VSC7440 datasheet(162/444 Pages) MICROSEMI

162 / 444 page

Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

145

In stage 2, the analyzer processes each frame copy from VD0, and L3 forwards each frame copy to its

egress VLAN.

In stage 1, ANA_L3 uses the VCAP LPM to determine the forwarding of the frame. Typically, two types of

VCAP LPM multicast entries exist: source-specific and source-independent.

•

Source specific IP multicast groups, where the same group IP address can be used by multiple

sources. Such groups are denoted (S,G).

•

Source independent IP multicast groups, where the group IP address uniquely identifies the IP

multicast flow. Such groups are denoted (*,G).

IPv4 multicast groups occupies two entries in the VCAP LPM, and IPv6 multicast groups occupies eight

entries. Source-specific multicast groups must be placed with higher precedence than source-

independent multicast groups.

A matching entry in the VCAP LPM results in an index (L3MC_IDX) to an entry in the L3MC Table.

Each L3MC table entry contains a list of egress router legs to which frames are routed. Upon removing

the ingress router leg from the list, the required number of routed multicast copies is calculated and this

number (L3MC_COPY_CNT) is sent to VD0.

Stage 1 L2 forwards the frame. ANA_ACL can be used to limit the L2 forwarding, for example, to support

IGMP/MLD snooping.

For each L3MC entry, it can optionally be configured that routing is only performed if the frame was

received by a specific ingress router leg. This is termed reverse path forwarding check and is configured

using L3MC:L3MC_CTRL.RPF_CHK_ENA and L3MC:L3MC_CTRL.RPF_VMID.

Note that this RPF check is unrelated to the SIP RPF check. For more information about the SIP RPF

check, see SIP RPF Check, page 147.

Reverse path forwarding check does not affect the L2 forwarding decision.

In stage 2, VD0 replicates the frame according to L3MC_COPY_CNT, and L3MC_IDX is written into the

frame’s IFH.

For each copy, which ANA_L3 receives from VD0 during stage 2, L3MC_IDX from IFH is used to lookup

the L3MC table entry and thus find the list of egress router legs that require a routed copy. The egress

router leg is used to determine the egress VID which in turn is used to perform (VID, DMAC) lookup in

the MAC table for L2 forwarding within the egress VLAN.

For each egress router leg, it can be configured that frames are only L3 forwarded to the router leg if the

TTL/HL of the packet is above a certain value. This is configured using

VMID:VMID_MC.RLEG_IP4_MC_TTL and VMID:VMID_MC.RLEG_IP6_MC_TTL.

In stage 2, both learning and ingress filtering is disabled.

The following illustration shows an IPv4 multicast routing example. IPv6 multicast routing works

identically to IPv4 multicast routing, with the exception that the IPv6 entries occupies four times as much

space in VCAP LPM.

VSC7440 датащи(PDF) 162 Page - Microsemi Corporation