Specifications

Multilayer Switching Overview
Guidelines for External Routers
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In contrast, if the MLS-SE is already Layer 3 switching a flow and the access list is created on the
MLS-RP, MLSP notifies the MLS-SE, and the MLS-SE immediately purges the affected flow from the
MLS cache. New flows are created based on the restrictions imposed by the access list.
Similarly, when the MLS-RP detects a routing topology change, the MLS-SE deletes the appropriate
MLS cache entries, and new flows are created based on the new topology.
Guidelines for External Routers
When using an external router, follow these guidelines:
• We recommend one directly attached external router per Catalyst 5000 series switch to ensure that
the MLS-SE caches the appropriate flow information from both sides of the routed flow.
• You can use Cisco high-end routers (Cisco 7500, 7200, 4500, and 4700 series) for MLS when they
are externally attached to the Catalyst 5000 series switch. You can make the attachment with
multiple Ethernets (one per subnet), by using Fast Ethernet with the ISL, or with Fast Etherchannel.
• You can connect end hosts through any media (Ethernet, Fast Ethernet, ATM, and FDDI) but the
connection between the external router and the Catalyst 5000 series switch must be through standard
10/100 Ethernet interfaces, ISL links, or Fast Etherchannel.
Features That Affect MLS
This section describes how certain features affect MLS.
Access Lists
The following sections describe how access lists affect MLS.
Input Access Lists
Router interfaces with input access lists cannot participate in MLS. If you configure an input access list
on an interface, all packets for a flow that are destined for that interface go through the router (even if
the flow is allowed by the router it is not Layer 3 switched). Existing flows for that interface get purged
and no new flows are cached.
Note Any input access list can be translated to an output access list to provide the same effect on the interface.
Output Access Lists
If an output access list is applied to an interface, the MLS cache entries for that interface are purged.
Entries associated with other interfaces are not affected; they follow their normal aging or purging
procedures.
Applying an output access list to an interface, when the access list is configured using the log,
precedence, tos, or establish keywords, prevents the interface from participating in MLS.