Static, RIP, EIGRP, OSPF, BGP, IBGP, IS-IS and any other routing.
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Hi all, Im studying on MPLS to get some idea and ran into some questions. In labs we configure mpls and tags get generated automatically. Im wondering how does ISPs add lables to customers in real world. is it automatic by the router? If its automatic how does lable passthrough from ISP to another without conflicts in lables with different ISPs. Please explain. Thanks in advance.
Hi Malith, labels are being assigned automatically, though there is a static approach as well. Please learn about mpls static binding command.Also, from operational point of view mpls label range command could be used to simplify troubleshooting of an LSP. It is not very common for ISPs to run MPLS on border routers/links, but it is possible in some scenarious. However, there is no such a thing like "labels conflict", in fact, an LSP could be built using the same label values across all PE/P routers. Imagine traffic flows from S to D and links from PE1 to PE2 are MPLS-aware. For simplicity let say they are in frame-mode: S----PE1----P1----P2----PE2----D PE2 tells P2 use pop label, P2 tels P1 use label = 17, P1 tells PE1 use label = 17 as well. Now how packets will be handled? PE1 will consult its LFIB table and impose label=17 on top of the packet and will forward it to P1. P1 then will swap label=17 with label=17 as well, as P2 told him that it expects label=17 for traffic to D. Once received, P2 will dispose label 17 and forward plain IP packet to PE2 for futher operations. Hope that clarified. Cheers,Ivan.
Hi Ivan, Thank you very much for the wonderful explanation. Malith Perera.
Labels are distributed to other routers by protocols named LDP and TDP which is cisco specific where LDP is an open standard.http://www.faqs.org/rfcs/rfc3036.html Take for example three routers R1--------------R2-------------------R3. When I enable ldp on R1, R2and R3 . these routers generates labels to all routes in routing table. example R1 has 10 routes it will generate 10 labels for the 10 routes, like wise R2 and R3 does. then it will exhange the labels between the routers. R2 recieves labels from R1. for routes comming from R1 R2 select labels send by R1 like wise R3 does the same thing this way LSP path is estableished. Say one loopback of R1 188.8.131.52 R1 sends pop to R2, R2 has generated locally one label 16 for it but once it recieves pop from R1, R2 knows it is connected route of R1 R1 accecpts that where as the label 16 is given to R3. R3 accestps the label 16 because to reach 184.108.40.206 it has to go through R2 like wise lsp is bulit in both direction. say R1 220.127.116.11 wants to reach R3 18.104.22.168 R1-------(16)------------------------R2(pop)-----------------------------R3
Hi Malith, There won be any conflict in exchanging labels to different SP as mpls labels are locally significant.. HTH-bava-