Linux Kernel not responding to ARP requests

Nitin Yadav Nitin.Yadav at
Thu Sep 27 02:51:01 EDT 2012



From: Nitin Yadav 
Sent: Thursday, September 27, 2012 12:16 PM
To: 'netdev at'
Subject: RE: Linux Kernel not responding to ARP requests


Hi All,

    I am facing loss of connectivity between Linux system (2.6.18
kernel) & Cisco switch (6509) when HSRP is enabled. The Cisco switches
(STAND BY) ARP queue were flushed and new MAC address were requests, but
the kernel did not answer to this requests. 

                After investigation I found out that the Cisco switch
(STAND BY) is flushing the MAC address of the kernel port. Based on
Cisco they are flush the MAC address of inactive port every 8 minutes. 


A small note about the protocol (HSRP):-

Hot Standby Router Protocol (HSRP) is a Cisco proprietary redundancy
protocol for establishing a fault-tolerant default gateway, and has been
described in detail in RFC 2281.

The protocol establishes a framework between network routers in order to
achieve default gateway failover if the primary gateway becomes
inaccessible,[1] in close association with a rapid-converging routing
protocol like EIGRP or OSPF. By multicasting packets, HSRP sends its
hello messages to the multicast address (all routers) for
version 1, or for version 2[2], using UDP port 1985, to
other HSRP-enabled routers, defining priority between the routers. The
primary router with the highest configured priority will act as a
virtual router with a pre-defined gateway IP address and will respond to
the ARP request from machines connected to the LAN with the MAC address
0000.0c07.acXX where XX is the group ID in hex. If the primary router
should fail, the router with the next-highest priority would take over
the gateway IP address and answer ARP requests with the same mac
address, thus achieving transparent default gateway fail-over. A HSRP
Basics Simulation visualizes Active/Standby election and link failover
with Hello, Coup, ARP Reply packets and timers.


My queries :-

Is there any way Kernel is dropping the ARP requests (from the Stand by
Router to the Kernel)? 

If it's not dropping, is there any other reason for not replying to ARP?



Nitin Yadav


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