[c-nsp] LTL?

[Tim Stevenson]

Hi Ray,

You're right, LTLs are also known as "port selects", it's basically a 
table in various ASICs that identify which ports should receive a 
frame. For a unicast frame, the LTL would select a single port; for 
multicast, could be many or all. What the destination LTL of a packet 
is inside the box is typically dictated by the forwarding engine lookup.

It's a bit more complex than that when you dig into it, but that's it 
in a nutshell.

Note that every physical port in the system has an LTL (destination 
index) set aside for it so you can't really "run out" in that case. 
But in other cases LTLs are a dynamic, limited resource, eg for 
multicast, there's a fixed number available, each describing a 
particular "fanout" or "receiver set", so if there are many unique 
receiver sets then each will burn an LTL from that pool.

Some platforms can share those LTLs so if two mcast groups share the 
same fanout they can share an LTL. Others don't have that capability.

The specific capacities & capabilities etc are very hardware 
dependent. The primary shipping platforms that use LTL today that I 
know of are 6500/7600, and Nexus 7000.

What leads you to think you have run into an overflow condition, some 
error msg I take it?

Hope that helps,
Tim

At 01:13 PM 6/30/2010, Ray Van Dolson contended:

>Newbie question here.
>
>LTL -- in the context of the Cisco world -- what is it and what does it
>do?
>
>Sounds like it's some sort of an index to track which ports to forward
>frames to.
>
>However, I believe we may have run into an LTL "overflow" (older
>hardware, 32-bit data-size?).  Anyone know what might trigger this?
>
>Just wanting to get a little background / understanding.  What I've
>read makes it sound like the LTL wouldn't contain all that much data.
>
>Thanks,
>Ray
>_______________________________________________
>cisco-nsp mailing list  cisco-nsp at puck.nether.net
><https://puck.nether.net/mailman/listinfo/cisco-nsp>https://puck.nether.net/mailman/listinfo/cisco-nsp
>archive at 
><http://puck.nether.net/pipermail/cisco-nsp/>http://puck.nether.net/pipermail/cisco-nsp/




Tim Stevenson, tstevens at cisco.com
Routing & Switching CCIE #5561
Technical Marketing Engineer, Cisco Nexus 7000
Cisco - http://www.cisco.com
IP Phone: 408-526-6759
********************************************************
The contents of this message may be *Cisco Confidential*
and are intended for the specified recipients only.