers and achieving this in a scalable manner. As the service grows, more and more VPN routes need to be advertised using the backbone control-plane infrastructure. The amount of information could become significant as the service becomes more and more successful.

Although USCom's future expansion projections for its Layer 3 MPLS VPN service do not indicate any kind of saturation point in terms of routing information capacity, it is clear that over time, as the service matures, the design of the backbone control-plane infrastructure will be critical.

The current network deployment has 255 PE routers providing Layer 3 MPLS VPN services. With this number of PE routers, and the requirement to carry an ever-expanding VPNv4 address space, USCom chose to deploy VPNv4-specific RRs (the details of which are discussed in section "VPNv4 Route Reflector Deployment Specifics") to help scale the distribution of routes. RRs help scale the network infrastructure in a number of ways. You will see in other chapters that additional functionality may be added to further increase this scaling. However, USCom chose to use RRs primarily to ease the network's operational complexity as the number of MP-BGP TCP sessions required by the PE routers into the backbone could be reduced to two (one to each RR), as opposed to every other PE router in the network.

Each PE router is required to maintain at least two MP-BGP peering sessions into the USCom backbone network. These sessions will be used to exchange VPNv4 prefix information with other PE routers via the VPNv4 RRs. Two sessions are necessary for redundancy in case an RR fails or connectivity to that RR becomes unavailable.

PE Router Path MTU Discovery

In Cisco IOS, by default, all PE routers have Path MTU Discovery [see PMTU] disabled. This means that the default TCP Maximum Segment Size (MSS) is used for all TCP sessions. This default normally is based on the outgoing interface MTU size minus the IP header/options and TCP header/options (for a total of 40 bytes). For example, for an Ethernet interface with an MTU of 1500 bytes, the MSS is calculated as 1460.

BGP on Cisco routers uses a default MSS value of 536 bytes regardless of the outgoing interface type. The problem with this small value is that BGP signaling information sent across a given BGP session needs to be segmented into a much higher number of packets, substantially increasing convergence times. However, [PMTU] provides a mechanism in which the PE router can discover the optimum MSS to use for its BGP sessions, and therefore reduce the number of messages generated. USCom enables [PMTU] on all its VPN PE routers, and VPNv4 RRs, using the configuration shown in Example 3-3.

Example 3-3 PE Router PMTU Configuration Template

hostname USCom.cityname.PErouter-number
ip tcp path-mtu-discovery

VPNv4 Route Reflector Deployment Specifics

Two tools are available to assist in the scaling of the TCP sessions required to support the VPNv4 address family for Layer 3 MPLS VPN service—confederations and route reflectors. USCom chose to deploy RRs in its Layer 3 MPLS VPN design; these are completely separate from the RRs used for its Internet service. This separation provides improved convergence times, as well as scalability in terms of CPU and hardware memory requirements. USCom did not have a requirement to deploy confederations, because its service requirements did not necessitate multiple sub-autonomous systems to split the MP-BGP topology.

While reviewing the needs of the Layer 3 MPLS VPN service from a control-plane perspective, it was clear that the rules for RR deployment were different from those followed for the Internet service, because the VPN traffic would be label-switched rather than routed. The primary difference between label switching and IP forwarding within the backbone network is that label switching allows the RRs to be deployed outside the packet-forwarding path, because the forwarding decision for a given packet is made at the edge of the network rather than on a hop-by-hop basis. This paradigm is a little different from the typical Internet design used for IPv4 route distribution, in which the common practice is to place the RRs so that they follow the network's physical connectivity. This type of design avoids any forwarding loops that could be caused by bad route placement. Figure 3-10 shows what can happen if these rules are not followed when forwarding IP traffic natively instead of via label switching.

Figure 10

Figure 3-10 Forwarding Loop with Incorrectly Designed IPv4 Route Reflection

Figure 3-10 shows that the Denver POP believes that can be reached via a Washington next-hop address, whereas the Chicago POP believes it can be reached via a San Francisco next-hop address. This is clearly a bad design, because both POPs should peer with their geographically closest RRs. In this case, packets loop between Chicago and Denver.

This issue is eliminated when packet forwarding is achieved through label switching (or IP tunneling), because the packets' original destination IP address is no longer examined in the network core.

Deployment Location for VPNv4 Route Reflectors

The Internet RR topology described previously is not well suited to the Layer 3 MPLS VPN service because the topology assumes that all RRs will carry the same set of routes. This may not be necessary, or even desirable, for the VPN service, because not all PE routers will need the same set of routes. Hence, it would not scale as well as a partitioned VPN RR design, which may become necessary for a large-scale VPN topology. Another drawback of this Internet topology for the VPN service is that it introduces a number of BGP hops that increase the convergence delay for routing updates. This may be detrimental to the Layer 3 MPLS VPN SLA, and therefore the topology of the VPNv4 RRs is a little different, as shown in Figure 3-11.

Figure 11

Figure 3-11 VPNv4 Route Reflector Deployment

The VPNv4 RRs are deployed only within Level 1 POPs and are connected directly to both core backbone P routers via OC-3 links. The topology does not follow the network's physical path; this is unnecessary because of the deployment of LDP in the backbone. Level 2 and Level 3 POPs do not house any RRs but instead peer directly with their local Level 1 POP. All VPNv4 RRs peer within a full mesh.

The initial deployment has VPNv4 RRs in six locations: San Francisco, Los Angeles, Denver, Chicago, New York, and Washington. Each PE router has peering sessions with a pair of RRs that are within its local regional vicinity. For example, a PE router in Boston may peer with an RR in New York and another in Chicago. A maximum of 200 peering sessions has been defined within the engineering deployment guidelines for the RRs. Although the currently deployed hardware could support more than this number, USCom has validated only up to 200 peering sessions within its labs. Because the current network has 255 existing VPN PE routers, and each PE router peers to local RRs based on geography, no RR within the topology has close to this maximum number of peering sessions.

USCom takes advantage of update groups, which are enabled by default in the level of Cisco IOS it is running on its routers. Therefore, USCom can dynamically build groups of MP-BGP peering partners that have the same outbound policy. The update group does not consider the extended communities used by the PE routers for import/export policy; therefore, all the PE routers belong to the same group. This provides the ability to build one MP-BGP update (instead of one per PE router) and to replicate it to all members of the update group. This functionality provides improved performance at the RRs. Each RR, just like the PE routers, also uses [PMTU].

The design of the control plane must provide the ability for all Layer 3 MPLS VPN PE routers to learn routes from the centralized VPNv4 RRs. Ideally each PE router should peer based on geography as much as possible, and to different Level 1 POPs. This is useful so that a particular Level 2/3 POP does not lose all routing information in the event of a catastrophic failure within a given Level 1 POP, such as a complete power outage.

Figure 3-12 illustrates the topology of the VPNv4 RRs. It shows that the Boston Level 2 POP peers to both the Chicago and New York Level 1 POPs to provide geographic redundancy. All PE routers within a Level 1 POP (for example, the New York POP) peer to their local RRs, because a local power failure would mean that they would not be able to maintain a peering session with another Level 1 POP because all connectivity would be lost. Therefore, there is little point in following the same design rule as the Level 2/3 POPs.

Figure 12

Figure 3-12 Physical Topology of VPNv4 Route Reflection

Preventing Input Drops at the VPNv4 Route Reflectors

Each RR has an interface-level queue, referred to in Cisco IOS as the input hold queue, that may not by default be large enough to prevent input drops at the interface. Dropping TCP packets reduces the protocol's efficiency and causes retransmissions to occur. This behavior can slow down the convergence of MP-BGP at the VPNv4 RRs. For this reason, USCom tunes the queue value using the following algorithm:

  • Input hold queue = (TCP window size / mss) * number of MP-BGP peers

  • where TCP window size is the TCP window size for the MP-BGP session, mss is the TCP maximum segment size, and number of MP-BGP peers is the number of route reflector clients.

  • The window size (sndwnd) and mss (max segment size) values can be found using the show ip bgp neighbor command in Cisco IOS.

PE Router and Route Reflector VPNv4 MP-BGP Peering Template

USCom uses the template shown in Example 3-4 for the VPNv4 MP-BGP configuration of the PE routers and RRs.

Example 3-4 PE Router and Route Reflector VPNv4 BGP Configuration Template

! PE-router configuration
hostname USCom.cityname.PErouter-number
ip tcp path-mtu-discovery
interface Loopback0
 description ** interface used for BGP peering **
 ip address 
router bgp 32765
 no bgp default ipv4-unicast
 neighbor address-for-1st-RR remote-as 32765
 neighbor address-for-1st-RR update-source Loopback0
 neighbor address-for-1st-RR remote-as 32765
 neighbor address-for-1st-RR update-source Loopback0
 address-family vpnv4
 neighbor address-for-1st-RR activate
 neighbor address-for-1st-RR send-community extended
 neighbor address-for-1st-RR activate
 neighbor address-for-1st-RR send-community extended

! VPNv4 Route Reflector configuration
hostname USCom.cityname.RRrouter-number
interface Loopback0
 description ** interface used for BGP peering to RR-clients **
 ip address range address and network mask
router bgp 32765
 neighbor address-1st-PE-router remote-as 32765
 neighbor address-1st-PE-router update-source Loopback0
 address-family vpnv4
 neighbor address-1st-PE-router activate
 neighbor address-1st-PE-router route-reflector-client
 neighbor address-1st-PE-router send-community extended

PE-CE Routing Protocol Design

As discussed in Chapter 1, various routing protocols (and static routing) are available for connectivity between the CE routers and PE routers. When assessing the requirements for the Layer 3 MPLS VPN service design, the set of routing protocols to offer was evaluated. The initial deployment of the service included static routing and BGP-4 support only on the PE-CE links. However, RIPv2 was added fairly shortly afterwards to provide service to customers who were unable to run BGP-4, such as those with PE-CE links backed up via ISDN to a Network Access Server (NAS).

USCom avoids using RIPv2 as much as possible because of its periodic update behavior and the implications of this on the PE routers' CPU cycles. For customers who require RIPv2, USCom configures flash-update-threshold 30 to prevent Flash updates from being sent before the regular periodic updates. Flash updates send new routing information as soon as something changes in the customer topology and therefore can increase CPU requirements substantially during customer routing instability events. Also, USCom imposes the use of BGP-4 for dual-attached sites to avoid having to configure RIP tagging for loop prevention.

Static Routing Design Considerations

VPN sites that are single-homed to the USCom network may use static routing. However, this depends on the number of routes (a low number is mandatory, usually no greater than 5) and whether these routes are likely to change on a regular basis. Static routing is particularly suitable if route summarization is easily achievable for the set of routes that can be reached for a particular VPN site. In the majority of cases, only a few routes can be accessed via a single-homed site, such as a local /24 LAN segment, so static routing is adequate.

Clearly static routing does not provide any dynamic rerouting capability. Although static routing provides good stability while requiring minimal router resources, USCom actively encourages its larger Enterprise customers to run a dynamic routing protocol. The overhead of managing static routing in this case is considerable, especially at the central sites, where route summarization is often impossible.

In many cases, even if the customer has only a single connection to the Layer 3 MPLS VPN service, if the customer takes Internet service from somewhere else within the site, whether from USCom or some other Internet service provider, it is likely that the customer will follow a default route toward the Internet exit point. This means that the CE router needs to have all the relevant static routes from the VPN pointing toward the PE router. An appropriate addressing scheme that allows some summarization simplifies the configuration exercise but nevertheless is prone to errors and typically is avoided.

For stability reasons, USCom prefers to configure the static routes with the permanent keyword. This prevents the static routes from being withdrawn in MP-BGP in the event that a PE-CE link flaps or fails. The downside of this design decision is that traffic continues to be attracted toward the failed link, even if the PE router is unable to forward traffic from other sites across the link. However, because the customer site is single-homed, the added backbone stability is preferred over the suboptimal (unnecessary) packet forwarding.

Current statistics show that approximately 40 percent of USCom's PE-CE connections use static routing.

PE-CE BGP Routing Design Considerations

50 percent of VPN PE-CE connections use external BGP (eBGP). This is the protocol of choice for USCom, because it is used to dealing with this protocol (with experience from the Internet service), and it can easily add policy on a per-VPN basis. Some end users are already familiar with the BGP protocol and have been running it within their network before migrating to the VPN service, although this is normally restricted to large Enterprises. Also, many of these end users already subscribe to an Internet service and therefore are familiar with how the protocol is used. Therefore, standardizing on BGP is an obvious choice.

To protect the PE routers, every customer BGP-4 peering session is configured to accept only a maximum number of prefixes. This is achieved through the use of the neighbor maximum-prefix command on each PE-CE BGP peering session. USCom also uses route dampening (with the same set of parameters) for all its customers who attach to the VPN service via external BGP. This is stringently applied to all customers because route flaps (constant routing information changes) can cause instability in the control plane of the USCom network. The policy applied for dampening is as follows: Any route that flaps receives a penalty of 1000 for each flap. A reuse limit of 750 is configured so that a route, once suppressed, can be readvertised when the limit reaches 750. After a period of 15 minutes (the half-life time), the total value of the accumulated penalty is reduced in value by 50 percent. If the accumulated penalty ever reaches a suppress limit of 3000, MP-BGP suppresses advertisement of the route regardless of whether it is active.

Both of these parameters are configured using the template shown in Example 3-5.

Example 3-5 Restricting the Number of Prefixes on PE-CE BGP Links Template

router bgp 32765
 address-family ipv4 vrf vrf-name
  neighbor remote-as customer-ASN
  neighbor activate
  neighbor maximum-prefix 100
  no auto-summary
  no synchronization
  bgp dampening route-map vpn-dampen
route-map vpn-dampen permit 10
 set dampening 15 750 3000 60

PE-CE IGP Routing Design Considerations

In recent months USCom has seen an increase in the number of customers requesting either OSPF or EIGRP support on their PE-CE links. These customers typically have large, and often complex, IGP topologies.

A number of benefits may be gained by running IGP on the PE-CE links:

  • The service provider MPLS VPN network may be used for WAN connectivity while remaining within the customer's IGP domain. This provides a "drop and insert" approach to migrating the existing network onto the new infrastructure.
  • A relatively seamless routing domain from the attached customers' perspective may be obtained. This avoids the extra costs associated with staff retraining to support an additional routing protocol such as BGP-4.
  • IGP fast convergence enhancements can be deployed, especially in the case of multihomed sites, which may be useful in the case of a PE router or PE-CE link failure.
  • External routes can be prevented within the IGP topology.
  • IGP routing metrics can be maintained across sites, and the USCom network can remain transparent to the end user from a routing perspective.
  • In the presence of customer back-door links (direct connectivity between customer sites, such as via leased lines), superior loop-avoidance and path-selection techniques can be used, such as sham links (OSPF) and site of origin (EIGRP).

A provider could offer a specific routing protocol as the only choice to avoid the costs associated with provisioning, maintaining, and troubleshooting different routing protocols. However, such an offering might force the VPN customers to compromise their design requirements and would ultimately hurt the provider through restriction of its customer base. If multiple routing protocol choices are to be offered on the PE-CE links, it is important to carefully consider the convergence characteristics (which are important to the VPN customer) and the service's scalability (which is important to both VPN customer and service provider).

USCom chose to offer RIPv2, EIGRP, and OSPF, all of which are provided on a restricted basis (in terms of the number of sites permitted to attach to a given PE router for each protocol). These restrictions are currently set at 25 for each protocol, although this figure is not a hard rule. It depends on the specific customer attachment needs (such as the number of routes and so forth) and is monitored to obtain more deployment experience. The IGPs are configured on a per-customer basis. The complexity of the configuration is driven by the complexity of the attached customer topology.

Specifics of the OSPF Service Deployment

USCom currently has two large customers who run OSPF on their PE-CE links. A number of features are included in the service provider design at the PE routers to support these customers.

A different OSPF process ID is used for each VPN. By default the same process ID is used for the VPN on all PE routers that have attached sites for that VPN. This is important. Otherwise, the OSPF routes transported across the MPLS VPN network are inserted as external routes (Type 5 LSAs) at a receiving OSPF site. This is typically undesirable because externals are by default flooded throughout the OSPF domain. Using the same process ID causes the PE router to generate interarea (Type 3 LSAs) routes instead, which are not flooded everywhere and therefore are bounded.

USCom uses the following command for all OSPF deployments. It protects the PE router from a large flood of Link-State Advertisements (LSAs) from any attached CE router.

[no] max-lsa maximum [threshold] [warning-only] [ignore-time value]
  [ignore-count value] [reset-time value]

Restricting the number of LSAs at the PE router is important because it protects the OSPF routing process from an unexpectedly large number of LSAs from a given VPN client. That might result from either a malicious attack or an incorrect configuration (such as redistributing the global BGP-4 table into the customer OSPF process).

Using this functionality, the PE routers can track the number of non-self-generated LSAs of any type for each VPN client that runs OSPF on the PE-CE links. When the maximum number of received LSAs is exceeded, the PE router does not accept any further LSAs from the offending OSPF process. If after 1 minute the level is still breached, the PE router shuts down all adjacencies within that OSPF process and clears the OSPF database.

USCom leaves the threshold, ignore time, ignore count, and reset time at their default values of 75 percent, 5 minutes, 5, and 2 times ignore time, respectively. Because only two OSPF clients exist at this time, the maximum LSA count is set to 10,000. USCom will continue to monitor this as new OSPF deployments arrive so as to optimize the default value.

Each router within an OSPF network needs to hold a unique identifier within the OSPF domain. This identifier is used so that each router can recognize self-originated LSAs and so that other routers can know during routing calculation which router originated a particular LSA. The LSA common header has a field known as the advertising router. It is set to the originating router's router ID.

The router ID used for the VRF OSPF process within Cisco IOS is selected from the highest loopback interface address within the VRF or, if no loopback interface exists, the highest interface address. This may be problematic if the interface address selected for the router ID fails, because a change of router ID is forced, and the OSPF process on the router must restart, causing a rebuild of the OSPF database and routing table. This clearly may cause instability in the OSPF domain. Therefore, USCom allocates a separate loopback address for each VRF that has OSPF PE-CE connectivity. This address is used as the router ID as well as for any sham links that may be required.

Specifics of the EIGRP Service Deployment

USCom found that a number of large Enterprise customers requested EIGRP connectivity with their PE routers. This protocol is widely deployed within Enterprise networks. Therefore, USCom felt that offering support for this protocol was a "service portfolio" differentiator. USCom deploys a number of features at the PE routers to support this protocol.

Automatic summarization is disabled as a matter of course for all EIGRP customers. The default behavior is for this functionality to be enabled. However, because the MPLS VPN backbone is considered transparent, USCom uses the no auto-summary command to disable it.

To support external routes within a customer EIGRP domain, a default metric of 1000 100 255 100 1500 is used, but this may be changed on a per-customer basis.

USCom supports the EIGRP site-of-origin (SoO) cost community. This community attribute is applied automatically at the point of insertion (POI) (the originating PE router) when an EIGRP route is redistributed into MP-BGP. Supporting this functionality allows USCom to support back-door links within a customer EIGRP topology by affecting the BGP best path calculation at a receiving PE router. This is achieved by carrying the original EIGRP route type and metric within the MP-BGP update and allowing BGP to consider the POI before other comparison steps.

USCom also supports the SoO attribute. This is configured by default for every site that belongs to a given EIGRP customer. This feature allows a router that is connected to a back-door link to reject a route if it contains its local SoO value. Example 3-6 shows this default configuration.

Example 3-6 EIGRP SoO Attribute Configuration Template

interface Serial 1/0
 ip vrf forwarding vrf-name
 ip vrf sitemap customer-name-SoO
route-map customer-name-SoO permit 10
 set extcommunity soo per-customer-site-id

USCom protects the PE routers from saturation of routing information by using the maximum-prefix feature. The following shows the syntax of this command:

maximum-prefix maximum [threshold] [warning-only]
  [[restart interval] [restart-count count] [reset-time interval] [dampened]]

At this point in time the default values for threshold, restart, restart-count, and reset-time are used. These values are 75 percent, 5 minutes, 3, and 15 minutes, respectively.

IP Address Allocation for PE-CE Links

USCom decided within its design that it would allocate the PE-CE link IP addresses from one of its registered blocks. This allows more flexibility in determining a filtering template that can be applied to all PE routers so that unwanted traffic can be dropped at the edge. It also avoids any conflicts with customers' IP address space, because many will have selected IP addressing from the [PRIVATE] private ranges.

The block of addresses chosen for this purpose is taken from the address block. Because the customer access routers are unmanaged, each PE-CE link is assigned a network mask that allows two hosts. For example, provides IP addresses and with which to address the PE-CE link of a given VPN customer. These addresses are redistributed into MP-BGP so that they are available within the VPN for troubleshooting purposes.

Controlling Route Distribution with Filtering

Each PE router within the USCom network has finite resources that are distributed between all services that are offered at the edge. Because many VPN clients will access the network via the same PE routers, USCom would like to be able to restrict the number of routes that any one customer can carry within its routing table. This is achieved by applying the maximum routes command to all VRFs, as shown in Example 3-7.

Example 3-7 Maximum Routes Configuration Template

hostname USCom.cityname.PErouter-number
ip vrf vpn-name
 rd 32765:1-4294967295
 route-target export 32765:101-65535
 route-target import 32765:101-65535
 maximum routes maximum-#-of-routes {warning-threshold-% | warning-only}

USCom considered what values should be set within this command. It noticed that if the value of the limit imposed were set too low, valid routes would be rejected, causing a denial of service for some customer locations. Also, USCom noted that the maximum routes value must be able to cater to all types of routes injected into the VRF, including static routes, connected routes, and routes learned via a dynamic protocol. USCom decided to start with a maximum routes limit that was set for each VRF to be 50 percent more than the actual number of routes in steady state, with a warning at 20 percent more than the actual number of routes in steady state.

USCom decided not to use any filtering for customer route distribution during its initial deployment of the Layer 3 MPLS VPN service. Because of this, all RRs carry the same set of routes, and each PE router relies on the Automatic Route Filtering (ARF) feature to ignore any routing updates that contain routes that are not locally imported into any attached VRFs.

Security Design for the Layer 3 MPLS VPN Service

Security of the network infrastructure is one of the most important considerations when designing any robust network. [ISP-security] provides an excellent overview of security best practices for ISP networks. Most of the material presented is also relevant to the USCom Layer 3 MPLS VPN service, because it presents basic router security tips, and USCom already follows these for its Internet service.

Although the Layer 3 MPLS VPN service separates customer routing from backbone routing, existing tools such as traceroute provide a method of revealing the core topology of the USCom network from within a customer VPN. Because of this, USCom chose to disable this behavior through the use of the mpls ip propagate-ttl forwarded command throughout the network. This command is discussed in detail in Chapter 13 of [VPN-Arch-Volume-1]. It basically has two effects: It propagates the IP TTL into the label header, and it propagates the label TTL into the IP header (where the packet exits the MPLS backbone). By disabling the TTL propagation (via the no mpls ip propagate-ttl forwarded command), USCom can hide its internal infrastructure from the output of any customer-initiated traceroutes that are sourced from within a Layer 3 VPN. This command, however, does not protect any of the Internet PE routers because packets are IP-forwarded rather than label-switched toward the Internet. Therefore, USCom has a policy of not advertising the IP address blocks used for its internal infrastructure toward the Internet. It also applies packet filters toward these addresses at the external-facing interfaces of the Internet PE routers.

Although the core addressing is hidden from traceroute through the use of the no mpls ip propagate-ttl forwarded command, the same cannot be guaranteed for the subnet used for PE-CE circuit addressing. DoS attacks can always be performed if the VPN client knows one or more of the IP addresses of the PE router. This is easy to determine through the use of the traceroute command. Visibility of PE router circuit information could allow the VPN client to intrude or perform DoS attacks on the PE router.

If the CE router is managed, which is not the case for USCom in its initial deployment, the PE router circuit address can be hidden by a filter that prevents it from being redistributed into the customer network. In addition, various inbound filters can be applied at the PE router to restrict CE router access; this is what USCom has adopted for its unmanaged service. Example 3-8 shows the filter template chosen for deployment.

Example 3-8 PE-CE Link Filter Template

ip access-list extended PE-CE-Filter
 permit icmp host CE-host-address host PE-router-PE-CE-link-address
 permit bgp host CE-interface-address host PE-interface-address
 deny ip any
 permit ip any any

The first line of the access list (the second line of Example 3-8) permits ICMP packets such as pings to be sent from the VPN customer to the directly connected PE router interface. Allowing such packets is useful for the customers, because they can perform diagnostics and management activity. The second line permits the BGP routing protocol to exchange routes by allowing communication between the CE router and PE router interface addresses. If a VPN customer is using a different protocol, this needs to be explicitly allowed within the filter.

The third line of the access list blocks any IP packets that are addressed to a destination within the USCom backbone network. The last line permits all other IP traffic to pass-through the PE router.

5. Quality of Service Design | Next Section Previous Section

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