Static Routing Two Paths but....

Amador Torrez

I'm having an issue with static routing. I have a topology setup similar to the one shown in the CCENT series of videos. I'm practicing static routing. I'm trying to setup the static routes in such a way that where two paths exist, if one fails the router will take the alternate path. Here is my topology.

alt text

The 4th octet of every interface is the number of the router so for example on router 3 interface f0/0 has an ip address of 192.168.3.3 and R5 interface f0/0 is 192.168.3.5

Problem:

I'm on Router 5 and prior to shutting down interface g1/0 I was able to ping Router 4's IP of 192.168.4.4

I shut down interface g1/0

Now I'm unable to ping Router 4 ip 192.168.4.4

I had a feeling that Router 5 is attempting to take the path towards Router 3 as opposed to taking the available "alternate path" and here is the proof:

R5#traceroute 192.168.4.4
Type escape sequence to abort.
Tracing the route to 192.168.4.4
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.3.3 32 msec
192.168.5.6 52 msec
192.168.3.3 56 msec
2 * * *
3 * * *
4 * * *
......timeout

Also:

R5#ping 192.168.4.4
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.4.4, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R5#

I can ping Router 6 interface g1/0 that is in the 192.168.4.0 network:

R5#ping 192.168.4.6
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.4.6, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/44/48 ms
R5#

From Router 4 I cannot ping/trace to Router 5 192.168.5.6

R4#ping 192.168.5.6
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.5.6, timeout is 2 seconds:
UUUUU
Success rate is 0 percent (0/5)

R4#traceroute 192.168.5.6
Type escape sequence to abort.
Tracing the route to 192.168.5.6
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.2.2 32 msec
192.168.4.6 48 msec
192.168.2.2 56 msec
R4#

Router 4 is taking the path toward Router 2 instead of heading towards router 6.

Here is my routing table on Router 5:

Gateway of last resort is not set

S 192.168.0.0/24 [1/0] via 192.168.3.3
S 192.168.1.0/24 [1/0] via 192.168.3.3
S 192.168.2.0/24 [1/0] via 192.168.3.3
192.168.3.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.3.0/24 is directly connected, FastEthernet0/0
L 192.168.3.5/32 is directly connected, FastEthernet0/0
S 192.168.4.0/24 [1/0] via 192.168.5.6
[1/0] via 192.168.3.3
192.168.5.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.5.0/24 is directly connected, FastEthernet1/0
L 192.168.5.5/32 is directly connected, FastEthernet1/0
R5#

As you can see we have to routes to the 192.168.4.0 network. One path toward Router 3 and the other path toward router 6.

Here is Router 4's Routing table:

Gateway of last resort is not set

S 192.168.1.0/24 [1/0] via 192.168.2.2
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.2.0/24 is directly connected, GigabitEthernet2/0
L 192.168.2.4/32 is directly connected, GigabitEthernet2/0
S 192.168.3.0/24 [1/0] via 192.168.2.2
192.168.4.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.4.0/24 is directly connected, GigabitEthernet1/0
L 192.168.4.4/32 is directly connected, GigabitEthernet1/0
S 192.168.5.0/24 [1/0] via 192.168.4.6
[1/0] via 192.168.2.2
R4#

Here is Router 6's routing table:

Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.0.0/24 is directly connected, GigabitEthernet0/0
L 172.16.0.1/32 is directly connected, GigabitEthernet0/0
S 192.168.0.0/24 [1/0] via 192.168.5.5
S 192.168.1.0/24 [1/0] via 192.168.5.5
S 192.168.2.0/24 [1/0] via 192.168.5.5
[1/0] via 192.168.4.4
S 192.168.3.0/24 [1/0] via 192.168.5.5
192.168.4.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.4.0/24 is directly connected, GigabitEthernet1/0
L 192.168.4.6/32 is directly connected, GigabitEthernet1/0
192.168.5.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.5.0/24 is directly connected, GigabitEthernet2/0
L 192.168.5.6/32 is directly connected, GigabitEthernet2/0
R6#

Question:

Why isn't Router 5 taking the alternate route if it sees that the route to R4 towards Router 3 is broken?

Is it because static routing doesn't know how to handle a situation when one of two paths fail?

Please look at my diagram and explanation. Let me know if you want me to provide anything else.

Maybe I need to add a metric to one of the routes so that if the primary fails the secondary kicks in?

My previous post regarding inter layer 3 switch routing was never solved, I hope Ronnie or someone can provide some expertise on this question.

Thanks

NMF

Amador Torrez

@Amador-Torrez

I believe I answered my own question. After doing re-watching the CCENT video on static routing with Ronnie and Don I did have to change the admin distance metric from the default value of 1 to 2 on the second path. In addition I need to be careful of the direction of my routes and which route is primary and secondary.

After adjusting the admin distance metric I am able to successfully ping from Router 5 to Router 4 via Router 6 when the link downstream from Router 3 goes down.

Going through this exercise clearly shows the advantage dynamic routing has over static routing when there are multiple paths. As Don stated Static routes are only as smart as the person who configured them because we as humans have to manually tell the router what to do.

If anyone needs clarity in this area, I would strongly recommend re-watching the CCENT videos on static routing along with performing the configuration on live or virtual equipment.

Regards

NMF

Ronnie Wong

@Amador-Torrez ,

Great job figuring it out! Static routing's administrative distance is always a 1, if you want to default to another path, if the route fail, it's something called a "floating" static route. This is essentially a backup route for the router if the route from the routing table disappears!