Section 11.3. OSPF Extensions for Traffic Engineering


11.3. OSPF Extensions for Traffic Engineering

The role of OSPF and IS-IS in MPLS TE is to communicate TE interface parameters throughout an area to populate the traffic engineering database. In this, the role of these protocols is the same as their role for IGPs; in fact, this is just an extension of basic link state IGP behavior. The OSPF extensions for TE are specified in RFC 3630.[4]

[4] Dave Katz, Kireeti Kompella, and Derek M. Yeung, "Traffic Engineering (TE) Extensions to OSPF Version 2," RFC 3630, September 2003.

OSPF carries TE interface parameters in Traffic Engineering LSAs (Figure 11.9), an adaptation of type 10 Opaque LSAs. As first discussed in Section 10.1.2, Opaque LSAs are general-use LSAs intended for future extensions; the extension-specific information is carried in TLVs in the payload portion of the LSA.

Figure 11.9. The Traffic Engineering LSA.


This LSA performs essentially the same function as Router (type 1) LSAs: It identifies the originating router, the router's neighbors, and characteristicsin particular the TE parametersof the links to those neighbors. Because the necessary TE parameters are carried in this LSA for interfaces to both point-to-point and multi-access links, there is no need or a special "TE version" of Network (type 2) LSAs. The existing type 2 LSAs are sufficient for the CSPF calculations.

The Opaque Type of the TE LSA, as shown in Figure 11.9, is 1.

Instance[5] differentiates this LSA from other TE LSAs. Because this field is 24 bits (unlike a regular LSA ID field, the Opaque type takes up the first 8 bits) there can be a maximum of 216 = 16,777,216 TE LSAs in a given traffic engineering area.

[5] Recall from Section 10.1.2 that this field in the general Opaque LSA format is called the Opaque Type field and is defined as an identifier specific to the LSA application.

The payload portion of the TE LSA is one or more TLVs of one of the following types:

  • Router Address TLV (TLV type 1) carries in its value field an always-reachable IPv4 loopback address of the originating router. This address is normally also the RID of the originator, but of more importance here is that the address serves as the endpoint of any LSP egressing the originator.

  • Link TLV (TLV type 2) describes the TE parameters of a single link. The value of this TLV is a set of sub-TLVs. The format of a sub-TLV is the same as any other TLV; it is a sub-TLV only by virtue of the fact that it is in the value field of another TLV.

The sub-TLVs of the Link TLV, and their types, are as follows:

  • Link Type (type 1) carries as its value a 1-byte field that specifies the type of link being described: point to point (link type 1) or multi-access (link type 2).

  • Link ID (type 2) serves the same purpose, and uses the same semantics, as the Link ID in Router LSAs: It identifies the LSR at the other end of the link. If the link type is 1 (point-to-point link), the link ID is the RID of the neighbor. If the link type is 2 (multi-access), the Link ID is the interface address of the DR.

  • Local Interface IP Address (type 3) specifies the IP address of the originator's interface to the link. This sub-TLV can carry multiple IP addresses if the interface has more than one address.

  • Remote Interface IP Address (type 4) specifies the IP address or IP addresses of the neighbor's interface to the link, if the link is point to point. If the link is multiaccess, the value of this sub-TLV is 0.0.0.0 or, alternatively, the sub-TLV is not included at all.

  • Traffic Engineering Metric (type 5) carries a 4-byte TE metric as described in Section 11.2.1.

  • Maximum Bandwidth (type 6) carries the maximum bandwidth as described in Section 11.2.1. This is a 4-byte value specifying the bandwidth in bytes (not bits) per second.

  • Maximum Reservable Bandwidth (type 7) carries the maximum reservable bandwidth, as described in Section 11.2.1. This is also a 4-byte value specifying the bandwidth in bytes per second.

  • Unreserved Bandwidth (type 8) carries the unreserved bandwidth for each of the eight setup priority levels 0 through 7, as described in Section 11.2.1. You can observe these values in Figures 11.7 and 11.8; they are listed in the sub-TLV in order from 0 to 7. Because each bandwidth size is described by a 4-byte number (again in bytes per second), the total length of the value field of this sub-TLV is 32 bytes.

  • Administrative Group (type 9) specifies the administrative group (link color) or groups to which the link is assigned. The value is a 32-bit field, with each of the bits representing one of 32 possible administrative groups. If a bit is set, the link belongs to the group corresponding to that bit position. The most significant bit corresponds to administrative group 31, and the least significant bit to group 0. In Figure 11.7, the value of that link's affinity bit (yet another name for administrative group) is 0x3, so the link belongs to administrative groups 1 and 0 (and hence to whatever "colors" the network administrator has associated with those two numbers). In Figure 11.8, this same TLV value is labeled as "color," and the value of 0 indicates that the links in the database do not belong to any administrative groups.

Every Link TLV must have a Link Type and Link ID sub-TLV, but the other sub-TLVs might or might not appear in the Link TLV depending on whether the TE parameter is specified.

A significant point is that type 10 Opaque LSAs, on which the TE LSAs are built, have area flooding scope. That means that when you design a TE domain using OSPF, its boundaries must correspond to the boundaries of an OSPF area. Typically, because a TE domain is in the core of a network, the domain boundary corresponds to OSPF area 0.

And because the TE LSAs flood throughout the area they are originated in, all routers in the area, whether they individually participate in TE or not, must recognize and flood these LSAs.




OSPF and IS-IS(c) Choosing an IGP for Large-Scale Networks
OSPF and IS-IS: Choosing an IGP for Large-Scale Networks: Choosing an IGP for Large-Scale Networks
ISBN: 0321168798
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Jeff Doyle

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