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I heard that it would be easy for someone to mess up the whole HSRP process if they came in with a laptop that had Linux and with the use of some shell scripts they could listen for an HSRP multicast and then talk to any of those HSRP group #'s with a priority of 255 (the highest priority) Then the shell script would route those packets to a "black hole" thus "hacking" the HSRP process. I've heard that a defense against this hack would be to set my priority router to 255 and the secondary to 254. That way if someone did try this hack the worst that could happen is possibly a UDP flood on the HSRP group (vlan) because of the multicast chattering that could possibly take place with the re-election process of primary and standby??? Has anyone heard about this, furthermore how do you feel about it?



Re: HSRP HACK ????

The destination address of HSRP hello packets is the all routers multicast address ( and the port is UDP 1985, so it could be susceptible to malicious activities.

Another weak defense against this, other than the one you suggested, is to use a HSRP password. An example command is "standby 1 authentication cisco". However, this is sent in clear text, so a sniffer could defeat this. I would use priority and passwords to help protect your HSRP.

VTP is also vulnerable to this sort of thing (a PC can become root and sniff your network) as well so you can use VTP password as well.

In general common network defenses you can use are: disabling unused ports, using dedicated native vlans on trunks (ie vlans that aren't used by users), port security, bpdu guards and private vlans.

Hope it helps.


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