Administrator: Created page with "{{Versions|v3, v4, v5 +}} File:ospf-forwarding.png Lets assume that router '''R1''' has static route to external network ''192.168.0.0/24''. OSPF is running between R1,..."

2015-01-29T19:16:10Z

Created page with "{{Versions|v3, v4, v5 +}} File:ospf-forwarding.png Lets assume that router '''R1''' has static route to external network ''192.168.0.0/24''. OSPF is running between R1,..."

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{{Versions|v3, v4, v5 +}}

[[File:ospf-forwarding.png]]

Lets assume that router '''R1''' has static route to external network ''192.168.0.0/24''. OSPF is running between R1,R2 and R3 and static route is distributed across the OSPF network.

The problem in such setup is obvious, R2 can not reach external network directly. Traffic from '''R2''' will be forwarded to router '''R1'''

[[File:ospf-forwarding-traffic.png]]

<pre>
[admin@R2] /ip route> print
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
...
8 ADo 192.168.0.0/24 10.1.101.10 110
</pre>

Fortunately OSPF has mechanism to solve such situations. OSPF router can set '''forwarding-address''' to something other than itself which indicates that alternate nexthop is possible. Mostly forwarding address is set to '''0.0.0.0''' suggesting that the route is reachable only via the advertising router.

Forwarding address is set in LSA, if following conditions are met:
<ul class="bullets">
<li>OSPF must be enabled on next-hop interface
<li>Interface is not passive
<li>Interface is not p2p or p2mp
<li>Next-hop address falls into network provided in ospf networks
</ul>

So knowing conditions, we can make router '''R1''' to set forwarding address. We simply need to add 10.1.101.0/24 network to OSPF networks in router's '''R1''' configuration:
<pre>
/routing ospf network add network=10.1.101.0/24 area=backbone
</pre>

{{Note | OSPF adjacency between routers in 10.1.101.0/24 network is not required}}

Now lets verify that forwarding address is actually working:
<pre>
[admin@R2] /ip route> print
Flags: X - disabled, A - active, D - dynamic,
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
# DST-ADDRESS PREF-SRC GATEWAY DISTANCE
...
8 ADo 192.168.0.0/24 10.1.101.1 110
</pre>

On all OSPF routers you will see LSA set with forwarding address other than 0.0.0.0
<pre>
[admin@R2] /routing ospf lsa>print detail
instance=default area=external type=as-external id=192.168.0.0
originator=10.1.101.1 sequence-number=0x80000073 age=16 checksum=0x8510
options="E" body=
netmask=255.255.255.0
forwarding-address=10.1.101.1
metric=30
route-tag=0x0
type1
</pre>

{{cont}}

[[Category:Manual|O]]
[[Category:Routing|O]]
[[Category:OSPF|O]]
[[Category:Case Studies|O]]

Manual:OSPF Forwarding Address - Revision history

Administrator: Created page with "{{Versions|v3, v4, v5 +}} File:ospf-forwarding.png Lets assume that router '''R1''' has static route to external network ''192.168.0.0/24''. OSPF is running between R1,..."