Theoretical foundations and architecture
Laboratory session: deploying IGP Traffic Engineering Extensions (TE Extensions). Standard IGPs only understand static hop metrics; they are blind to link capacity, reserved bandwidth, or link colors. TE extensions (OSPF Opaque LSA Type 10 and IS-IS Extended IS Reachability TLV 22) broadcast vital TE attributes: Maximum Bandwidth, Maximum Reservable Bandwidth, Unreserved Bandwidth across 8 priority levels, and Administrative Groups (link coloring/affinity). Every LSR aggregates these advertisements into an independent Traffic Engineering Database (TED). The CSPF (Constrained Shortest Path First) engine computes optimal paths across the TED satisfying strict bandwidth and affinity constraints.
Why the IGP has to know more than a metric
A regular IGP stores the topology and one scalar metric. That is not enough for Traffic Engineering: to compute a path that respects bandwidth and link "colors", the headend router must know for every link in the area its physical capacity, how much of it may be reserved, how much is already reserved, and its administrative attributes. These TE extensions (RFC 3630 for OSPF, RFC 5305 for IS-IS) are flooded by the same mechanism, and each node stores them in a separate database, the TED (Traffic Engineering Database). The CSPF (Constrained SPF) algorithm works on the TED: it first discards links that do not satisfy the constraints (not enough free bandwidth, a forbidden color) and then runs SPF on the rest. The result is an explicit path which is then signaled with RSVP-TE.
The OSPF format
TE information is carried in Opaque LSA Type 10 (flooding scope - area), Opaque Type 1 (the TE LSA). It holds two kinds of TLV: Router Address (TLV 1, the TE router-id, usually the loopback) and Link (TLV 2, one per link). The Link TLV contains sub-TLVs: 1 - link type (1 = point-to-point, 2 = multi-access), 2 - Link ID, 3 - local interface address, 4 - remote interface address, 5 - TE metric, 6 - Maximum Bandwidth, 7 - Maximum Reservable Bandwidth, 8 - Unreserved Bandwidth (8 values for priorities 0-7), 9 - Administrative Group (a 32-bit mask). Bandwidth is carried as a 32-bit IEEE floating-point number in bytes per second. For a router to generate and accept Opaque LSAs at all, opaque-capability must be enabled (Huawei opaque-capability enable; on Cisco and Junos enabling TE for OSPF does it).
The IS-IS format
IS-IS carries the same data as sub-TLVs inside TLV 22 (Extended IS Reachability): 3 - Administrative Group, 6 - IPv4 interface address, 8 - IPv4 neighbor address, 9 - maximum bandwidth, 10 - maximum reservable bandwidth, 11 - unreserved bandwidth, 18 - TE default metric. The TE router-id is carried in TLV 134. All this is possible only with wide metrics (TLV 22/135): with metric-style narrow TLV 22 is not used and the TED stays empty. On Junos IS-IS distributes TE attributes by default while OSPF requires an explicit traffic-engineering; on Cisco mpls traffic-eng router-id and mpls traffic-eng level-2 are needed under the IS-IS process.
TE parameters set by the engineer
| Parameter | Meaning | Where it is set |
|---|---|---|
| Max / Reservable bandwidth | Link capacity and the share available for reservation | Cisco ip rsvp bandwidth, Junos rsvp interface bandwidth, Huawei mpls te bandwidth |
| Unreserved bandwidth [0-7] | Free bandwidth for each holding priority | computed from reservations |
| Admin group / affinity | A 32-bit mask of link "colors" | Cisco attribute-flags, Junos admin-groups, Huawei mpls te link administrative group |
| TE metric | A separate metric for CSPF (for example, latency) | Cisco administrative-weight, Junos te-metric, Huawei mpls te metric |
Bit 0 of the mask 0x1 in Cisco corresponds to an admin-group with value 0 in Junos (Junos takes the bit number, not the mask). A numbering mismatch is a frequent reason why a tunnel "excludes" the wrong link. RFC 7308 extends the mask to 256 bits (Extended Admin Group).
When and how the TED is flooded
A change in free bandwidth cannot be flooded on every signaling event - the LSAs would fill the network. So thresholds are introduced: Cisco sends an update when configurable utilization levels are crossed (by default a series of thresholds from 15% up to 100%, and downward) and periodically every 180 seconds; link events (up/down, attribute change) are flooded immediately. Updates that are too frequent overload the IGP, updates that are too rare give the headend stale data, and CSPF sends a tunnel over an overloaded link. In parallel there is the MinLSInterval/arrival limit: an LSA about one link is not sent more often than once every few seconds.
TED scope
The TED is built from the LSDB of one OSPF area or one IS-IS level. CSPF cannot compute a tunnel across several areas in full: either loose hops are specified (ABRs recompute the segment) or a PCE is used. In a flat core (Area 0 or a single Level 2) this problem does not appear, which is why multi-area designs are usually avoided in an SP core with TE.
Reservation is not traffic
Unreserved bandwidth decreases according to tunnel signaling (RSVP), not according to the real interface load. If 8G is reserved on a 10G link while real traffic is 1G, CSPF still will not place a new 3G tunnel there. The opposite situation: no tunnels are signaled at all, the TED shows 100% free, and the link is loaded. To tie it to real load, use auto-bandwidth (later lessons) and DS-TE, where the BC0/BC1 pools separate traffic classes (RFC 4124).
Diagnostics and typical failures
A link is missing from the TED if TE is not enabled on the interface or the TE router-id does not match the loopback: CSPF reports "No path" although an IGP route exists. A zero max-reservable bandwidth (no ip rsvp bandwidth or rsvp interface bandwidth) makes the link unusable for any tunnel with non-zero bandwidth. Without opaque-capability (Huawei) or traffic-engineering (Junos OSPF) Opaque LSA Type 10 are simply not created. A node with narrow metrics in IS-IS does not publish TLV 22. After a bandwidth or color change the TED must update within a few seconds: check show mpls traffic-eng topology, and on the neighbor look for an LSA with a new sequence number.
The lab below enables IGP TE in OSPF Area 0, assigns capacity, colors and TE metrics to the two paths PE1-P1 and PE1-P2, and checks that the TED on PE1 sees both links with the right attributes: this is the basis for CSPF and RSVP-TE in the following lessons.
Architecture Diagram & Topology
+-----------+ [Path A: Cost 10, BW 10G] +-----------+
| PE-1 | =======================================> | PE-2 |
| (Headend) | -------- [Path B: Cost 30, BW 100G] ---> | (Tailend) |
+-----------+ +-----------+Traffic Flow & Label Stack Scheme
Traffic Engineering Database (TED) Synchronization:
1. IGP TE Floods: Link Max BW, Unreserved BW (per priority 0..7), TE Metric, Administrative Group (Colors).
2. Headend PE builds separate TED alongside standard IGP topology database.