IS-IS Traffic Engineering (TE) is an extension of IS-IS to support MPLS TE. As specified in RFC 5305 and RFC 4205, IS-IS TE defines new TLVsand sub-TLVs in IS-IS LSPs to carry TE information, floods LSPs to implement the flooding and synchronization of TE information, and transmits TE information to the CSPF module.
IS-IS TE supports MPLS to set up and maintain the Constraint-based Routed Label Switched Paths (CR-LSPs).
When constructing the Constraint-based Routed LSP (CR-LSP), MPLS needs to learn the traffic attributes of all the links in this area. MPLS can acquire the TE information of the links through IS-IS.
Traditional routers select the shortest path as the master route regardless of other factors, such as bandwidth. In this manner, the traffic is not switched to other paths even if a path is congested.
As shown in Figure 1, assume that each link has the same metric. The shortest path from CX-A/CX-H to CX-E is CX-A/CX-H -> CX-B -> CX-C -> CX-D -> CX-E. Data is forwarded along this shortest path though other paths exist. In this manner, the path CX-A/CX-H -> CX-B -> CX-C ->CX-D -> CX-E may be congested, and the path CX-A/CX-H -> CX-B -> CX-F -> CX-G -> CX-D -> CX-E may be idle.
To solve the preceding problem, you can adjust the link metric. After analyzing the topology, we adjust the metric of the path CX-B-CX-C to 3. In this manner, the traffic can be led to the path CX-A/CX-H -> CX-B -> CX-F -> CX-G -> CX-D -> CX-E.
This method eliminates the congestion on the link CX-A/CX-H -> CX-B -> CX-C -> CX-D -> CX-E; however, the other link CX-A/CX-H -> CX-B -> CX-F -> CX-G -> CX-D -> CX-E may be congested. On the network of a complicated topology, the metric is difficult to adjust because the change of a link may affect multiple routes.
As an overlay model, MPLS can set up a virtual topology over the physical network topology, and then map the traffic to this virtual topology. Thus, MPLS TE that integrates MPLS with TE emerges.
MPLS TE has advantages in solving the problem of network congestion. Through MPLS TE, the provider can precisely control the traffic path and prevent the traffic from passing through congested nodes. Meanwhile, MPLS TE can reserve resources to ensure the quality of services during the establishment of LSPs.
To ensure the continuity of services, MPLS TE introduces the LSP backup and fast reroute (FRR) mechanisms. When faults occur on the link, the traffic can be switched immediately. Through MPLS TE, service providers (SPs) can fully utilize the current network resources to provide diversified services, optimize network resources, and scientifically manage the network.
To achieve the preceding purpose, MPLS needs to learn TE information of all routers in this network. MPLS TE lacks such a mechanism through which each router floods its TE information in the entire network to implement the synchronization of TE information. This mechanism is provided by the IS-IS protocol. Thus, MPLS TE can advertise and synchronize TE information with the help of the IS-IS protocol. The IS-IS protocol needs to be extended to support MPLS TE.
IS-IS TE is an extension of IS-IS to support MPLS TE. IS-IS TE defines new TLVs in IS-IS LSPs to carry TE information and floods LSPs to implement the flooding and synchronization of TE information. IS-IS TE then extracts TE information from all LSPs and then transmits the TE information to the CSPF module of MPLS for calculating tunnel paths.
To put it simply, IS-IS TE collects TE information in the IS-IS network and then transmits the TE information to the CSPF module.
Basic Principle
IS-IS Traffic Engineering (TE) is an extension of IS-IS to support MPLS TE. As specified in RFC 5305 and RFC 4205, IS-IS TE defines new TLVs in IS-IS LSPs to carry TE information and helps MPLS implement the flooding, synchronization, and resolution of TE information. IS-IS TE then transmits the resolved TE information to the CSPF module. IS-IS TE plays the role of a porter in MPLS TE. Figure 2 shows the relationships between IS-IS TE, MPLS TE, and CSPF.
To carry TE information in LSPs, IS-IS TE defines the following TLVs in RFC 5305:
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Extended IS reachability TLVThis TLV takes the place of IS reachability TLV and extends the TLV formats with sub-TLVs. Sub-TLVs are implemented in TLVs in the same manner as TLVs are implemented in LSPs. Sub-TLVs are used to carry TE information configured on physical interfaces.
Table 1 Sub-TLVs defined in Extended IS reachability TLV
| Name | Type | Length (Byte) | Value |
| Administrative Group | 3 | 4 | Indicates the administrative group. |
| IPv4 Interface Address | 6 | 4 | Indicates the IPv4 address of a local interface. |
| IPv4 Neighbour Address | 8 | 4 | Indicates the IPv4 address of a neighbor interface. |
| Maximum Link Bandwidth | 9 | 4 | Indicates the maximum bandwidth of a link. |
| Maximum Reserved Link Bandwidth |
10 | 4 | Indicates the maximum reserved bandwidth of a link. |
| Unreserved Bandwidth | 11 | 32 | Indicates the unreserved bandwidth. |
| Traffic Engineering Default Metric |
18 | 3 | Indicates the default metric of TE. |
| Bandwidth Constraints sub-TLV |
22 | 36 | Indicates the TLV of the bandwidth constraint. |
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Traffic Engineering router ID TLVIt is of TLV type 134, with a 4-byte Router ID. It is used as the MPLS LSR ID. In MPLS TE, a Router ID uniquely identifies a router. Each router has a Router ID.
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Extended IP reachability TLVThis TLV takes the place of IP reachability TLV and carries routing information. It extends the length of the route cost field and carries sub-TLVs.
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Shared Risk Link Group TLVIt is of TLV type 138 and used to carry information about the shared risk link group. This TLV can carry information about multiple shared links, each of which is a 4-byte positive integer.
IS-IS TE is implemented in two processes.
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Process of responding to MPLS TE configurations.IS-IS TE functions only after MPLS TE is enabled.IS-IS TE updates the TE information in IS-IS LSPs based on MPLS TE configurations.IS-IS TE transmits MPLS TE configurations to the CSPF module.
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Process of handling TE information in LSPs.IS-IS TE extracts TE information from IS-IS LSPs and transmits the TE information to the CSPF module.
Application Environment
In typical applications, IS-IS TE helps MPLS TE set up TE tunnels. As shown in Figure 3, a TE tunnel is set up between CX-A and CX-D.
The requirements are as follows:
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Enable MPLS TE on CX-A and enable MPLS TE CSPF to calculate the path.
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Enable MPLS TE on CX-B, CX-C, and CX-D.
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Run IS-IS on CX-A, CX-B, CX-C, and CX-D to realize communication between the routers and enable IS-IS TE on each router.
After the configuration, IS-IS on CX-A, CX-B, CX-C, and CX-D respectively sends LSPs carrying TE information configured on each router. CX-A then obtains the TE information of CX-B, CX-C, and CX-D from the received LSPs. The CSPF module can calculate the path required by the TE tunnel based on the TE information in the entire network.
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