Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Community Member

Proxy Arp and Switches without "ip default-gateway" configured

1) How do switches without "ip default-gateway" configured respond when a ping is initiated from the switch to an ip address that is not in the same local subnet as the switch's management vlan? Does it do a proxy arp for the ip address?

2) If a default route has been defined in the routing table, will routers with proxy arp enabled answer to arp request for ip addresses that do not belong to any network found in the routing table?

Eg. RouterA's routing table contains connected interface and routes to some local subnet. In addition, it has the default route (to Internet) configured. If it receives an arp request for an Internet ip address, will it answer with its ethernet mac address (proxy arp)?

Thanks.

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Proxy Arp and Switches without "ip default-gateway" configur

Yes if the switch ARPs out the interface for any remote destination the router on the same subnet would reply with proxy ARP if a specific route to the destination or a default route exists in it's routing table.

HTH

Sundar

4 REPLIES
Community Member

Re: Proxy Arp and Switches without "ip default-gateway" configur

1) In any situation where the next hop ip address is not known, but outgoing interfae is known, ARP request is sent asking the MAC address of the Destination IP.The Destination IP may be in a different subnet also.(This is proxy ARP situation)

2) If your default route points to an interface, proxy ARP comes in to picture.In this case the router should respond *ONLY* if there is a matching entry in its routing table.Else the router should not respond.

If the default route contains an IP address then normal ARP resolution process happens.

Community Member

Re: Proxy Arp and Switches without "ip default-gateway" configur

1)First of all you need a L3 device to ping an ip address in a different subnet. If both the switch and the L3 device reside in the same broadcast domain (VLAN), the sw will arp the router. If a match is found, the router will return a mac address associated with the destination ip address.

Let`s assume that the router is connected to the sw via an access link and is assigned to the sw`s management vlan. The sw also has a port assigned to a different broadcast domain (hence different VLAN), the router won`t be able to communicate with the port in different vlan. So your ping will be unsuccessful even if the sw arps the router.

Well if the router is connected to the switch via a trunk link and has subinterfaces each assigned to a different VLAN on switch, your ping will be successful even without "ip default-gateway". Now the router can reply the arp request from the switch because it has an entry for the destination of the ping in its route table.

2) The router will perform a route look-up even if it has a default route. The destination ip adress will match the default adress in the route table(eg 0.0.0.0) so in fact it will return an arp reply with a source MAC address of its interface on which it received the arp request from.

Hope it`s halped.

Community Member

Re: Proxy Arp and Switches without "ip default-gateway" configur

Switch(10.1.1.1)-----(10.1.1.254)RouterA(180.10.10.10)------(180.10.10.11)ISP_Router

*RouterA has proxy arp enabled.

If a default route has been configured in RouterA ("ip route 0.0.0.0 0.0.0.0 180.10.10.11"), does it mean the switch will be able to reach Internet ip addresses even if "ip default-gateway" is not configured? Since the Switch will arp for all ip addresses, will the router answer to the arp requests since the router has a route (default route) to these ip addresses?

Thanks !

Re: Proxy Arp and Switches without "ip default-gateway" configur

Yes if the switch ARPs out the interface for any remote destination the router on the same subnet would reply with proxy ARP if a specific route to the destination or a default route exists in it's routing table.

HTH

Sundar

1860
Views
0
Helpful
4
Replies
CreatePlease to create content