Traffic on a network is always sent over a
least cost path computed by the routing protocols. On the bellow figure,
traffic, either from A to D or from A to G, will always flow over
A->B->C portion because it is on is the least cost
path. Even if the preferred path is oversubscribed, no packet will flow over
the portion (A->E->F->C). Clearly, this is not an optimal way to do.
Traffic Engineering (TE) is the ability to
send traffic over a non least cost path (steering traffic)
Traffic Engineering
(TE) is an MPLS feature, it is a powerful tool used to spread traffic more
efficiently throughout a network, avoiding oversubscription or under subscription.
Implementing TE on the previous infrastructure, one can make traffic from A to D
to flow on A->B->C->D path, and traffic from A to G to flow on
A->E->F->C->G path.
The path calculated by TE is called an LSP
Tunnel. The tunnel is unidirectional (A->D tunnel and D-> A Tunnel), it
is always maintained by the Head-End router (Router A in the previous example)
The TE Tunnel
is calculated based on constraints (Bandwidth, Latency...). The Bandwidth
constraint is the one mostly used; a tunnel is built on path that satisfies a
certain amount bandwidth
In order to
build an TE Tunnel based on bandwidth constraint, the information on current
available bandwidth on each link must be distributed to all the routers that
performs path computation, this is done by TE-specific extension made to Link
state routing protocols (OSPF and ISIS). In this way each node has knowledge of
the current available bandwidth on all links in the network. This information
is stored in TED (Traffic Engineering Database) on each router
TE makes use of 2 protocols
- CSPF (Constrained Shortest Path First): This is the protocol that compute the shortest path based on constraint (Bandwidth).The path is computed much like SPF do, it uses the information stored in the TED.
- RSVP -TE (Ressource ReserVation Protocol): After the path was computed, RSVP-TE is used to set up the path
and to lock (reserve) the requested resources on each node.
It is important to note that this reservation is done on the control plane only, RSVP is not aware about what is going on on the data plane. If, for example, 10 Mbps was reserved for A->D tunnel, RSVP can’t prevent A from, actually, sending more traffic.
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