VSC7440 datasheet(160/444 Pages) MICROSEMI

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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

143

Traffic destined outside a subnet is sent to the router using the router leg’s MAC address. The router

performs the routing decision as follows:

Known local host. If the destination IP address is known to be a local host (with the full IP address

installed in the VCAP LPM table), the router takes the local host DMAC address and egress router

leg VMID (EVMID) from the ARP table entry pointed to by the ARP_IDX of the VCAP Action. For

example, if traffic sent from Host A1 to B1 is routed using LPM entry m+2 (2.2.2.1/32) and ARP entry

m + 2, the local host’s DMAC address is found to be 0x000002020201 and egress router leg to be

RLEG2 (that is, VMID = 2).

Known longest prefix. If the full destination IP address is not known, the router finds the longest

matching IP prefix and uses it to determine the next hop DMAC address and egress router leg. For

example, traffic sent from host A1 to host 4.4.4.2 is routed using LPM entry n + 2 (the 4.4.0.0 subnet

is reached through Router B2) and using ARP entry 0, the next hop’s DMAC address is found to be

0x000002020202 and egress router leg to be RLEG2 (for example, VMID = 2).

Known equal cost multiple paths (ECMP). If the full destination IP address is not known by the

router and the longest matching IP prefix is an equal cost path, the next hop DMAC address and

egress router leg is derived from one of up to 16 ARP entries. For example, traffic sent from host A1

to host 5.5.5.13 is routed using VCAP LPM entry n + 1. The corresponding VCAP action has

ECMP = 1, so the ARP table entry is chosen to be either n +2 or n + 3, based on a pseudo random

number. If entry n + 2 is chosen, the next hop’s DMAC address is found to be 0x000001010103

(Router A3) and egress router leg to be RLEG1. If entry n + 3 is chosen, the next hop’s DMAC

address is found to be 0x000002020202 (Router B2) and egress router leg to be RLEG2.

Default gateway. In order to forward packets destined to unknown subnets to a default router, a

default rule can be configured for the VCAP LPM. This is illustrated in the IP Unicast Routing

example as a “0.0.0.0” rule. For example, traffic sent from host A1 to host 8.9.4.2 is routed using

LPM entry 0 (the default router is reached through Router B2) and ARP entry 0, the next hop’s

DMAC address is found to be 0x000002020202 and egress Router Leg to be RLEG2.

When routing packets, the router replaces the frame’s DMAC with the ARP table’s DMAC and replaces

the SMAC with MAC address associated with the egress router leg. The classified VID is replaced by the

egress VID (VMID:RLEG_CTRL.RLEG_EVID). The TTL/HL is decremented by 1. Note that the VMID

tables (ANA_L3:VMID and REW:VMID) and the router leg MAC address must be configured consistently

in the analyzer Layer 3 and rewriter.

If host A2 wants to send traffic to a host in subnet 3.3.0.0/16, the host issues an ARP request and gets a

response to send traffic to router A3. Host A2 can choose to ignore this request, in which case, the router

routes to router A3 in the same VLAN.

Local networks 1.1.1.0/24 and 2.2.2.0/24 are also configured. Hosts and routers located in these

networks can be directly L2 accessed. This includes IP addresses 1.1.1.0 and 2.2.2.0, which are the IP

addresses of the router in the respective subnets to be used for IP management traffic, for example.

3.19.2.4.1 Encoding ARP Entry in VCAP Action

Instead of using an entry in the ARP table, the ARP information can alternatively be written into the

VCAP action of the LPM entry so as to not be limited by the size of the ARP table.

ARP entries written into a VCAP actions have the following limitations compared to ARP entries in the

ARP table.

•

ECMP cannot be supported. If there are multiple paths to a given subnet, then ARP table entries

must be used.

•

RGID cannot be configured. If RGIDs of SIP RPF are used, then ARP table entries must be used for

such routes.

•

Alternative Next Hop Configuration cannot be supported. If Alternative Next Hop Configuration is

required, then ARP table entries must be used.

3.19.2.4.2 ARP Pointer Remap

An ARP Pointer Remap table is available to support fast fail-over when the next hop changes for a large

group of LPM VCAP entries.

By setting ARP_PTR_REMAP_ENA=1 in LPM VCAP action, the ARP_PTR in the VCAP action is used

to address an entry in the ARP_PTR_REMAP table and through this, new ARP_PTR and ECMP_CNT

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