VSC7440 datasheet(27/444 Pages) MICROSEMI

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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

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VLAN Classification VLAN-based forward filtering includes source port filtering, destination port

filtering, VLAN mirroring, and asymmetric VLANs.

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Security Enforcement The security decision made by the VCAP IS2 can, for example, redirect the

frame to the CPU based on security filters.

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MSTP The VLAN identifier maps to a Multiple Spanning Tree instance, which determines MSTP-

based destination port filtering.

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MAC Addresses Destination and source MAC address lookups in the MAC table determine if a

frame is a learn frame, a flood frame, a multicast frame, or a unicast frame.

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Learning By default, the device performs hardware learning on all ports. However, certain ports

could be configured with secure learning enabled, where an incoming frame with unknown source

MAC address is classified as a “learn frame” and is redirected to the CPU. The CPU performs the

learning decision and also decides whether the frame is forwarded.

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Learning can also be disabled. If learning is disabled, it does not matter if the source MAC address

is in the MAC table.

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Link Aggregation A frame targeted to a link aggregate is processed to determine which physical

port of the link aggregate group the frame must be forwarded to.

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Mirroring Mirror probes may be set up in different places in the forwarding path for monitoring

purposes. As part mirroring, a copy of the frame is sent either to the CPU or to another port.

3.2.6

Layer 3 Forwarding

The device supports Layer 3 forwarding (routing), which is performed by the analyzer. With Layer 3

forwarding, the IPv4 or IPv6 addresses determine the destination port set and Layer 3 processing is

performed on IP header. The Layer 3 forwarding process also replaces the arrival Layer-2 Ethernet

header (including MAC addresses and VLAN tags) with a new Layer 2 Ethernet header after departure

from the switch.

The analyzer supports 32 router legs, and supports Virtual Router Redundancy Protocol (VRRP). Ingress

router legs are addressed using classified arrival VLAN and DMAC address.

If an arrival frame is determined to belong to an ingress router leg and routing is enabled, Layer 3

forwarding is applied. Note that this is in addition to any Layer 2 forwarding, so for example, an arrival

multicast frame may be Layer 2 forwarded on the classified arrival VLAN and also Layer 3 forwarded to

one or more additional VLANs. IP unicast frames only use Layer 2 forwarding or Layer 3 forwarding, but

never both. Layer 3 forwarding always uses the classified arrival VLAN.

Layer 3 forwarding first checks the IP header for validity. IP header checks include IP version, header

length, and header checksum. The Time-to-Live (TTL) or Hop Limit values are also checked for validity

and decremented if the packet is forwarded.

The analyzer then searches the Longest Prefix Match (LPM) table for an exact match of the destination

IP address. If there is not an exact match, the table is searched for the best partial match. If there is no

partial match in the LPM table, the frame is Layer 3 forwarded to the location of the default gateway.

With Layer 3 forwarding, each egress frame copy uses an egress router leg to determine the per-copy

egress encapsulation. There can be up to 16 egress router legs associated with one forwarding entry in

support of Equal Cost Multipath (ECMP) functionality, where multiple forwarding paths are used between

adjacent routers for increased forwarding bandwidth.

Reverse path forwarding (RPF) is optionally performed on multicast and unicast (uRPF) packets as a

security check. This source IP address check helps detect and prevent address spoofing.

Multicast frames can be Layer 2 forwarded on the arrival VLAN as well as Layer 3 forwarded to different

VLANs. Layer 3 forwarding of IP multicast is different from Layer 2 forwarding of IP multicast using

IGMP/MLD snooping in the following ways:

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IP header checks and TTL processing are performed

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The Ethernet header is swapped

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The egress VLANs may be different from the ingress VLANs

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Multiple frame copies can be generated per physical port

Network control such as ICMP and ARP are redirected to the CPU, along with malformed packets,

packets with expired TTL, and packets with options.

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