Junos OS

If there are multiple routes that fall within the aggregated range that share the same route

preference, the route with the lowest number prefix, not the lowest prefix length, is selected as the primary contributing route.

what is the difference between “the route with the lowest number prefix” and “lowest prefix length” ?

HI,

Primary contributing route and it is based on:

- route with lowest protocol preference (eg: statics are preferred)
- route with numerically lowest IP address as tie breaker (eg: 10.1.1.0/24 is preferred over 11.1.1.0/30)

For further ref:-

http://forums.juniper.net/t5/Junos/aggregates-statics-and-generated-routes/m-p/16284

Regards,

Anand 

Hi, 

Consider the 2 examples below:

Example 1:

192.168.0.0/22 *[Aggregate/130] 00:01:59
 > to 10.10.10.2 via ge-0/0/4.0
192.168.0.0/24 *[Static/5] 00:01:59
 > to 10.10.10.2 via ge-0/0/4.0
192.168.2.0/23 *[Static/5] 00:01:59
 > to 10.20.20.2 via ge-0/0/5.0

Example 2:

192.168.0.0/22     *[Aggregate/130] 00:17:37
                    > to 10.10.10.2 via ge-0/0/4.0
192.168.0.0/23     *[Static/5] 00:00:05
                    > to 10.10.10.2 via ge-0/0/4.0
192.168.2.0/24     *[Static/5] 00:00:05
                    > to 10.20.20.2 via ge-0/0/5.0

The primary contributing route is lowest number prefix [192.168.0.0], irrespective of the prefix length [/23 v/s /24] as long as it is a contributing route.

Cheers,

Ashvin 

Thanks for the help on this issue. It makes a lot of sense.

Do you have a link to the Juniper documentation where this is located for context?

These sure seem like they would be the same to me too.  The lower prefix number is the same as saying the lowest length.  IP prefix is defined as the number of ones in the subnet mask not the route itself.

I'm wondering if this might by a typo trying to contrast to the longest match as opposed to lowest length.

Hi, 

Re: "The lower prefix number is the same as saying the lowest length.IP prefix is defined as the number of ones in the subnet mask not the route itself."

My understanding is:

Number of 1s in subnet mask is Prefix size / length

The term prefix is used interchangably for network prefix ie. Prefix 192.168.0.0 for 192.168.0.0/24 in analogy to Telephone number prefixes.

From RIPE: https://www.ripe.net/about-us/press-centre/understanding-ip-addressing

Also in wireshark decoded BGP pcaps, we can see prefix referred to 1.1.0.0 and prefix length to /21 for 1.1.0.0/21.

I see this question has been raised in other forums too:

https://learningnetwork.cisco.com/thread/39520

I guess the meaning is open to interpretation, however going by the logic of telephone number prefixes, I would assume an IP prefix = Network prefix.

Cheers,

Ashvin