The MPLS-OPS Archive[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index][Thread Index][Author Index][Subject Index] Fwd: Label Space
Vasu, think of it this way: If a standard IP-based router can decide internally where a packet needs to be sent in order to reach a destination, then that router only has to have one address for every other router to reach it. Any other router can send a packet, and the router in question then decides what to do with the packet based on standard routing. This is about what happens if you are using unnumbered interfaces and a loopback address; the routing protocol knows of only one address for the router. One address, from the point of view of labels, is one label. For any other router to reach Router X, Router X needs to send only one label to all other connected routers to allow those routers to send labeled packets to Router X on their way to a final destination. One label serves the entire Router X, the platform regardless of which interface the packet comes in on. This is platform-wide addressing. A switch, on the other hand, works differently. An ATM switch takes cells coming in on a given interface and, based on that interface alone, applies a matching outgoing label to the cells and sends them out a related outgoing interface. An end-to-end path is unknown to the switch; it can only apply an outgoing label based on the incoming interface. But MPLS labels are supposed to, in some way, reflect an end-to-end path, each label reflecting a portion of that path. Some routing intelligence needs to have generated the labels to reflect that end-to-end idea, and standard switches have no capability of doing that. The result is that for a given switch, no one label can represent the entire switch. You need separate labels, each representing a portion of an end-to-end path, even if many paths pass through that one switch. As to label length, all MPLS labels are 32 bits. Some writers say the 20-bit portion that is effectively the address portion is the "label" and that the 32-bit whole item (containing the 20-bit address, the 3-bit "experimental" Class of Service bits, the 1-bit bottom-of-label-stack bit, and the 8-bit TTL field) is the "label stack". I find that confusing, as others state that a group of labels (e.g. local, VPN, and TE Tunnel labels) is referred to as a label stack. You have to understand those words in the context they are being used. I use "label" for the 32-bit item, and "label stack" for any group of these items. Uniqueness of labels is a fuzzy issue. Labels can be used for a number of different situations as I mentioned above. A single VPN label, for instance, may be carried across an entire network, whereas the local label works as a physical next-hop marker. No label has to be unique across the entire network. The label is issued by Router X for use by other entities addressing Router X. These are, in the case of local labels, directly attached routers. Let's say Router X issues Label 15 as a platform-wide label to be used by all directly connected routers. This does not mean that Router Y couldn't do the same thing. Why? Because the router receiving a label from Router X couples that label with an interface attached to Router X. A different interface is attached to Router Y. Effectively, there is nothing saying that the receiving router couldn't couple the same label number (15 in this case) with 2 different interfaces, but it would depend on how the OS worked internally. I hope this helps Roger Williams Resent-Date: Wed, 25 Sep 2002 17:47:17 -0400------- The MPLS-OPS Mailing List Subscribe/Unsubscribe: http://www.mplsrc.com/mplsops.shtml Archive: http://www.mplsrc.com/mpls-ops_archive.shtml |
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