The MPLS WG Archive

[Matt Squire <mattsquire@acm.org>]

David Charlap wrote:
> 
> Punj, Arun wrote:
> > Matt Squire wrote:
> >>
> >> What prevents a packet with a router alert flag set from being
> >> sent over an LSP?  I don't recall anything in the specs that
> >> specifies router alter packets can't be LSP'd.
> >
> > Interesting.. Should it not be in the spec? If not, than are you
> > not changing the meaning of router alert. If yes, Eric's solution
> > seems perfectly OK, although it would require some sort of
> > additional support on the intermediate nodes.
> 
> Logically, it would not make sense to forward a router-alerted packet
> into an LSP.
> 
> The purpose of router alert is so that intermediate nodes will closely
> examine the packet, and possibly intercept it for processing.  The
> purpose of an LSP is to quickly forward a packet with a minimum of
> processing by the transit routers.  These are two diametrically opposed
> goals.
> 
> Nevertheless, I don't think there is anything forbidding a router from
> sending a router-alert packet into an LSP.
> 

LSPs can be used to provide L2 VPN connectivity.  LSPs can be used to
provide virtual p2p connections between two virtual routers.  LSRs might
not even be L3 forwarders.  When used in such ways, it is conceivable
and probably desired that router alert messages be sent down LSP as the
LSP is not spanning multiple L3 routers from the perspective of the
ingress - the LSP is just a single hop.  

Strictly outlawing router alert messages from LSPs would not be a
desirable end goal.

- Matt