3750, 3845 ARP, Routing Issue

[Kishore Chennupati]

Hi All,

I have  the following set up.  A layer2 switch and 3 routers. The routers have eigrp between them.

1. My question here is that although the default gateway on the switch is different, I am still able to log into it  via vlan300. how is this happening?

The SVI ip address is in 10.300.67.1 and the dfault gateway should be in the same subnet which is 10.300.67.0/24.Right?

2. Why is R1 using the vMAC of R3's HSRP for its own HSRP and sending it to the switch?

If you need more info please let me know and i will source it. IP addresses have been changed for security

                    ---eigrp------

sw1 ------->R1----->R2----R3

3750       3845     3845  3845

===sw1

sw1#sh ip int brief | i 67.1
Vlan300               10.300.67.1     YES manual up                    up                 <<< mgmt interface

sw1#sh ip route
Default gateway is 10.300.76.254                                <<  Gateway  assigned incorrectly in a different subnet
Host               Gateway           Last Use    Total Uses  Interface
ICMP redirect cache is empty

sw1#sh ip arp

Protocol  Address          Age (min)  Hardware Addr   Type   Interface

Internet  10.300.76.254         101  0000.0c07.acc7 ARPA   Vlan300

====R1

R1#sh ip arp | i 10.300.67.1
Internet  10.300.67.1            69   001b.0c43.3c41  ARPA   GigabitEthernet0/1.300

R1#ping  10.300.67.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.300.67.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

R1#sh ip arp | i 10.300.67.254              <<<<< Virtual ip of HSRP group
Internet  10.300.67.254           -   0000.0c07.acc7  ARPA   GigabitEthernet0/1.300

===R3

R3#sh ip arp | i 10.300.76.254            <<<<Virutal IP of HSRP group
Internet  10.300.76.254           -   0000.0c07.acc7  ARPA   GigabitEthernet0/1.300

Any  ideas would be appreciated

Regards,

Kishore

[milan.kulik · Accepted solution]

Hi,

ad Q1)

As the sw1 and R1 are in the same subnet, you can Ping sw1 from R1 no matter of incorrectly configured default GW on the sw1.

I suppose you are not able to Ping sw1 from R3 though?

ad Q2) The MAC address assigned to the virtual IP address of HSRP group is derived from the HSRP group number.

"This MAC address will be a virtual MAC address composed of 0000.0C07.ACxy, where xy is the HSRP group number in hexadecimal based on the respective interface."

See http://www.cisco.com/en/US/docs/ios/ipapp/configuration/guide/ipapp_hsrp_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1054586

for details.

In your case you are using HSRP group 199 on both routers probably which leads to the 0000.0c07.acc7 virtual MAC address.

HTH,

Milan

[Kishore Chennupati]

Thanx Milan...I should have paid more attention to the subnets ..n thanx 4 the info about HSRP too..

Regards,