Hello Carl,
As Rolf has explained, you can indeed summarize networks redistributed into OSPF using the summary-address command on the router that performs the redistribution. It does not matter where do the redistributed routes come from (i.e. connected, static, from another protocol...)
My only concern regarding redistributing the directly connected networks is the fact that these networks can also be injected into OSPF as internal networks using the network command (or in newer IOSes, the ip ospf interface level command). I see a couple of disadvantages of redistributing directly connected networks into OSPF:
- They will be redistributed as OSPF External Type-1 or Type-2 networks. If there is any other router in these networks that injects these networks as internal networks into OSPF, it will be used instead to reach those networks. OSPF strictly prefers intra-area and inter-area routes to external routes.
- Each redistributed network is injected into the OSPF link-state database as a standalone LSA-5 (unless summarization is configured). If you have 100 VLANs, you will create 100 instances of LSA-5, each of them describing a single redistributed network. All 100 LSA-5 have to be flooded, acknowledged, refreshed and maintained in the link-state database, possibly leading to unnecessary consumption of routers' resources. With these networks injected as internal networks, there are chances these networks will all be listed in the injecting router's single LSA-1. If these networks are to be advertised across area boundaries, an extra LSA-4 will need to be created and flooded for the redistributing router. Limiting the number of link-state database entries is a sensible precaution in OSPF.
- Redistribution is more appropriately used when retaking routes learned from a different routing protocol. Redistributing directly connected networks is by all means possible but I do not consider it to be a best practice approach because it tends to lead people into thinking that it can be compared to using the network command while it can not. If nothing else, those directly connected networks will be advertised as external, i.e. foreign, even though in reality they are still your internal networks, so why advertise them as "not coming from our own world"? Even if you require these networks to be summarized, you can still place them into a separate OSPF area and have that area subsequently summarized, resulting in these networks being advertised as summarized inter-area routes.
Best regards,
Peter
