Cannot route from specific interface

[Steffen Frederiksen]

I have a C2900 router with 3 gigabit interfaces:

GigabitEthernet0/0         10.31.48.9      YES NVRAM  up                    up 

GigabitEthernet0/1         172.20.1.2      YES NVRAM  up                    up 

GigabitEthernet0/2         172.16.0.1      YES manual up                    up 

Kiva-Access#sh ip route

      10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks

S        10.31.0.0/16 [1/0] via 10.31.48.2

C        10.31.48.0/24 is directly connected, GigabitEthernet0/0

L        10.31.48.9/32 is directly connected, GigabitEthernet0/0

      172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

C        172.16.0.0/24 is directly connected, GigabitEthernet0/2

L        172.16.0.1/32 is directly connected, GigabitEthernet0/2

      172.20.0.0/16 is variably subnetted, 2 subnets, 2 masks

C        172.20.1.0/29 is directly connected, GigabitEthernet0/1

L        172.20.1.2/32 is directly connected, GigabitEthernet0/1

S     192.168.11.0/24 [1/0] via 172.20.1.1

S     192.168.12.0/24 [1/0] via 172.20.1.1

S     192.168.14.0/24 [1/0] via 172.20.1.1

S     192.168.15.0/24 [1/0] via 172.20.1.1

S     192.168.16.0/24 [1/0] via 172.20.1.1

S     192.168.17.0/24 [1/0] via 172.20.1.1

S     192.168.18.0/24 [1/0] via 172.20.1.1

S     192.168.25.0/24 [1/0] via 172.20.1.1

S     192.168.28.0/24 [1/0] via 172.20.1.1

S     192.168.29.0/24 [1/0] via 172.20.1.1

S     192.168.30.0/24 [1/0] via 172.20.1.1

S     192.168.37.0/24 [1/0] via 172.20.1.1

S     192.168.49.0/24 [1/0] via 172.20.1.1

S     192.168.52.0/24 [1/0] via 172.20.1.1

S     192.168.102.0/24 [1/0] via 172.20.1.1

I can access 192.168.16.0/24 from the 10.31.48.9 gigabit 0/0 interface:

Kiva-Access#trace 192.168.16.9 source 10.31.48.9

Type escape sequence to abort.

Tracing the route to 192.168.16.9

VRF info: (vrf in name/id, vrf out name/id)

  1 172.20.1.1 0 msec 4 msec 0 msec

  2 172.21.101.149 12 msec 24 msec 52 msec

  3 172.21.101.1 0 msec 0 msec 0 msec

  4 172.21.101.2 4 msec *  0 msec

But fron the 172.16.0.1 gigiabit 0/2 i dont even get to the first hop (172.20.1.1)

Kiva-Access#trace 192.168.16.9 source 172.16.0.1

Type escape sequence to abort.

Tracing the route to 192.168.16.9

VRF info: (vrf in name/id, vrf out name/id)

  1  *  *  *

  2  *  *

It's done from the same router - with the same routing table. The only explanation i have is the first hop 172.20.1.1 don't have any (return) route for the 172.16.0.1/24 network.

Can you confirm this?

Regards, Steffen

[jawad-mukhtar · Accepted solution]

Check

192.168.16.9 has a route for 172.16.0.0 Network

[mlund · Accepted solution]

Hi Steffen

Yes, You are correct. The most obvious reason is that next hop router does not have a route to 172.16.0.0 network.

A not so common reason can be that there is an access-list or similar that prevent the 172.16.0.0 network to communicate, either on your side , or on the óther side.

/Mikael

[Giuseppe Larosa · Accepted solution]

Hello Steffen,

your understanding looks like correct : when using source 172.16.0.1 the first hop router in the path to the destination 192.168.16.9 has no route back to your device or sends its answer to somebody else because it has a route for 172.16.0.1 pointing to another router.

The source address really counts!

Hope to help

Giuseppe

[Steffen Frederiksen]

Thank you all :-)