From: Huan Pham (pnhuan@yahoo.com)
Date: Mon Nov 10 2008 - 10:47:45 ARST
Hi GS,
The IOS DHCP server feature allows us to specify up to 8 default gateways, in
order of preference. My question are:
- How DHCP clients keep track of the the primary default gateway, so that when
it dies, clients can switch to the backup gateway?
- How to verify if this feature works.
I have set up a similar scenario, with R6 as the primary gateway, and R4 as
the backup. R1 is a DHCP client. All three belong to VLAN146. R1 correctly
picks up R6 as the default gateway. But I do not know how to verify if R1
picks up R4 as the backup gateway. When R6 dies, it seems that R1 still points
its default gateway to R6 :((
Thanks in advance for your help.
Rack1R6#
ip dhcp pool VLAN146_POOL
network 155.1.146.0 255.255.255.0
default-router 155.1.146.6 155.1.146.4
!
interface FastEthernet0/0.146
description **** connect to VLAN146
encapsulation dot1Q 146
ip address 155.1.146.6 255.255.255.0
! to simulate as DHCP client
Rack1R1#
!
interface FastEthernet0/0
description **** connect to VLAN146
ip address dhcp
no ip routing
Rack1R1#sh ip int fa0/0
FastEthernet0/0 is up, line protocol is up
Internet address is 155.1.146.2/24
Broadcast address is 255.255.255.255
Address determined by DHCP
Rack1R1#sh ip route
Default gateway is 155.1.146.6
Host Gateway Last Use Total Uses Interface
ICMP redirect cache is empty
Rack1R1#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 155.1.146.1 - 000b.5f76.7280 ARPA FastEthernet0/0
Rack1R1#ping 155.1.146.6
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 155.1.146.6, timeout is 2 seconds:
...
Success rate is 0 percent (0/3)
Rack1R1#sh arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 155.1.146.1 - 000b.5f76.7280 ARPA FastEthernet0/0
Internet 155.1.146.6 0 Incomplete ARPA
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