Hop: Router or interface?

binarysoul

I always thought a hop meant the physical router, but now I have a feeling that when packets arrive at one interface and exit another one on a router that is regarded two hops.

Is this accurate?

dynamik

I believe a hop refers to a change in the network or subnet, which would require a router (or perhaps an L3 switch).

subnet a <> router <> subnet b <> router <> subnet c

Subnet c is two hops away from subnet a and one hop away from subnet b.

The short entry in Wikipedia seems to confirm this.

Wikipedia wrote:

With routing a distance in terms of topology and of a length that may be not specified topographically, i.e. one hop is the step from one router to the next, on the path of a packet on any communications network (on the Internet often discovered with pings or traceroutes). The hop count then is the number of subsequent steps along the path from source to sink.

http://en.wikipedia.org/wiki/Hop_%28telecommunications%29

RTmarc

I've always considered a hop to be any point at which it passes through an administered interface.

Your computer 192.168.1.2 > f0/0 R1 192.168.1.1 > f0/1 R1 4.2.2.2 > f0/1 R2 6.3.3.3 > f0/2 R2 192.168.2.1 > Computer B 192.168.2.2

Personally, I'd consider computer B four hops away even though there are two physical routers. I know what dynamik is saying regarding changing networks/subnets and I think it boils down to a matter of personal preference.

mikej412

RTmarc wrote:

Personally, I'd consider computer B four hops away even though there are two physical routers. I know what dynamik is saying regarding changing networks/subnets and I think it boils down to a matter of personal preference.

I don't think you have a "personal preference" when it comes to TCP/IP and how it handles TTL processing.

Same with Distance Vector Routing protocols. If hop count becomes "personal preference" then you'd be introducing lots of asymmetric routing issues into lots of networks -- or at least any network you introduced your "modified TCP/IP protocol" into.

dynamik

>tracert google.com

Tracing route to google.com [64.233.187.99]
over a maximum of 30 hops:

  1     1 ms    <1 ms    <1 ms  192.168.0.1
  2     6 ms     7 ms     6 ms  [my public ip]
  3     8 ms     *       11 ms  68.85.165.101
  4     7 ms     8 ms     *     68.86.232.177
  5     6 ms     7 ms     *     68.87.174.17
 ...
 17    48 ms    50 ms    49 ms  4.78.208.114
 18    50 ms    52 ms    51 ms  72.14.236.15
 19    70 ms    53 ms    62 ms  216.239.49.222
 20     *       54 ms    51 ms  jc-in-f99.google.com [64.233.187.99]

Trace complete.

The trace route output on my machine makes it look like it's each hardware device, not interface. I wonder if it's a difference between Cisco and MS terminology. I'm curious to see what others have to say.

mikej412

dynamik wrote:

The trace route output on my machine makes it look like it's each hardware device, not interface.

Except if there is an MPLS network (or several) in the path configured to "hide the hops." And switches are hardware devices too, but they don't count unless they are configured as layer 3 switches (and the packet goes though a layer 3 configured interface).

And then there are tunnels that can hide lots of hops between tunnel endpoints.

RTmarc

mikej412 wrote:

RTmarc wrote:

Personally, I'd consider computer B four hops away even though there are two physical routers. I know what dynamik is saying regarding changing networks/subnets and I think it boils down to a matter of personal preference.

I don't think you have a "personal preference" when it comes to TCP/IP and how it handles TTL processing.

Same with Distance Vector Routing protocols. If hop count becomes "personal preference" then you'd be introducing lots of asymmetric routing issues into lots of networks -- or at least any network you introduced your "modified TCP/IP protocol" into.

I don't seem to have been making any commennts on how "TCP/IP and how it handles TTL processing." I was commenting on the terminology of the word "hops" and how I perceive it from within an Operating System.

networker050184

What mikej is saying is there is no room for perception on hops, it is a standard.