Cisco Catalyst Multilayer Switching

The following section is a summary of the major points you should be aware of in this chapter.

By default, hosts in one VLAN cannot directly communicate with hosts in another VLAN
Routing is required to allow interVLAN communication between hosts
Three solutions may be used to allow and enable interVLAN routing on switches
1. InterVLAN Routing Using Physical Router Interfaces
2. InterVLAN Routing Using Router Subinterfaces
3. InterVLAN Routing Using Switched Virtual Interfaces
Using physical router interfaces for interVLAN routing is simple but not scalable
Using subinterfaces for interVLAN routing means shared bandwidth
Using Switched Virtual Interfaces for interVLAN routing is the recommended solution

The control plane is responsible for updating and populating the routing table
The data plane is responsible for the actual forwarding of packets
The Supervisor 720 module is comprised of three integrated core components:
1. The Multilayer Switch Feature Card 3
2. The Policy Feature Card 3
3. The Switch or Switching Fabric
The MSFC 3 runs all software processes
The MSFC 3 supports both the Switch Processor (SP) and Route Processor (RP)
The RP supports Layer 3 features and functionality such as routing protocols
The SP supports Layer 2 features and functionality such as the Spanning Tree Protocol
The MSFC 3 builds the CEF FIB table in software
The MSFC downloads the FIB to the hardware ASICs on the PFC 3
The PFC makes forwarding decisions in hardware
The PFC supports routing and bridging, QoS, IP Multicast packet replication, and ACLs
The switch fabric is the connection between multiple ports or slots within a switch

Demand-based switching is also referred to as flow-based switching
Demand-based switching is a legacy method of implementing MLS in Catalyst switches
A flow is a unidirectional sequence of packets between a source and destination
Demand-based switching requires the following components:
1. Multilayer Switching Engine (MLS-SE)
2. Multilayer Switching Route Processor (MLS-RP)
3. Multilayer Switching Protocol (MLSP)
The MLS-SE is responsible for the packet switching and rewrite functions in the ASICs
The MLS-SE represents the data plane
The MLS-RP informs the MLS-SE of MLS configuration, and runs routing protocols
The MLS-RP represents the control plane
MLSP is a Multicast protocol used by the MLS-RP to communicate with the MLS-SE

The Supervisor 720 supports the CEF architecture for forwarding packets
CEF operates at the data plane and is a topology-driven proprietary switching mechanism
CEF uses two data structures for switching:
1. Forwarding Information Base (FIB)
2. Adjacency Table
CEF uses a FIB to make IP destination prefix-based switching decisions
The FIB is conceptually similar to a routing table or information base
The FIB maintains a mirror image of the forwarding information contained in the RIB
The adjacency table is created to contain all connected next hops
An adjacent node is a node that is one hop away, i.e. directly connected
The adjacency table is populated as adjacencies are discovered
By default, all CEF-based Cisco Catalyst switches use a central Layer 3 switching engine
Cisco Catalyst switches allow for the optimization of CEF using two methods:
1. Accelerated CEF (aCEF)
2. Distributed CEF (dCEF)
Accelerated CEF allows only a portion of the FIB to be distributed to capable line cards
With aCEF, if FIB entries are not found in the cache, requests are sent to the Layer 3 engine
Distributed CEF refers to the use of multiple CEF tables distributed across line cards
With dCEF, the entire FIB contents are downloaded to all line cards with dCEF support
CEF technology offers the following benefits:
1. Improved performance
2. Scalability
3. Resilience

The PFC can forward up forwarding rates to 30 Million Packets per Second (pps)
The MSFC is typically capable of forwarding only up to 500,000 pps
This may result in the MSFC being oversubscribed, which could lead to:
1. Routing protocols getting out of sync with the rest of the network
2. The console on the switch may lock up
3. Other Route Processor (RP) based processes may cease operation altogether
MLS rate limiters limit the PPS rate of certain packets that are punted to the MSFC
Catalyst 6500 series switches support the following CEF rate limiters:
1. CEF Receive
2. CEF Glean
The Catalyst 6500 series PFC 3also supports the following hardware rate limiters (HWRLs):
1. Ingress-Egress ACL Bridged Packets (Unicast Only)
2. uRPF Check Failure
3. TTL Failure
4. ICMP Unreachable (Unicast Only)
5. Layer 3 Security Features (Unicast Only)
6. ICMP Redirect (Unicast Only)
7. VACL Log (Unicast Only)
8. MTU Failure
9. Layer 2 PDU
10. Layer 2 Protocol Tunneling
11. IP Errors
12. Layer 2 Multicast IGMP Snooping

Cisco Catalyst Multilayer Switching – Chapter Summary