tailieunhanh - The Complete IS-IS Routing Protocol- P52

The Complete IS-IS Routing Protocol- P52: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | 512 17. Future of IS-IS Figure . Each provisioned tunnel in the switching hierarchy N is represented as a forwarding adjacency of switching capability N 1 G-MPLS 513 isolated because there have been no paths established for them by the lower-layer switching layers. Forwarding adjacencies are powerful tools for bringing up the network incrementally. Note that in the previous example a packet-over TDM-over lambda switching path has been used to illustrate the concept of multiple switching hierarchies. A network does not need to support all switching layers. If network service providers want to eliminate their SONET SDH TDM network one could also make for example the routers signal the lambdas directly. IS-IS G-MPLS Extensions The G-MPLS extensions to IS-IS are sub-TLVs to the extended IS Reach TLV 22. The sub-TLVs are listed in Table . The first sub-TLV 4 is a redefinition. Originally defined in Internet Draft draft-ietf-isis-traffic-05 sub-TLV 4 was intended as a link-local identifier that should carry a unique number to identify unambiguously a link in the traffic engineering database. The sub-TLV is intended for unnumbered interfaces those lacking IP addresses and used to carry a 4-byte value. Most implementations used to fill that sub-TLV with their loopback address. There is one problem with using a loopback address as link-identifier the loopback address does not uniquely identify a link between a pair of routers. The current G-MPLS Internet draft draft-ietf-isis-gmpls-extensions-19 extends that sub-TLV to 8 bytes. Now the combination of the loopback addresses between a given pair of routers can be used to uniquely identify the link between the two. The Link Protection Type sub-TLV 20 tells other routers how risky it is to use a certain link. It does that by advertising the protection switching scheme of the underlying media. Values indicating that the underlying topology runs over a shared fibre that other circuits run unprotected as well as .

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