Routing With Layer 2 And Layer 3 Modules - Avaya P580 User Manual

Chapter 1

Routing with Layer 2 and Layer 3 Modules

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When a switch contains a combination of layer 2 and layer 3 modules, IP
and IPX routing is performed by the layer 3 media modules or the
supervisor module using special ASICs present on those modules. These
ASICs contain an address cache (forwarding table) that can contain a
maximum of 20,000 cache entries. The address cache entries consist of
packet addressing information and next hop information that enable the
switch to effectively route the packets to their destination.
The supervisor module also maintains a master routing table that is kept in
the its memory. The master routing table can contain up to 28,000 entries.
This routing table enables the supervisor module to keep track of which
entries are in each address cache. As a result, each time a change occurs in
the master routing table, the supervisor module updates the appropriate
address caches. For example, if a unicast route is removed from the master
routing table, all matching entries in address caches are also removed.
Consequently, when you connect a switch to the network, it begins to
receive frames from the network and builds a master routing table
(supervisor module) and forwarding tables (address caches of media
modules) based on those frames.
This process creates three distinct results:
All known (learned) layer 3 traffic that requires routing, is routed
directly by the 50-series layer 3 or 80-series media module without
a need to traverse the switching fabric to get to the supervisor
module's software routing function. This is referred to as the FIRE
path, or Fast Inband Routing Engine, since the routing is
accomplished in the hardware in the media modules.
All unknown (not learned) layer 3 traffic must first be sent to the 50-
series layer 3 or 80-series supervisor module, where information on
the frame is added to the supervisor module's master routing table,
then it is added to the address cache of each applicable layer 3
media module. This is referred to as the Slow Path, since the
routing is accomplished by the CPU and software in either
supervisor module instead of the hardware in the media
modules.The Slow Path is used when the destination is unknown
for IP and IPX packets and for all AppleTalk routing.
Since layer 2 modules have no routing capability, packets that are
received by a layer 2 module and require routing are forwarded by
sending the packet to the 50-series layer 3 or 80-series supervisor
module. The routing engine on the supervisor module then performs
the routing operation for the layer 2 modules and sends the packet
back through the switching fabric to the destination port.This is
User Guide for the Avaya P580 and P882 Multiservice Switches, v6.0