RIP Maximum-Paths

(Rene Molenaar) #1

This topic is to discuss the following lesson:

0 Likes

(NetRuso) #2

Hi Rene,

I have been trying to test the RIP load balancing with an ACL and seems like the source router is choosing one path although there is two same metric paths.

in this lab, R3 is choosing always the path trough R1 (90.0.0.2) instead of share the traffic with R2 (80.0.0.2). Do you know why?

For your information this is a GNS3 lab.

R3#sh ip route 4.4.4.4
Routing entry for 4.4.4.4/32
Known via “rip”, distance 120, metric 2
Redistributing via rip
Last update from 90.0.0.2 on GigabitEthernet1/0, 00:00:07 ago
Routing Descriptor Blocks:
90.0.0.2, from 90.0.0.2, 00:00:07 ago, via GigabitEthernet1/0
Route metric is 2, traffic share count is 1
(*) 80.0.0.2, from 80.0.0.2, 00:00:12 ago, via FastEthernet0/0
Route metric is 2, traffic share count is 1

This is the ACL result:
3.3.3.3 --> R3
4.4.4.4 --> Remote destination

R1#sh ip access-lists
Extended IP access list 100
10 permit icmp host 3.3.3.3 host 4.4.4.4 (1015 matches)
20 permit ip any any (174 matches)

R2#sh access-lists
Extended IP access list 100
10 permit icmp host 3.3.3.3 host 4.4.4.4
20 permit ip any any (31 matches)

0 Likes

(Lazaros Agapides) #3

Hello Jose

When a routing like RIP protocol determines that there are two equal cost routes to a destination, it employs load balancing. How that load balancing is managed depends not on the routing protocol but on the type of switching that is configured on the device. Specifically, it depends on whether process switching or fast switching has been enabled.

According to Cisco:

You can usually use the show ip route command to find equal cost routes. For example, below is the show ip route command output to a particular subnet that has multiple routes. Notice there are two routing descriptor blocks. Each block is one route. There is also an asterisk (*) next to one of the block entries. This corresponds to the active route that is used for new traffic. The term ‘new traffic’ corresponds to a single packet or an entire flow to a destination, depending on the type of switching configured.

  • For process-switching—load balancing is on a per-packet basis and the asterisk (*) points to the interface over which the next packet is sent.
  • For fast-switching—load balancing is on a per-destination basis and the asterisk (*) points to the interface over which the next destination-based flow is sent.

The position of the asterisk (*) keeps rotating among the equal cost paths each time a packet/flow is served.

This was obtained from this Cisco documentation.

So, apparently, your RIP configuration is correct, however, the method of load balancing seems to be set up as a fast switching. This means that the load balancing mechanism employed depends on the destination of your data.

The method of switching depends on the platform you are using as well as the specific configuration you have set up.

If your tests are sent to only a single destination IP, then the result will be that all traffic will go via only one of the two available routes. Try one of the following:

  1. Attempt to connect to a different destination address via the same route (i.e. over the load balanced route)
  2. Attempt to change the switching method if the platform you use allows you to do so. Information on how to do this can be found in the link shared above.

I hope this has been helpful!

Laz

0 Likes

(NetRuso) #4

Crystal clear Laz!

Thank you for the full explanation.

BR,

0 Likes