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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.

Network Recovery Design

Network recovery is undoubtedly a key component of the overall network design because it impacts the network availability of the various service offerings and consequently the SLAs presented by USCom. In particular, USCom had an objective to offer high availability for both the Layer 3 MPLS VPN and Internet services. USCom's existing customers clearly required network availability for its VPN traffic equivalent to that given by the regular Layer 2-based network (Frame Relay, ATM, and so on). As far as the Internet traffic was concerned, although the requirements of this service are usually less stringent, USCom decided to arbitrarily provide high network availability to both the Layer 3 MPLS VPN and Internet traffic. As specified earlier in the SLA section, a network availability of 99.4 percent is guaranteed for both types of traffic.

Before determining its network recovery design, USCom had to consider several objectives and network design constraints, such as the required network availability, the failure scope coverage (link/SRLG/node failure), the requirement for covering single versus multiple failures, the traffic rerouting time, QoS during failure, and single versus multiple class of recovery (CoR). Other criteria, such as the operational constraints and cost aspects, were also taken into account.

Network Availability Objectives

When considering the network failure scope, USCom had a requirement that the network be able to survive any single failure, including the failure of an SRLG (which is considered a single failure). In terms of rerouting time, the goal was to provide a 50-ms convergence time upon an inter-POP link or SRLG failure.

As mentioned in the "USCom's Network Environment" section, although a very limited set of data was available for the newly deployed optical network, USCom expected that link failures would be by far the most common failure scenario (90 percent of the failures were expected to be link failures). Consequently, the objective was to provide a rerouting time similar to SONET (60 ms) in case of link and SRLG failures only.

Because the USCom network was designed to engineer customer traffic flows based on the computed set of IS-IS metrics, link utilization does not exceed 40 percent during steady state and 70 percent during a single link/SRLG failure. Because of this, QoS can be guaranteed during failure (along the backup path) without the requirement of any type of DiffServ deployment in the core network. Hence, the only objective that USCom had in the design was to provide a backup path and to implement fast recovery (50 ms) upon link and SRLG failure. Based on this design, the rerouted traffic flows should not suffer from QoS degradation.

In terms of class of recovery, USCom decided to provide equivalent network availability to all traffic without any discrimination between types. In other words, both the Internet and Layer 3 MPLS VPN traffic should benefit from the same rerouting time objectives.

Operational Constraints on Network Recovery Design

One of USCom's key objectives was to carefully minimize the network management complexity for all service offerings. The adoption of a new technology, such as MPLS Fast Reroute, could not be justified if the cost of such an implementation unreasonably increased the network management complexity. Although such criteria might be somehow subjective, trying to keep the network as simple as possible was a clear objective, and it is reflected in the resulting network design.

Cost Constraints for the Network Recovery Design

Obviously USCom could have selected from a large set of network recovery mechanisms to be able to reach its particular network availability objectives. Every mechanism has some benefits and drawbacks in terms of efficiency, complexity, scalability, scope of recovery, and so on (as discussed in the Chapter 2 section "Core Network Availability"). For USCom, the cost of the chosen network recovery strategy to meet the set of objectives for network availability was of the utmost importance and had to be kept as low as possible. In particular, the purchase of additional equipment at any layer (optical or IP/MPLS) was to be avoided if at all possible.

Network Recovery Design for Link Failures

SONET link failures are handled at the SONET layer. In the case of the optical links, USCom decided to deploy unprotected light paths and MPLS-based Traffic Engineering Fast Reroute (FRR) to provide 50-ms rerouting time upon link and SRLG failure for every unprotected light path. An important objective was to keep the operation as simple as possible. Moreover, the only constraint to be taken into account as far as the backup tunnel path was concerned was the SRLG diversity. Indeed, as pointed out in the "Traffic Engineering Within the USCom Network" section, thanks to the in-house IGP metric computation tool, the network was designed such that any recovery path offers an acceptable QoS during failure.

USCom elected to pursue the following MPLS Traffic Engineering Fast Reroute design for each light path that is to be protected:

  • Configuration of a one-hop unconstrained primary TE Label-Switched Path (LSP)
  • Dynamic configuration of an SRLG diverse next-hop (NHOP) backup tunnel

Before reviewing each of these aspects, it is useful to revisit the definitions of the terms one-hop, NHOP, and next-next hop (NNHOP).

As shown in Figure 3-17, a one-hop TE LSP is defined as a TE LSP that starts on router X and terminates on router Y, where Y is a direct neighbor of X. The signaling aspects of such a TE LSP are identical to any other TE LSP. The forwarding is different because it does not require any additional MPLS labels. Indeed, when a packet is sent to a one-hop TE LSP, no additional label is pushed (because of the penultimate hop popping operation).

Figure 17

Figure 3-17 One-Hop, NHOP, and NNHOP TE LSP

An NHOP backup tunnel simply refers to the fact that a backup tunnel originating on router X terminates on a direct neighbor of X (router Y in Figure 3-17). As shown in Figure 3-17, such a backup tunnel can be a one-hop tunnel if it protects a link via another parallel link or a multihop backup tunnel.

An NNHOP backup tunnel is a backup tunnel that originates on router X and terminates on router Z, where Z is one of X's neighbor's neighbors.

If you review each of these elements in more detail (along with the corresponding parameter tuning), you can see that configuration of a one-hop unconstrained primary TE LSP is possible because MPLS Traffic Engineering is just used with the aim of providing Fast Reroute protection. A single one-hop primary TE LSP is required so as to carry all the traffic routed through the link in question. (This is ensured because the TE LSP does not have any constraint, so its path just follows the IS-IS shortest path.) The use of such a primary one-hop TE LSP allows for the automatic protection of all the IP prefixes routed by the IGP along the same link that the one-hop tunnel follows.

Dynamic configuration of an SRLG diverse NHOP backup tunnel is made possible by flooding SRLG-related information within the IGP, as specified in [ISIS-GMPLS] and [OSPF-GMPLS]. In turn, this allows every router acting as a Point of Local Repair (PLR) to dynamically and automatically compute an NHOP backup tunnel path, SRLG diverse from the protected link (a path that does not have any SRLG in common with the protected link). USCom decided to make use of such technology to reduce the management complexity.

Figure 3-18 shows an example of SRLG (the links St. Louis–Chicago and St. Louis–Washington share SRLG 1). It also illustrates an example of a one-hop unconstrained primary TE LSP and NHOP SRLG diverse backup tunnel for the St. Louis–Washington OC-192 link.

Figure 18

Figure 3-18 USCom MPLS Traffic Engineering Fast Reroute Design

In this example, the router in St. Louis has to compute an SRLG diverse path for the backup tunnel B1 that will be used to protect the link St. Louis–Washington.

The first step in deploying this design is to configure the SRLG membership on each router. This information is then flooded throughout the network by means of the relevant IS-IS extensions. Example 3-10 provides the necessary Cisco IOS configuration used by USCom for this process.

Example 3-10 Configuration of SRLG Membership

hostname USCom.StLouis.P1
interface POS0/0
 description ** St Louis  Washington OC-192 link
 mpls traffic-engineering srlg 1
interface POS1/0
 description ** St Louis  Chicago OC-192 link
 mpls traffic-engineering srlg 1

In addition, each router has been configured to automatically configure and set up a one-hop unconstrained primary TE LSP and an NHOP SRLG diverse backup tunnel for each protected link (the unprotected light path).

Example 3-11 illustrates how to automatically configure and set up unconstrained one-hop TE LSPs to each neighbor. These TE LSPs terminate at the IP address that is connected to each next-hop neighbor. They are fast-reroutable (protected by means of Fast Reroute). They do not have any other constraints such as bandwidth, affinities, and so on. This is because the aim of these TE LSPs is to use MPLS TE Fast Reroute as a fast local protection mechanism as opposed to using MPLS TE to effectively perform some traffic engineering functions.

Example 3-11 Automatic Configuration of the One-Hop Primary Unconstrained TE LSP

hostname USCom.StLouis.P1
mpls traffic-engineering auto-tunnel primary onehop

Example 3-12 shows the configuration that triggers the setup of one SRLG diverse backup tunnel per protected interface.

Example 3-12 Automatic Configuration of NHOP SRLG Diverse Backup Tunnel

hostname USCom.StLouis.P1
mpls traffic-engineering auto-tunnel backup nhop-only
mpls traffic-engineering auto-tunnel backup srlg exclude

Referring back to Figure 3-18, given the previous configuration, all traffic routed to the St. Louis–Washington link according to the IS-IS routing table is carried to the primary tunnel, T1.

As shown in Figure 3-19, the router in St. Louis has an NHOP backup tunnel B1 configured to protect any fast reroutable TE LSP traversing the protected link St. Louis–Washington. Hence, upon a failure of the St. Louis–Washington link, T1 is rerouted to B1 within a few tens of milliseconds. Consequently, in the case of a failure of this link, all the traffic routed to the link is rerouted along the path followed by the backup tunnel B1. In a second step occurring right after the rerouting, the primary tunnel T1 is reoptimized by the PLR in St. Louis along a more optimal path. Because T1 is unconstrained, that path corresponds to the IS-IS shortest path along the new topology, as shown in Figure 3-19.

In the case of a link failure, the design selected by USCom guarantees a traffic restoration time within a few tens of milliseconds. This meets USCom's rerouting time requirements.

Figure 19

Figure 3-19 MPLS Traffic Engineering Mode of Operation in the Case of the St. Louis–Washington Link Failure

Prefix Prioritization Within the USCom Network

When FRR is triggered, the fast reroutable traffic engineering LSP T1 is immediately rerouted to the selected backup tunnel. At a lower level of detail, this means that all the IP prefixes routed by means of T1 (shown in Figures 3-17and 3-18) must have their forwarding entries updated to reflect the path change. Upon failure detection, MPLS TE Fast Reroute is triggered by the PLR. This operation consists of updating the forwarding entry for each affected IP prefix in a serialized fashion. Consequently, some prefixes are rerouted faster than others. (Note that the total rerouting time for all prefixes still occurs within a very short period.) USCom adopted an interesting design solution that consists of giving a higher priority to important prefixes so that they get rerouted before less-important prefixes.

Given the Layer 3 MPLS VPN service offered by USCom, and the desire to maintain the same level of service for its Internet customers, USCom chose to use prefix prioritization during the FRR process. To ensure that IP and VPNv4 traffic is restored first in the case of a link failure, the IP addresses that represent a BGP next hop (a loopback from either an Internet or Layer 3 MPLS VPN PE router) were chosen for prioritization. This optimizes the reroute of these services, because these addresses are used by recursive resolution to reach all IP and VPNv4 prefixes advertised by the USCom PE routers. IP addresses of internal links, such as those between P routers, were considered less important, or at least did not require such a stringent convergence time.

This prioritization is achieved using the configuration shown in Example 3-13.

Example 3-13 Configuration of Prefix Prioritization

hostname USCom.StLouis.P1
mpls traffic-engineering fast-reroute acl prefix-priority
ip access-list standard prefix-priority

Temporary Loop Avoidance

The MPLS TE Fast Reroute design elected by USCom allows the company to meet its 50-ms rerouting time objective in case of link failure, but it requires a bit of extra work to be entirely satisfactory. By their very nature, IGP link-state protocols may lead to temporary loops during network convergence. (Until all the routers have synchronized their Link-State Database [LSDB] and have converged, a loop-free state cannot be guaranteed.) By default, the knowledge of a TE LSP is kept local to the router that is the headend for that TE LSP. The FRR design chosen by USCom does not escape this rule (the TE LSP [T1] is not visible to any other router in the USCom network). The consequence of this upon a link/SRLG failure is that the router in St. Louis locally reroutes all the traffic traversing the St. Louis–Washington link. After a period of time (determined by the IS-IS timer tuning, discussed later in this chapter), both the routers in St. Louis and Washington originate a new IS-IS LSP that reflects the new network topology and, in particular, the loss of adjacency between those two routers. The IS-IS LSP is then flooded throughout the network, and each router triggers a new routing table calculation.

IP routing is distributed, so the sequence of events is not deterministic. In particular, although the Dijkstra algorithm guarantees the computation of a loop-free path during steady state, this may not be the case during network convergence, when the routers' LSDB may not be synchronized.

To help illustrate this point, consider the following sequence of events:

  • Time t0—The St. Louis–Washington link fails. (More precisely, the interface connecting the link to Washington fails on the router in St. Louis.)
  • Time t1—The router in St. Louis detects the failure and triggers a local reroute by means of MPLS Fast Reroute. IS-IS originates a new IS-IS LSP and recalculates its routing table and forwarding database (note that MPLS Fast Reroute and IS-IS operate independently).
  • Time t2—The router in Chicago receives the newly originated IS-IS LSP and recalculates its routing table and forwarding database.

During the time interval (t2–t1), the LSDBs of the routers in St. Louis and Chicago are not synchronized with each other. Assume that the IS-IS link metrics have been computed by the in-house tool such that

  • The shortest path from Chicago to Washington is Chicago–St. Louis–Washington (in the absence of failure).
  • In case of failure of the St. Louis–Washington link, the shortest path from St. Louis to Washington is via Chicago and New York.

During t2–t1, a temporary loop appears between the St. Louis and Chicago routers for the traffic sent from Chicago to Washington. This happens because during t2–t1, the St. Louis router sends the traffic for Washington back to the Chicago router, as shown in Figure 3-20.

Figure 20

Figure 3-20 Temporary Loop Effect During IS-IS Network Convergence

Forwarding Adjacency for Loop Avoidance

The solution to this problem is to configure the primary tunnel T1 as a forwarding adjacency (FA). Configuring a primary TE LSP as an FA has the effect of flooding the TE LSP as an IP link within the IGP. This means that as long as the TE LSP is operational, the node in St. Louis advertises T1 as a physical link in its IS-IS LSP. Consequently, when the physical link St. Louis–Washington fails, T1 is rerouted and then reoptimized along the path St. Louis–Dallas–Washington. T1 is still advertised in the IS-IS LSP originated by the St. Louis node as a physical link. Hence, upon link failure, no new IS-IS LSP is advertised, and the other routers in the USCom network do not detect any network topology change. (Of course, this requires configuring the FA with the same cost as the primary link it traverses.) This avoids the undesirable temporary loop effect just described. This process is illustrated in Figure 3-21.

Figure 21

Figure 3-21 Avoiding the Temporary Loop with Forwarding Adjacency

The advantage of using a forwarding adjacency is that any temporary loop during network convergence can be avoided. Moreover, in the case of a temporary failure of a few seconds, FA prevents the generation of two network convergence sequences throughout the network (which impacts hundreds of routers in the case of USCom).

As with all design choices, there are trade-offs. In the case of a forwarding adjacency, some failures such as fiber cuts may last for several days or even weeks, although USCom has not yet gathered a significant optical failure history. In such a case, the path followed by the traffic routed across the St. Louis–Washington link may follow a nonoptimal path for a long period of time because IS-IS is unaware of any topology changes. For instance, the traffic following the path Denver–St. Louis–Washington would actually follow the path Denver–St. Louis–Dallas–Washington, because the router in Denver does not actually see the failure of the link St. Louis–Washington. However, the path Denver–Dallas–Washington might have been more optimal. This is because T1 is still advertised as a physical link; hence, the other routers do not see any network topology change. In the USCom case, this was not considered an issue, because the network is overprovisioned. Therefore, the backup path, although potentially not optimal for some period of time, still provides the required QoS guarantees. Moreover, USCom has a monitoring system capturing the SNMP traps. Therefore, a procedure can be put into place to detect link failures and potentially deconfigure forwarding adjacencies on some primary tunnels if the failure lasts too long, such as in the case of a fiber cut. Of course, the deconfiguration of forwarding adjacency triggers an IGP convergence and is equivalent to the previous case where temporary loops occur.

Reuse of a Restored Link

An important consideration in the USCom FRR design was the reuse of a restored link after failure. Once a link is restored, a couple of strategies can be put into place:

  • Reuse the restored resources as soon as possible.
  • Wait for a period of time before reusing the link to maximize the network stability.

A multitude of link failure profiles are possible. For example, a link can fail and then be reliably restored (an up-down effect caused by a temporary network element failure, such as a laser desynchronization). On the other hand, a link can become unstable, experiencing a set of successive failures (in other words, the link is flapping). In such cases, waiting for a period of time before reusing a link helps determine whether it is safe to reuse the link. A flapping effect is highly undesirable; it can generate network instabilities, triggering storms of LSP flooding, SPF computation on each router, and so on. To solve such issues, multiple techniques can be used, such as back off and dampening. These can be implemented at various layers (such as the interface level, IGP, MPLS Traffic Engineering, and so on). In a nutshell, the key idea is to dampen the use of a link that suffers from instabilities to preserve network stability.

Various back-off/dampening algorithms have been designed (based on accumulated penalties such as BGP dampening, exponential back off, and so on). In the case of MPLS Traffic Engineering, on a Cisco router, the triggering of the TE LSP reoptimization always drives the reuse of a link. When a router tries to reoptimize a TE LSP path by means of a CSPF algorithm, it first determines whether a more optimal path other than the path currently in use can be found. If it can, the TE LSP is gracefully rerouted along the more optimal path. A Cisco router has various configurable reoptimization triggers that can be individually activated and deactivated:

  • Timer-based trigger—Every (Tr) seconds, a headend router attempts to reoptimize its set of TE LSPs.
  • User-triggered—Reoptimization forced by the user.
  • Link-up—Each time the IGP signals a link, every router tries to see whether its set of TE LSPs can benefit from that new link.

USCom therefore had to make a decision regarding the following trade-offs:

  • Reuse a restored link as soon as possible to quickly alleviate some congestion, but with the potential risk of generating network instability


  • Wait for a period of time before reusing a restored link whose state does not change

Because the USCom backbone is overprovisioned, the immediate reuse of a restored link was not considered a priority when compared to preserving network stability. Hence, USCom decided to be conservative in the reuse of a restored link. It would rely on IS-IS to declare the link operational according to the IS-IS back-off mechanism, described in the Chapter 2 section "Use of Dynamic Timers for LSA Origination and SPF Triggering." No TE LSP reoptimization is triggered on a link-up event. On the traffic engineering side, USCom decided to use the timer-based approach, with a timer value of 15 minutes. Hence, every 15 minutes, a router tries to see whether the link is restored so as to reoptimize the one-hop unconstrained primary TE LSP along the link. As soon as a link is restored, in the worst case, the TE LSP is rerouted to it in 15 minutes.

Multiple Failures Within the USCom Network

When assessing its multiple-failure requirements, USCom found that the only cause of concern was multiple failures provoked by the failure of an SRLG. Such a situation is handled by the design because every backup tunnel path is dynamically computed to be "SRLG diverse" from the protected link. (The links visited along the backup tunnel path do not share any SRLG with the link protected with FRR.) Any other case of multiple failures (such as an SRLG failure followed by the failure of another link that does not belong to the failed SRLG) was not considered a requirement because of the low probability of multiple simultaneous failures of independent elements. It is worth pointing out that the USCom network could survive double failures without experiencing a disconnected network but would not have any guarantee in terms of rerouting times and QoS during those multiple failures.

Link Failure Detection Within the USCom Network

The main challenge when protecting links in a switched environment (such as the intra-POP Gigabit Ethernet links) is quickly detecting a link failure. In the case where two routers are interconnected via a direct Gigabit Ethernet link, in only a few milliseconds the neighbor can detect a link failure caused by a fiber cut or a router interface failure. On the other hand, if the routers are interconnected by means of an intermediate Layer 2 switch, as in the case of the USCom Level 3 switched POP, this presents the challenge of link failure detection, because it requires the use of a fast hello (keepalive) protocol. Indeed, consider the following two cases:

  • Two routers connected by a direct PoS or Gigabit Ethernet link. The failure of the link or one of the router interfaces is quickly detected by means of the alarms provided by Layer 1 or 2.
  • Two routers connected by means of a Layer 2 switch. In this case, the failure of the link or router interface is seen only by the switch and the router connected to the failed element. Hence, the routing neighbor of the router attached to the failed element cannot detect such failures other than with a hello protocol between the two routers.

Because router interface failures are pretty rare and intra-POP links also very rarely fail, USCom decided not to protect these intra-POP links and to just rely on the IS-IS convergence.

Node Failures Within the USCom Network

When assessing the requirements for protection against node failure within the network, USCom chose to differentiate between the case of planned router maintenance and unplanned router failure.

Planned Router Maintenance

A software or hardware upgrade may require a router to be taken out of operation for a period of time (typically 10 minutes on average in USCom's case, as indicated in Table 3-2). In this case, for the core routers, USCom adopted the approach of setting up the IS-IS overload bit of the router in question via an administrative procedure. This has the effect of triggering a network-wide IS-IS convergence (rerouting) of the traffic around the router in question. As soon as the network has fully converged, the upgrade can finally take place without any traffic disruption. Such an approach is particularly suited to the USCom environment. The network overengineering rules are such that the network does not experience any congestion, even in such circumstances as a single network element failure. After the router has been reloaded, the original link metrics are restored.

In the case of the planned router maintenance of edge routers (Layer 3 MPLS VPN and Internet PE routers), things are quite different. USCom considered three scenarios:

  • Internet customer sites that are dual-attached—Before upgrading the Internet PE routers, USCom relies on a script to automatically increase the MED value for all the BGP routes announced to the set of affected CE routers. This allows each CE router to smoothly reroute its traffic to the second PE router it is connected to; this avoids any traffic disruption. The actual PE router maintenance takes place 5 minutes after the BGP routing changes.
  • VPN customer sites that are dual-attached—This could be two colocated CE routers connected to two different Layer 3 MPLS VPN PE routers or a single CE router connected to two different Layer 3 MPLS VPN PE routers. A similar procedure is applied to the case of dual-attached CE routers if BGP is used as the routing protocol between CE router and PE router. No particular measure is taken for other routing protocols or the CE routers using static routing.
  • Internet and VPN customer attached to a single PE router—In this case, USCom handles the router maintenance, which inevitably provokes some traffic disruption, during a maintenance period.

In all these cases, USCom managed to get a maintenance period window of 4 hours on the first Sunday of every month from 3 a.m. to 7 a.m.

Unexpected Router Failures

USCom noted that several types of router failures have highly variable effects on data forwarding. These effects can vary from the traffic being black-holed to absolutely no consequences on the traffic, depending on the router platform, failure types, and so on.

Two examples are provided in the next section to illustrate how the USCom IS-IS design met the requirements of unexpected router failures:

  • The case of a power supply failure at a core router
  • The case of a router failure that does not trigger any link failure, or the failure cannot be detected by the neighbors

Convergence of IS-IS

When designing the tuning of IS-IS from a convergence perspective, USCom had the objective of providing a convergence of 5 seconds in the case of a router failure or intra-POP link failure (when Layer 2 switches are used to interconnect routers). Link failures were considered outside the scope of IGP tuning because MPLS Traffic Engineering Fast Reroute covers them. This convergence time includes detection of the failure, propagation of the topology change, and local convergence (computing a new routing table).

Of course, a number of IS-IS parameters come into play when tuning IS-IS for faster convergence. As mentioned in Chapter 2's "Core Network Availability" section, the IGP convergence time is basically made up of three main components:

  • The failure detection time
  • The flooding of the new IS-IS LSP reporting a topology change
  • The routing table computation on each router (SPF algorithm, Routing Information Base (RIB) update, and so on)

IS-IS Failure Detection Time

When a router failure occurs, also implying multiple link failures (such as a power supply failure), the SONET or light path link failure is detected within tens of milliseconds. On the other hand, as mentioned, the case of a link or a router failure within a switched POP (Level 3) requires a hello protocol. USCom decided to set the IS-IS hello frequency to 1 second (one IS-IS hello message [IIH]Drew, I changed back to parenthesis since this does not refer to a reference but to the name of the hello messages. Thanks. is sent to every adjacent neighbor every second). Note that in the USCom network topology the maximum number of adjacent neighbors stays within a very reasonable limit (less than 30). Hence, sending an IS-IS Hello message every second to each neighbor is not of concern. The Hold timer is set to 3 seconds. If no IS-IS message is received during this period, the routing adjacency is declared down.

Flooding of New IS-IS LSPs

The flooding of new IS-IS LSPs is basically a function of the LSP origination time (discussed later in this section), the propagation delay, and the processing time at each router hop. In the USCom network, because the optical network is pretty dense, the worst-case propagation delay from coast to coast is 50 ms. Based on several internal tests, USCom determined that the worst-case processing delay of an IS-IS LSP even on a pretty heavily loaded router would rarely exceed 10 ms. This calculation supposes that the flooding of the newly received IS-IS LSP always occurs before the triggering of the new SPF.

Furthermore, every router has to be configured to ensure that the queuing delay experienced by the IS-IS control messages is bounded and negligible so it won't severely impact the total convergence time. It is of the utmost importance to provide a high priority to the IS-IS control messages. This applies to hello messages to avoid losing a routing adjacency in case of congested links (not in the case of USCom, however). It also ensures a quick LSP update because hello messages may reflect a topology change (if the LSP is not a refresh), which is required to quickly converge to reroute the traffic to alternate paths. Because IS-IS control messages do not rely on IP, internal mechanisms need to ensure that IS-IS messages get the relevant precedence over other user traffic.

Based on the previous flooding time analysis, USCom determined that the total flooding time should never exceed 200 ms. (This is the time to originate the new IS-IS LSP plus the total propagation delay between the originating routers and the routers where the traffic is rerouted along an alternate path by IS-IS.)

Routing Table Computation on Each Node

The final component of the IS-IS convergence to consider is the routing table computation time, which is itself made up of two components:

  • The SPF computation
  • The routing table computation and update to router line cards (in the case of a distributed router architecture)

Some testing on the USCom network showed that the SPF computation time was 100 ms, and the complete routing table update was 500 ms.

IS-IS Configuration Within the USCom Network

Example 3-14 provides the configuration of the St. Louis P router shown in Figure 3-1 to achieve the IS-IS convergence objective of 5 seconds upon a node failure. Similar configurations are adopted for all the routers in the network.

Example 3-14 Fast IS-IS Configuration

hostname USCom.StLouis.P1
interface pos0/0
 isis hello-interval 1
 isis hello-multiplier 3
router isis
 lsp-gen 5 50 20
 spf-interval 5 50 20
 prc-interval 5 50 20

Considering the syntax lsp-gen A B C, USCom decided to set B to 50 ms so that every router would get a chance to detect all the possible local failures (caused by SRLG failure) before originating a new IS-IS LSP. Indeed, upon SRLG failure, multiple local links may fail, and these failures might not be detected simultaneously. Thus, the 50 ms of waiting time before originating the new IS-IS LSP provides an accurate network topology state. If a second failure occurs, the router originates a second LSP after 20 ms (C = 20).

This also applies to the triggering of the SPF. In the syntax spf-interval A B C, B is set to 50 ms. This gives a chance, in case of an SRLG failure, to receive all the IS-IS LSPs reflecting the new topology and consequently the SRLG failure before triggering a new (second) SPF. See the section "Core Network Availability" in Chapter 2 for an explanation of the various IS-IS tuning parameters.

To help illustrate the outcome of the IS-IS parameter settings, consider two extremes:

  • The case of a power supply failure at a core router—In this case the links attached to the router will also likely fail, which will provide a fast failure indication to the neighbors of the failing routers. Each neighbor originates a new IS-IS LSP that is flooded throughout the network, and each router converges. In such a case, the failure is detected before the expiration of the hold-time timer. The propagation delays and SPF/RIB computation time are such that the objective of 5 seconds total convergence time is easily met.
  • The case of a router failure that does not trigger any link failure, or the failure cannot be detected by the neighbors—For the sake of illustration, two situations should be considered:
    • A router fails, with impact on traffic forwarding, but the attached links do not fail.
    • A router fails, with impact on traffic forwarding. Its attached links also fail, but its neighbors cannot detect these failures. Typically this is the case with a switched POP.

In these two situations, the failure detection occurs by means of the IS-IS adjacency maintenance procedure—hence, within 3 seconds (until the hold-time timer expires). This still provides 2 seconds for the neighbors of the failing router to originate their new IS-IS LSP, for the new LSP(s) to be flooded throughout the network, and finally for all the nodes to converge. Hence, this guarantees that the 5-second rerouting time objective is also met with the previously mentioned IS-IS parameter tuning. Note that only a subset of the routers is required to converge for the impacted traffic (traffic routed through the failing router) to be restored.

It is worth mentioning that other router failures do not affect data forwarding, such as a control plane failure on a distributed platform. In such failures, if the control plane cannot be restored within 3 seconds (the value of the hold-time timer), the IS-IS neighbor declares a loss of adjacency, and IS-IS converges (the traffic is rerouted around the failing router). However, the user traffic is unaffected because the alternate paths offer an equivalent QoS in the case of USCom.

It is worth noting that an edge router failure always has an impact on the traffic originated by locally attached CE routers as well as the traffic to those sites. USCom decided not to initially implement any high-availability (HA) functionality on the Internet or Layer 3 MPLS VPN PE routers, but this will be assessed at a later stage. Hence, this applies to any type of router failure. Because the customer sites are out of the realm of the USCom operation (they are unmanaged), the customers, depending on the routing protocol in use and their parameter settings, control the convergence time.

8. Design Lessons to Be Taken from USCom | Next Section Previous Section

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