Multicast Cross-Vlan Layer 2 Switching; Size Of The Layer 3 Switch Forwarding Table; Interaction With The Firewall - Siemens RX1500 User Manual

RUGGEDCOM ROX II
User Guide
to MAC addresses. The same information is also needed by the Layer 3 switching ASIC when it switches IP
packets between subnets.
The destination or gateway MAC address is usually obtained through ARP. However, ARP entries can also be
statically configured in the Layer 3 Switch so that they do not time out. When configuring a static ARP entry, if no
value is entered for the MAC Address parameter, the address is automatically resolved through ARP and then
saved statically. This is preserved across reboots of the device.
For a static Layer 3 switching rule, the destination MAC address for the rule is always resolved, and is also saved
statically.
Section 5.31.1.5

Multicast Cross-VLAN Layer 2 Switching

Some RUGGEDCOM Layer 3 Switch models do not have full multicast Layer 3 switching capability and only
support multicast cross-VLAN Layer 2 switching. Multicast cross-VLAN Layer 2 switching differs from the normal
multicast Layer 3 switching in the following ways:
• Packet modification is not done. Specifically, the source MAC address and Time-To-Live (TTL) values in
forwarded packets do not change.
• Separate TCAM table entries are required for each egress VLAN in the multicast switching rule. For example, a
multicast stream ingressing VLAN 1 and egressing VLAN 2 and VLAN 3 requires two TCAM table entries: one
for VLAN 2 and one for VLAN 3.
• Supported bandwidth depends on the rule. Multicast traffic potentially has multiple egress VLANs, and the total
utilized ASIC bandwidth is the ingress bandwidth multiplied by the number of ingress and egress VLANs. For
example, a 256 Mbps multicast stream ingressing VLAN 1 and egressing VLANs 2 and 3 requires 768 Mbps
(256 Mbps × 3) of ASIC bandwidth.
• If a multicast packet should be forwarded to multiple egress VLANs, it egresses those VLANs sequentially
rather than concurrently. This means the packet will experience different latency for each egress VLAN.
Section 5.31.1.6

Size of the Layer 3 Switch Forwarding Table

The routing table in a software router is limited only by the amount of available memory; its size can be virtually
unlimited. However, the size of the TCAM in Layer 3 switching ASICs is significantly limited and may not be
sufficient to accommodate all Layer 3 switching rules. If the TCAM is full and a new static rule is created, the new
rule replaces some dynamically learned rule. If all of the rules in the TCAM are static, then the new static rule is
rejected.
Section 5.31.1.7

Interaction with the Firewall

If security is a concern and you use a firewall in a Layer 3 Switch, it is important to understand how the Layer 3
switch interacts with the firewall.
A software router always works in agreement with a firewall so that firewall rules are always applied. However, in
a Layer 3 Switch, if a switching rule is set in the switching ASIC (for example, due to a statically configured route),
the ASIC switches all the traffic matching the rule before the firewall inspects the traffic.
Multicast Cross-VLAN Layer 2 Switching
Chapter 5
Setup and Configuration
671