arp entry update

sleepingandrew · ‎03-16-2004

Hi,

Can any one tell me if there is a topology change in my LAN network and ethernet ip addresses of two LAN routers are going to swap over, how long will it take for the arp tables of the two routers to update themselves? Since the default arp timeout is 4 hrs, not sure how this works.

Will the command "clear arp interface" expedite the process? Thanks.

konigl · ‎03-17-2004

Not sure about your topology, but as long as the IP addresses on the host systems don't change (that is, they each stay on the same device with its unique MAC address), the routers' ARP tables shouldn't really need updating.

Generally speaking, it's when the IP address has already been learned on the router and associated with one MAC address, and then it shows up on another MAC address (like when you change machines but keep the same IP address) that you run into problems with the ARP table and the default 4-hour timeout.

And yes, clearing ARP will expedite the process. Depending on the router and its IOS, you can clear the entire ARP cache, the ARP cache for addresses associated with a particular interface, or you can drill right down and remove individual ARP table entries. (Each of these options requires an increasingly longer command line to accomplish the task, but it may be worth it to selectively purge some ARP entries while leaving others intact.)

Hope this helps.

sleepingandrew · ‎03-17-2004

Basically, my LAN has two WAN routers, one (router A)with an ethernet address 192.168.0.1 and the other (router B) has 192.168.0.2. What I am trying to do is to put 192.168.0.2 in router A and 192.168.0.1 in router B. All the PCs in the LAN have their IPs and default gateway (192.168.0.1) unchanged.

I know MS windows will update their arp entries for every 2 mins, so there shouldn't be any problem for them. What I want to know is how fast can the two routers update their entry for 192.168.0.1 and 192.168.0.2 respectively since there will be traffic flowing from one to another. Thanks.

konigl · ‎03-18-2004

Sounds like maybe using some other IP addresses could help you with the transition, especially if you are using dynamic routing protocols (RIP, IGRP, EIGRP, or OSPF). And, it would be a good time to implement Hot Standby Router Protocol between your two routers.

If you're on the LAN with a direct connection to both devices, then first change router A's IP to another 192.168.0.* address, one that's not currently being used (let's say 192.168.0.3). That frees up the 192.168.0.1 IP address on router A.

This next part has to be done fast, since the instant you make the change above, no one with a route or default gateway pointing at 192.168.0.1 will be able to go out the WAN. On router B, you can just put the 192.168.0.1 address as the LAN interface's primary IP address. But I would recommend configuring HSRP instead, for some redundancy. Add these commands to router B's LAN interface to enable HSRP:

standby 1 priority 110

standby 1 preempt

standby 1 ip 192.168.0.1

standby 1 track serial0 (or whatever interface your WAN is on)

To finish up, if you simply put 192.168.0.1 on router B then you can put 192.168.0.2 onto router A and you're done swapping IP addresses. Or if you chose to implement HSRP then you can put the following commands on router A's LAN interface:

standby 1 priority 105

standby 1 preempt

standby 1 ip 192.168.0.1

If you choose the HSRP approach, then the active router handling the 192.168.0.1 address will be router B, until either its serial connection goes down, its LAN interface is disconnected, or the rouer itself goes down. In any of those instances, router A will move from hot standby to active, automatically taking over routing responsibility for the 192.168.0.1 address. Failover is typically 10-15 seconds, and your clients with that IP address as their default gateway will never know the difference (other than the failover delay, during which they will not have access to the WAN).

By the way, even though router A would be the hot standby for the 192.168.0.1 address, it would still be actively routing traffic sent to it directly by router B or by clients who have 192.168.0.3 (or whatever address you chose) configured as their gefault gateway.

The time interval start-to-finish for either the simple swap or the HSRP implementation is however long it takes you to type those commands in, or cut and paste them. Having open telnet sessions to both routers before you start helps keep it short. (Serial console connection to the ones changing primary IP addresses is even quicker, since you won't cut yourself off and have to re-establish telnet session when you make the address change on the LAN interface.)

Hope this helps.