The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Hongwei wrote:
> 
> First thank David very much.  In RFC 3036, the basic discovery
> mechanism is:
> To engage in LDP Basic Discovery on an interface an LSR periodically
> sends LDP Link Hellos out the interface.  LDP Link Hellos are sent
> as UDP packets addressed to the well-known LDP discovery port for
> the "all routers on this subnet" group multicast address.
> 
> here I have something not understood:
> 1. what does the term "interface" mean? is it a physical port to
> which a link is connected to? if a router has four transimitters,
> it has four interfaces. am I right?

If they are four independantly addressible (not necessarily via IP
address), then yes.

I mention this qualifier to distinguish it from ports that are all
electrically identical.  For instance, you might have a multi-port
Ethernet card where all the ports are bridged together and therefore act
as a single layer-3 interface as far as the routing infrastructure is
concerned.

> 2. what is the "all routers on this subnet" group multicast address?

Multicast 101.  It is a well-known multicast IP address.  Its behavior
is similar to broadcast addresses, except that only routers should
process the packets - hosts should not.

> 3. if a LSR's neighbour router is not LSR, it doesn't understand
> the LDP Link Hellos, should it ignore the message or send a
> notification message back? if it sends a notification message back
> to the LSR,then how to make sure that the LSR understands the
> notification message? it seems like a dead loop.

The LDP Hello messages are sent to a specific wel-known UDP port.  If
the receiving router isn't running LDP (the only legitimate reason to
not understand an LDP Hello packet), then it won't be listening on that
port, and the packet will get ignored.

An LDP router sends its hello packets to the all-routers address on each
interface.  If there are no other LDP routers attached, then it will get
no answers.  If there are any LDP routers attached, then each one will
get the Hello, and each will respond.  Then the TCP sessions will be
created according to the RFC.  Once the TCP sessions are established,
the routers can be considered "LDP Peers", since they are now able to
send state-changing messages to each other.

-- David