Static Route Without Recursive Next-Hop Resolution Possible?

Posted on April 20, 2017 11:54 pm
 |  Asked by Erik Auerswald
 |  460 views
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Hi,

when adding a non-default static route with not yet reachable IP address as next-hop, the route is added to the routing table with the next-hop taken from the default route (and possibly another less specific route). This creates problems if the configured static route is intended to become active if and only if the directly connected next-hop subnet becomes active, and otherwise a less specific, but non-default blackhole route shall drop the traffic.

Example:

Added static route: ip route 192.0.2.128/25 198.51.100.111

Intended routing table:

destination
nexthop
remark

0.0.0.0/0
203.0.113.42
default route

192.0.2.0/24
Null 0
blackhole route

192.0.2.128/25
198.51.100.111
static route

198.51.100.110/31
Vlan 47
directly connected

Actual routing table with Vlan 47 down

destination
next-hop
remark

0.0.0.0/0
203.0.113.42
default route

192.0.2.0/24
Null 0
blackhole route

192.0.2.128/25
203.0.113.42
wrong next-hop

In the example case, traffic to 192.0.2.128/25 is sent to the default next-hop, instead of dropping it until the correct next-hop 198.51.100.111 is routed via the connected subnet 198.51.100.110/31 (Vlan 47).

Is there a way to prevent this from happening? I did not find anything in the manual or CLI help.

I have seen this on an older Arista 7050 switch running EOS 4.14.x, perhaps newer EOS versions or different hardware work differently?

Thanks,

Erik

Hm, the table formatting did not make it to the post, making it hard to read. I did not see that yesterday, as the question was held for moderation…

Anyway, using fully specified static routes (if I remember the terminology correctly) using both the interface and the next-hop results in interesting effects if interface and IP do not match (typo, configuration change, etc.).

(Erik Auerswald at April 21, 2017 6:50 am)
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Posted by Aesha Parikh
Answered on April 22, 2017 9:57 pm

Hi Erik,

If the SVI on the switch itself goes down, the static route should not recursively resolve to default  route next-hop post 4.12.4 version. How are you emulating vlan 47 to go down? 

I added a static route with next hop as remote svi ip and bringing the vlan down removed the static route from the table and did not recursively get resolve with default route next-hop. 

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Posted by Erik Auerswald
Answered on April 24, 2017 10:50 am

Hi Aesha,

I configured a transfer VLAN (no support for routed ports with dot1Q on the 7050) SVI and the static route with just an IP as next-hop, but did not have the other router connected yet. The output of “show ip route” displayed the new static route, but with wrong next-hop.

Thanks,

Erik

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Posted by Aesha Parikh
Answered on April 27, 2017 11:26 pm

Hi Erik,

This does not seem expected. Can you reach out to Arista TAC with the above details to see why static route is getting recursively resolved with default route next-hop? 

Thanks,

Aesha

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