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hsrp and traceroute

Hi all, when tracerouting with hsrp as a gateway, should the real interface ip be showing?


Re: hsrp and traceroute

Yes, you will see the physical IP Address (or IP Address of the SVI) in your traceroute, not the virtual ip address.



Community Member

Re: hsrp and traceroute

Dario is correct. An understanding of how exactly trace route works would yield the correct answer.

The source of the trace begins by sending a packet towards the destination with a TTL=1. Thus, the first L3 interface it hits decrements TTL by 1, getting it to 0. This forces that L3 interface to reply back with an ICMP message stating TTL expired during transit. That reply would be sent from that L3 interface. Thus, the source is able to obtain the IP from the reply packet. The source would then send a new packet with TTL=2, allowing it to go two hops before it would be decremented to 0, and that second hop would reply back with its own IP address. I hope this makes sense.

The point is, when the physical interface must generate a packet, it uses its physical IP address to do so.

As a matter of fact, when traffic is traversing this physical interface on the return, the L2 source would be the true MAC address of this interface. What I am getting at is that on the LAN for which HSRP is servicing, you shouldn't see the MAC address as a source address in any frame, unless it is a response to a specific request of the HSRP address (i.e. an ARP reply for an ARP request for the HSRP IP).



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