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OSPF taking wrong path despite cost (asymmetric)

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GlueGuy
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OSPF taking wrong path despite cost (asymmetric)

Mon Jun 30, 2014 8:44 pm

I think I'm running into an issue with a two-hop path versus a single hop path, but I don't know how to fix it.

We're interconnecting the site at R3 with another site with R1 and R2. R1 and R2 are interconnected with 1 Gbps at one location. R2 is connected to R3 with a 490 Mbps microwave link. So the interfaces going between R1->R2->R3 are all set to cost 10. The link between R1 and R3 is only 10 Mbps; useful as a backup, but not the primary link.

If I turn off ether4 at R3, routing works as expected. However, if I turn on R3/ether4, then routing goes as shown with the arrows. Traceroute to the cloud goes via the red path. Traceroute from the cloud to R3 goes via the blue path.

Looking at routes, the active route is the blue path.

The OSPF instance at all three sites is as below.

Can someone tell me why outbound traffic is taking the red path despite the cost?
Code: Select all
/routing ospf instance
set [ find default=yes ] \
	disabled=no \
	distribute-default=never \
	in-filter=ospf-in \
	metric-bgp=auto \
	metric-connected=20 \
   metric-default=1 \
   metric-other-ospf=auto \
   metric-rip=20 \
   metric-static=20 \
   name=default \
   out-filter=ospf-out \
   redistribute-bgp=no \
   redistribute-connected=no \
   redistribute-other-ospf=no \
   redistribute-rip=no \
   redistribute-static=no \
   router-id=<RTR ID>
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Last edited by GlueGuy on Tue Jul 01, 2014 9:47 pm, edited 1 time in total.
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roadracer96
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Re: OSPF taking wrong path despite cost (asymmetric)

Tue Jul 01, 2014 5:13 am

Post full /routing ospf export from each
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GlueGuy
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Re: OSPF taking wrong path despite cost (asymmetric)

Tue Jul 01, 2014 7:07 pm

Post full /routing ospf export from each
Here they are slightly sanitzed.
R1
Code: Select all
/routing ospf instance
set [ find default=yes ] router-id=10.13.0.2
/routing ospf interface
add interface=ether8-vpol authentication=md5 authentication-key=<pw> network-type=point-to-point
add interface=ether9-hpol authentication=md5 authentication-key=<pw> network-type=point-to-point
add interface=ether1 network-type=point-to-point
add interface=ether3 authentication=md5 authentication-key=<pw> cost=51 network-type=point-to-point
add interface=ether6 authentication=md5 authentication-key=<pw> network-type=point-to-point
add interface=ether2 cost=200 network-type=broadcast
/routing ospf network
add area=backbone comment="Loopback/Router ID" network=10.13.0.2/32
add area=backbone comment="Public - R5 primary" network=1.2.115.252/30
add area=backbone comment="Public - R5 backup" network=1.2.115.248/30
add area=backbone comment="Public - R4" network=1.2.115.240/30
add area=backbone comment="Public - R6" network=1.2.114.8/30
add area=backbone comment="Public - PTP S" network=1.2.114.128/28
add area=backbone comment="Public - V-pol MV" network=1.2.114.0/30
add area=backbone comment="Public - H-pol MV" network=1.2.114.4/30
add area=backbone comment="Public - PMP 7" network=1.2.114.32/27
add area=backbone comment="Public - PMP PA" network=1.2.114.192/26
add area=backbone comment="Public - R1->R2" network=1.2.115.220/30
add area=backbone comment="Public - PMP B1" network=1.2.112.0/25
add area=backbone comment="Private - R4" network=10.13.104.240/29
add area=backbone comment="Private - R6" network=10.13.114.240/29
add area=backbone comment="Private - R4 backup" network=10.13.108.8/29
add area=backbone comment="Private - PTP S" network=10.13.103.0/24
add area=backbone comment="Private - PMP B1" network=10.13.106.0/24
add area=backbone comment="Private - R4 primary" network=10.13.108.0/29
add area=backbone comment="Private - PMP B2" network=10.13.101.0/24
add area=backbone comment="Private - PMP PA" network=10.13.102.0/24
add area=backbone comment="Private - PMP 7" network=10.13.107.0/24
add area=backbone comment="Private - PTP MV" network=10.13.110.0/24
R2
Code: Select all
/routing ospf instance
set [ find default=yes ] router-id=10.13.0.22
/routing ospf interface
add interface=ether2 network-type=point-to-point
add interface=ether3 network-type=point-to-point
/routing ospf network
add area=backbone comment="Loopback address/router ID" network=10.13.0.22/32
add area=backbone comment="Public - R1->R2" network=1.2.115.220/30
add area=backbone comment="Public R2->R3" network=1.2.115.216/30
add area=backbone comment="Private R2->R3 BHM" network=10.13.119.252/30
R3
Code: Select all
/routing ospf instance
set [ find default=yes ] router-id=10.13.0.23
/routing ospf interface
add interface=ether9 network-type=broadcast
add interface=ether2 network-type=point-to-point
add interface=ether4 cost=250 network-type=point-to-point priority=2
/routing ospf network
add area=backbone comment="Router ID" network=10.13.0.23/32
add area=backbone comment="Public - R3->R2" network=1.2.115.216/30
add area=backbone comment="Public - PMP BG" network=1.2.112.128/27
add area=backbone comment="Public - PMP from R1" network=1.2.112.0/25
add area=backbone comment="Private - PMP BG" network=10.13.119.0/27
add area=backbone comment="Private R3->R2 BHS" network=10.13.119.248/30
add area=backbone comment="Private - SH PTP" network=10.13.119.232/29
add area=backbone comment="Private for SiteMonitor" network=10.13.119.244/30
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roadracer96
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Re: OSPF taking wrong path despite cost (asymmetric)

Tue Jul 01, 2014 9:27 pm

Is your default route being flooded from a router not in the picture? Is it being exported as type 2 and not type 1?
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GlueGuy
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Re: OSPF taking wrong path despite cost (asymmetric)

Tue Jul 01, 2014 9:37 pm

Is your default route being flooded from a router not in the picture? Is it being exported as type 2 and not type 1?
We have all the export settings = no (the default setting).

How would I tell if the default route is being flooded?
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roadracer96
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Re: OSPF taking wrong path despite cost (asymmetric)

Wed Jul 02, 2014 1:14 am

Post a /ip route print from each router.

Where is the default route coming from? Or is there no default?
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CelticComms
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Re: OSPF taking wrong path despite cost (asymmetric)

Wed Jul 02, 2014 5:24 am

I would check the routing table at R3 and also the LSAs being received at R3.
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GlueGuy
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Re: OSPF taking wrong path despite cost (asymmetric)

Wed Jul 02, 2014 7:22 pm

This fixes the problem:
Code: Select all
/ip dhcp-client set 0 default-route-distance=200
The thing that was confusing me was that the /routing ospf route table showed the correct route, but was being over-ridden by the default route in /ip routes table.
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CelticComms
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Re: OSPF taking wrong path despite cost (asymmetric)

Wed Jul 02, 2014 10:23 pm

Is that on R3? Is there a reason not to use static IP assignments on that router?
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GlueGuy
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Re: OSPF taking wrong path despite cost (asymmetric)

Wed Jul 02, 2014 10:49 pm

Is that on R3? Is there a reason not to use static IP assignments on that router?
That's what broke it, I believe. Switched it to DHCP, and you can set the distance when the static is applied.
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samsung172
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Re: OSPF taking wrong path despite cost (asymmetric)

Wed Jul 02, 2014 11:47 pm

dont run default route not comming from ospf it self With ospf
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roadracer96
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Re: OSPF taking wrong path despite cost (asymmetric)

Thu Jul 03, 2014 5:07 am

Why do you even have dhcp enabled?
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GlueGuy
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Re: OSPF taking wrong path despite cost (asymmetric)

Thu Jul 03, 2014 5:19 am

Why do you even have dhcp enabled?
It's a backup link on a PMP subnet.
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roadracer96
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Re: OSPF taking wrong path despite cost (asymmetric)

Thu Jul 03, 2014 10:05 pm

Yeah, OSPF should take care of the backup.
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  4. RouterOS
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