tailieunhanh - The Complete IS-IS Routing Protocol- P53

The Complete IS-IS Routing Protocol- P53: 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. | 522 17. Future of IS-IS TLV Type IANA TLV Length Flooding Reserved F D BGP Identifier Frag BGP Reserved Checksum Autonomous System s or confederation sub-AS s sub-TLV IPv4 IPv6 Peering Address sub-TLV AFI SAFI for mesh topologies sub-TLV AFI SAFI for reflection topologies sub-TLV Bytes 1 1 2 4 2 2 variable variable variable variable Figure . The BGP discovery TLV empowers an IS-IS speaker to automatically provision the iBGP distribution mesh into sub-sub-domains by introducing a level of route reflection in the sub-AS. That would also work in a Route Reflection environment. A cluster of routers can serve another cluster of routers. There is no need or desire here to further elaborate on the different methods of iBGP scaling techniques. However it is now obvious that the resulting iBGP mesh is a lot more complicated to describe once you are using one of the two iBGP scaling tools. Furthermore maintaining the iBGP mesh which means verifying that all routers are homed to the right cluster and are all producing consistent routing decisions is a daunting task for the operation teams at all service providers. The Internet Draft draft-raszuk-isis-bgp-peer-discovery describes a method that Route Reflectors and Confederation sub-ASBRs Autonomous System Border Routers can use to advertise their capabilities to terminate iBGP sessions. The magic carpet to transport those announcements is a TLV the number of which has not yet been determined by IANA. This is illustrated in Figure . The TLV is part of the LSP that is flooded inside an IS-IS level. Upon receipt of such a TLV the receiving router checks to see if it wants to connect to that router. Figure shows an example of how iBGP provisioning times can be radically reduced. All Level 2 routers are part of the full mesh and advertise that they want to be full-mesh speakers in the IS-IS Level 2. Once that information is received by any Level 2 router those routers try to connect to and bring up the BGP full .