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FTOS Configuration Guide for the C-Series - Force10 Networks

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UDP Helper with Broadcast-all Addresses<br />

When <strong>the</strong> destination IP address of an incoming packet is <strong>the</strong> IP broadcast address, <strong>FTOS</strong> rewrites <strong>the</strong><br />

address to match <strong>the</strong> configured broadcast address.<br />

In Figure 220:<br />

1. Packet 1 is dropped at ingress if no UDP helper address is configured.<br />

2. If UDP helper (using <strong>the</strong> command ip udp-helper udp-port) is enabled, and <strong>the</strong> UDP destination port<br />

of <strong>the</strong> packet matches <strong>the</strong> UDP port configured, <strong>the</strong> system changes <strong>the</strong> destination address to <strong>the</strong><br />

configured broadcast 1.1.255.255 and routes <strong>the</strong> packet to VLANs 100 and 101. If an IP broadcast<br />

address is not configured (using <strong>the</strong> command ip udp-broadcast-address) on VLANs 100 or 101, <strong>the</strong><br />

packet is <strong>for</strong>warded using <strong>the</strong> original destination destination IP address 255.255.255.255.<br />

Packet 2, sent from a host on VLAN 101 has a broadcast MAC address and IP address. In this case:<br />

1. It is flooded on VLAN 101 without changing <strong>the</strong> destination address because <strong>the</strong> <strong>for</strong>warding process is<br />

Layer 2.<br />

2. If UDP helper is enabled, <strong>the</strong> system changes <strong>the</strong> destination IP address to <strong>the</strong> configured broadcast<br />

address 1.1.255.255 and <strong>for</strong>wards <strong>the</strong> packet to VLAN 100.<br />

3. Packet 2 is also <strong>for</strong>warded to <strong>the</strong> ingress interface with an unchanged destination address because it<br />

does not have broadcast address configured.<br />

Figure 220 UDP helper with All Broadcast Addresses<br />

Packet 1<br />

Destination Address:<br />

255.255.255.255<br />

Ingress interface<br />

IP Address: 2.1.1.1/24<br />

UDP helper enabled<br />

1/1<br />

Packet 2<br />

Switched Packet<br />

UDP Helper with Subnet Broadcast Addresses<br />

1/3<br />

When <strong>the</strong> destination IP address of an incoming packet matches <strong>the</strong> subnet broadcast address of any<br />

interface, <strong>the</strong> system changes <strong>the</strong> address to <strong>the</strong> configured broadcast address and sends it to matching<br />

interface.<br />

328 IP Addressing<br />

1/2<br />

VLAN 100<br />

IP address: 1.1.0.1/24<br />

Subnet broadcast address: 1.1.0.255<br />

Configured broadcast address: 1.1.255.255<br />

Hosts on VLAN 100: 1.1.0.2, 1.1.0.3, 1.1.0.4<br />

VLAN 101<br />

IP address: 1.11.1/24<br />

Subnet broadcast address: 1.1.1.255<br />

Configured broadcast address: 1.1.255.255<br />

Hosts on VLAN 100: 1.1.1.2, 1.1.1.3, 1.1.1.4<br />

fnC0046mp

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