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4.5 IS-IS OSPF Redistribution
When interconnecting an SPB Core, using IPshortcuts (or L3VSNs) to existing networks running a routing
protocol such as RIP or OSPF, a redundant configuration will require two ERS 8800 routers to (a)
redistribute IP routes from RIP/OSPF into IS-IS (IP) and (b) redistribute from IS-IS (IP) into RIP/OSPF.
This is shown in the following figure.
In this scenario it is necessary to take extra care when redistributing via both 8005 & 8006.
By default the preference value for IP routes generated by SPB-IP(IS-IS) is 7 which is a higher preference
than OSPF (20 for intra-area, 25 for inter-area, 120 for ext type1, 125 for ext type2) or RIP (100). (Note:
the lower numerical value determines the higher preference).
In the diagram above both nodes 8005 & 8006 have an OSPF route to 10.8.x.0/24 with next-hop 8608.
The very moment that IP route is redistributed into IS-IS by 8005, 8006 will also learn that same route via
IS-IS (it already has the route via OSPF); since IS-IS has a higher preference, 8006 will replace its
10.8.x.0 OSPF route with an IS-IS one pointing at 8005 as the next-hop. This scenario is depicted in the
following diagram:
Avaya Inc. – External Distribution
March 2013
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