| When administrative policies or traffic engineering are
required to be applied, it is better to use explicit routing instead
of hop-by-hop routing. |
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Tunnels |
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| When the network administrator requires to forward certain classes of
traffic along certain pre-specified paths, where these paths are different
from the currently used hop-by-hop path, MPLS allows this by
means of Explicit Routed LSP Tunnels. This case, we need: |
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- A means of selecting the packets that are to be sent into the tunnel;
- A means of setting up the tunnel;
- A means of ensuring that packets sent into the tunnel will not loop
from the receiver to the sender endpoints.
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| To put a packet into the tunnel, the sender endpoint first replace
the label value at the top of the stack with a label value that was
distributed to it by the tunnel's receive endpoint. Then, it pushes
on the label which correspond to the tunnel itself, i.e., those that was
distributed to it by the next hop along the tunnel. To allow this,
the tunnel endpoints should be explicit label distribution peers.
The label bindings they exchange are of no interest to the LSRs along
the tunnel. |
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Important keywords to understand and remember |
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Explicit Routed LSP Tunnel |
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