Ipx Mls Operation - Cisco 7609 Configuration Manual

Understanding How IPX MLS Works
Received IPX packets are formatted (conceptually) as follows:
Layer 2 Frame Header
Destination
MSFC MAC Source A MAC
The PFC rewrites the Layer 2 frame header, changing the destination MAC address to the MAC address
of Host B and the source MAC address to the MAC address of the MSFC (these MAC addresses are
stored in the IPX MLS cache entry for this flow). The Layer 3 IPX addresses remain the same. The PFC
rewrites the switched Layer 3 packets so that they appear to have been routed by a router.
The PFC forwards the rewritten packet to Host B's VLAN (the destination VLAN is saved in the IPX
MLS cache entry) and Host B receives the packet.
After the switch rewrites an IPX packet, it is formatted (conceptually) as follows:
Layer 2 Frame Header
Destination
Destination B
MAC

IPX MLS Operation

Figure 20-1
VLAN (IPX address 01.Aa), Host B is on the Marketing VLAN (IPX address 03.Bb), and Host C is on
the Engineering VLAN (IPX address 02.Cc).
When Host A initiates a file transfer to Host C, an IPX MLS entry for this flow is created (this entry is
the second item in the table shown in
Host C in the IPX MLS entry when the MSFC forwards the first packet from Host A through the switch
to Host C. The PFC uses this information to rewrite subsequent packets from Station A to Station C.
Similarly, a separate IPX MLS entry is created in the MLS cache for the traffic from Host A to Host B,
and for the traffic from Host B to Host A. The destination VLAN is stored as part of each IPX MLS entry
so that the correct VLAN identifier is used when encapsulating traffic on trunk links.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide—12.1E
20-4
Layer 3 IPX Header
Source Checksum/
IPX Length/
Transport Control
n
Layer 3 IPX Header
Source Checksum/
IPX Length/
Transport Control
MSFC MAC n+1
displays a conceptual IPX MLS network topology. In this example, Host A is on the Sales
Figure
Chapter 20 Configuring IPX Unicast Layer 3 Switching on Supervisor Engine 1
Destination Net/
Node/
Socket
Destination B IPX
Destination Net/
Node/
Socket
Destination B IPX
20-1). The PFC stores the MAC addresses of the MSFC and
Data FCS
Source Net/
Node/
Socket
Source A IPX
Data FCS
Source Net/
Node/
Socket
Source A IPX
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