Secondary IP-addresses & Proxy ARP?

[tbandion]

What does exactly happen when I assign a secondary IP-address on the same LAN-interface (routers or Layer3-switches)?

I've something like this on my router connected to a flat LAN:

interface FastEthernet0

ip address 192.168.11.0 255.255.255.0

ip address 192.168.41.0 255.255.255.0 secondary

I understand that Proxy-ARP is enabled as a default-setting.

Are packets between the two /24-networks routed or handled by Proxy ARP?

I would like to understand this, because of possible routing thoughput issues.

Thanks in advance,

Thomas Bandion

[skarundi]

Proxy arp doesn't come into play here. Packets are routed through the same interface. So packets sent from 192.168.11.10 to 192.168.41.10 are routed through the same interface.

This can cause high cpu if you have a lot of packets routed between the primary and secondary networks. So usually it is recommended to configure the command "ip route-cache same-interface".

This way traffic going between the primary and secondary network uses CEF, which is more efficient then process switching.

A good document that explains Proxy ARP is doc ID 13718

( Just type 13718 in the cisco.com search field )

[tbandion]

Thank you very much. That's exactly what I also think happens.

But another question:

Cisco and Microsoft say (and I've seen this work in a live network) that with Proxy ARP enabled you can use the host's IP-address as its default-gateway (set by DHCP) in such a LAN (with more than one IP-range within the same broadcast-domain).

I tried this but had no luck: Communication between the two different IP-ranges was not possible without the hosts having their default-gateway set to the router.

What am I doing wrong? Did I miss anything essential?

Cisco: Proxy ARP - Document ID 13718:

Proxy ARP should be used on the network where IP hosts are not configured with default gateway or does not have any routing intelligence.

Microsoft: Microsoft Knowledge Base Article - 167686

The Dynamic Host Configuration Protocol (DHCP) server service in Windows NT 4.0 Service Pack 2 has a new parameter that will allow the client's default gateway to be set to its own IP address.

[tbaranski · Accepted solution]

Not to get soapbox-like here, but proxy ARP is in general an ugly hack that allows broken networks to function. It should be avoided unless absolutely necessary. The only reason it's somewhat common is because it's enabled by default in IOS to this day.

I don't see how it would benefit you in the situation you've described. Setting each host's gateway to the router is the way to go here -- all proxy ARP will do is increase network utilization via increased ARP traffic.