rip routing question

clarson

I've come across a routing question that you might be able to help me out with.

I've set my lab up to connect to the internet via a cable modem. The path the frames take is as I'd expect. they go from pc -> r1 -> r2 -> cable modem -> internet. The same happens when I do a tracert from the pc to the inside local address of the cable modem. But, r1 and r2 have two connections between them. one is fastethernet (both are connected to the same switch) and the other is a direct connection via serial.

Now, when I do a tracert to the internet, the path taken is always via the fastethernet connection. But, if I do a tracert to the inside local address of the cable modem, the path taken is always via the serial connection.

The default gateway for the pc is r1. Here is the routing table on r1.
Gateway of last resort is 192.168.1.4 to network 0.0.0.0


10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C 10.1.3.0/30 is directly connected, Serial0
R 10.1.2.1/32 [120/1] via 192.168.1.6, 00:00:20, FastEthernet0
[120/1] via 10.1.3.1, 00:00:22, Serial0
C 10.1.1.2/32 is directly connected, Serial1
C 10.1.3.1/32 is directly connected, Serial0
O 10.1.2.0/30 [110/129] via 10.1.1.2, 00:30:26, Serial1
C 10.1.1.0/30 is directly connected, Serial1
R 192.168.22.0/24 [120/1] via 192.168.1.4, 00:00:02, FastEthernet0
[120/1] via 10.1.1.2, 00:00:24, Serial1
C 192.168.1.0/24 is directly connected, FastEthernet0
O 192.168.2.0/24 [110/2] via 192.168.1.6, 00:31:03, FastEthernet0
R* 0.0.0.0/0 [120/1] via 192.168.1.4, 00:00:02, FastEthernet0
[120/1] via 10.1.1.2, 00:00:24, Serial1

The default static route is being sent via ripv2. and even though both have the same AD and metric, the router picks the fast ethernet connection as the default. And, all traffic intended for the internet will take that route. Just like I want it to.

But, when I do a tracert to the cable modem (192.168.22.1) it picks the serial connection. and again both connections have the same AD and metric.

So, why is the fast ethernet connection picked as the default when both connections have the same AD and metric?
and, why for the 192.168.22.1 address it picks the serial connection when again both have the same AD and metric?

C:\Documents and Settings\Administrator>tracert 192.168.22.1


Tracing route to 192.168.22.1 over a maximum of 30 hops


1 1 ms 1 ms 1 ms 192.168.1.2
2 2 ms 2 ms 2 ms 10.1.1.2 This is the serial connection to r2, this is always the output for this command
3 * 2 ms 2 ms 192.168.22.1

C:\Documents and Settings\Administrator>tracert 8.8.8.8


Tracing route to google-public-dns-a.google.com [8.8.8.8]
over a maximum of 30 hops:


1 2 ms 1 ms 1 ms 192.168.1.2
2 2 ms 1 ms 1 ms 192.168.1.4 This is the fast ethernet connection to r2
3 3 ms 2 ms 2 ms 192.168.22.1
4 * * * Request timed out.
5 11 ms 9 ms 10 ms te-0-3-0-5-sur02.shoreview.mn.minn.comcast.net

clarson

The best I can find out is with the same AD and metric, load balancing is done.

But, there is per packet load balancing and per destination load balancing.

on r1, cisco express forwarding (cef) is enabled. so the load balancing is going to be done with the cef algorithm. By default the cef algorithm uses per destination load balancing.

Well, knowing about the cef algorithm is above my pay grade. But, I'm guessing the traffic destined for the internet is getting the fast ethernet connection. And, the algorithm is load balancing the 192.168.22.1 destination on to the serial connection.

james43026

Your router may not have CEF enabled, or it may not even support CEF. Not all routers support CEF because it requires an ASIC and TCAM, which are expensive. But given what you described, it sounds like it may be CEF doing it's default per-destination load balancing. Do a "sh ip cef" on R1, and post the results. The output of this will show you the CEF FIB and will show us the next hop IP and interface that CEF has chosen.