Hi lads,
I had a strange failure scenario in one of my customer networks. I'm wondering if someone couldn't answer my questions.
Given the following network layout:
- Router A1 & A2
- runnning HSRP (vlan100 - 192.168.100.1 / vlan 66 - 192.168.66.1) A1 activ
- connected to vlan 100(gig0/1 - 192.168.100.0/24) + vlan 66(gig0/2 - 192.168.66.0/24)
- static default route over vlan100 - 192.168.100.254
- Router B1 & B2
- runnning HSRP (vlan100 - 192.168.100.254 / vlan200 - 192.168.200.254) B1 activ
- connected to vlan 100(gig0/1 - 192.168.100.0/24) + vlan 200(gig0/2 - 192.168.200.0/24)
- static route to vlan99(gig0/2 - 192.168.99.0/24) with the gateway 192.168.200.1
- Router C1 & C2
- runnning HSRP (vlan200 - 192.168.200.1 / vlan 99 - 192.168.99.1) C1 activ
- connected to vlan 200(gig0/1 - 192.168.200.0/24) + vlan 99(gig0/2 - 192.168.99.0/24)
- static default route over vlan200 - 192.168.200.254
Now the following failure occurs:
- a static route was added to router B1 & B2 - ip route 192.168.66.0 255.255.255.0 gig 0/1 192.168.200.1
As the result of this command all the traffic with the destination
of vlan99 - 192.168.99.0/24 was sent by B1 to router A1, because of a wrong ARP entry in router B1. This ARP entry was "192.168.200.1 <virtual HSRP MAC address of A1> gig 0/1".
I think the following scenario happens:
1. Because of the new route Router B1 send an ARP request out of int gig 0/1
2. Router A1 respond with his virtual HSRP MAC address
3. Router B1 updated his ARP table and routed all the traffic for the network 192.168.99.0/24 to Router A1
What I know for sure is that we had the wrong ARP entry on Router B1 (sh ip arp) and that Router B1 send all the traffic destined for network 192.168.99.0/24 to the MAC address of A1 (Ethereal). My describted scenario is only way how I can explain the wrong ARP entry. However I think the scenario is very strange and I don't understand why:
1. Router B1 sent a ARP request out of gig0/1 even if he knows that he's directly connected to a network including this ip address over int gig 0/2 ?!?!
2. Router A1 replied to this ARP request with his own MAC address (ip proxy-arp was DISABLED) ?!?!
3. Router B1 accept the ARP reply even if he knows that he's directly connected to a network including this ip address over int gig 0/2 ?!?!
Thx
