The MPLS-OPS Archive

[sthaug@nethelp.no]

> Can anyone help me with a small problem.
> 
> I have just started on a report  on " Conventional IP routing vs MPLS label
> switching" with a particular reference to table lookup delay.
> 
> This is where I have a small problem which I sincerely hope you somone would
> take the time to help me out.
> 
> Here it is:
> 
> I've got a Pentium IV 3GHz machine to be used as a core router maintaining
> 1000 entries in its routing table, and a visit to an entry in the routing
> table takes 1 ms.
> 
> My main query is:
> 
> "How would you calculate the table lookup delay for an incoming packet if
> conventional IP routing is used, and what would the table lookup delay be if
> MPLS label switching is used."

That may be a relevant question for a thesis or a report - but in real
life, it's a mostly uninteresting question.

All high speed routers use ASIC-based forwarding or similar these days,
and these boxes are engineered such that IP address lookup and MPLS
label lookup run in hardware at about the same speed (the difference is
small enough to be ignored). Vendors no longer claim that MPLS results
in faster forwarding.

If you look at it from another angle - IP is still a huge market, and
if any router was significantly slower at IP forwarding than at MPLS
forwarding, it would have a competitive disadvantage. I don't know of
any boxes that are MPLS-only (but would be interested to hear about
such).

> Further as I understand, MPLS label switching might not make routing faster
> but definitely efficient, in terms of reduced route lookup time. If one has
> large number of routes in the forwarding table and next hop for all of them
> is same, I understand that one single label replaces this entire table.

That doesn't seem to be the case for the vendor I'm most familiar with
(Cisco):

#show mpls forwarding-table
Local  Outgoing    Prefix              Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id        switched   interface
19     Pop tag     10.65.96.16/30    0          PO2/0      point2point
20     Pop tag     10.65.192.227/32  0          PO2/0      point2point
21     Pop tag     10.65.128.32/29   0          PO2/0      point2point
22     12332       10.65.96.4/30     0          PO2/0      point2point
23     12333       10.65.192.229/32  0          PO2/0      point2point
24     12304       10.65.112.44/30   0          PO2/0      point2point
25     12305       10.65.112.32/30   0          PO2/0      point2point
26     12306       10.65.112.24/30   0          PO2/0      point2point
...

Here we have lots of prefixes with the same next hop, but different
labels.

Steinar Haug, Nethelp consulting, sthaug@nethelp.no

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