[c-nsp] OSPF routing question

[Erik Sundberg]

Lee,


Change the Floating static route to an administrative distance of 254, so it is higher than OSPF.


router static
 address-family ipv4 unicast
 45.x.x.0/22 Null0 254


When the route is learned via OSPF it will have a metric of 110 and the ospf route will be installed into the routing table.

When the route is not learned via OSPF the floating static router on your Edge router will be active. This will still allow BGP to advertise the route.



Also, if you don't want to advertise the floating static route to other devices in your network you can do the following.

Add the tag 1 on the static route will stop it from being redistributed in your network.


router static
 address-family ipv4 unicast
 45.x.x.0/22 Null0 254 tag 1


router ospf 1
 log adjacency changes
 redistribute static route-policy IPV4-OSPF-REDIST-STATIC


route-policy IPV4-OSPF-REDIST-STATIC
  if tag eq 1 then
    drop
  endif
  done

If a static route has the tag of 1 it will not be redistributed into OSPF, so the rest of the network will not learn about the route.


---------------------

Side note, most ISP's will only advertise there Loopback and Core "Circuits" IPs in there IGP.  They will run iBGP between all of the there devices and allow BGP to redistribute the static and connected interfaces. BGP is also easier to manipulate routes on your network. Send me an email if you would like to know more.

Here is an old but still very relevant power point on this.

https://www.pacnog.org/pacnog2/track2/routing/a3-1up.pdf
3 - OSPF for ISPs - PacNOG<https://www.pacnog.org/pacnog2/track2/routing/a3-1up.pdf>
www.pacnog.org
© 2005 Cisco Systems, Inc. All rights reserved. 1 Session Number Presentation_ID Cisco Confidential Deploying OSPF for ISPs ISP/IXP Workshops













________________________________
From: cisco-nsp <cisco-nsp-bounces at puck.nether.net> on behalf of Lee Starnes <lee.t.starnes at gmail.com>
Sent: Tuesday, July 17, 2018 4:17:25 PM
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] OSPF routing question

Hello everyone,

I have a question about OSPF route redistribution. We have no issues
redistributing subnets in the network out of our /19 blocks. But we have a
/22 block that the entire /22 is allocated to a single client. The routes
redistribute across all the all switches except back to the edge routers
that announce them via BGP to our upstream carriers. This being because
there are holdown routes for the BGP on this of the same size IP block. Is
there a way to allow the /22 block to propagate to the edge routers and
still maintain the hold down routes we need to announce that /22 via BGP to
our various upstream carriers?

Edge routers are configured as such:

router static
 address-family ipv4 unicast
 45.x.x.0/22 Null0 19

router bgp ASNUMBER
address-family ipv4 unicast
network 45.x.x.0/22


router ospf NUMBER
 log adjacency changes
 redistribute connected
 redistribute static
 area W.X.Y.Z
  !
  interface TenGigE0/3/0/0
   passive disable
  !
  interface TenGigE0/3/3/0
   passive disable
  !


Any ideas are greatly appreciated.

-Lee
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