EDGEs OSPF redundancy and filtering troubles

 OSPF v2 ( IPv4) is supported for LAN and WAN interfaces.

I recently defined a redundant OSPF LAN connection between an VMware 6X0 edge and a Sophos UTM 9 Firewall


 The Edge forwards Overlay Routes to LAN as OSPF E1 routes, thus playing an ASBR (Autonomous System Boundary Router) within the OSPF area and domain.

On the UTM Firewall static routes and connected routes are advertised via OSPF E2, again playing the role of an ASBR.

During testing various features for Internet Backhaul we also played with advertising a default route into the Velocoud Overlay. 

In order to prevent the default route from being advertised via OSPF to the UTM FW we entered a filter on "Outbound Route Advertisement" on both SFP interfaces.

But on one of the connecting interfaces we forgot to check the "Exact Match" field which resulted in a Outbound filter of DENY ANY instead of the planned DENY DEFAULT ROUTE.

The result was not what we expected:

When testing we found out that routes adverised from the EDGE were sometimes available, then after some minutes disappeared and came back after again some minutes, but always some E1 were visible but not all.

By shutting down first SFP2 and testing with SFP1 everything was announced and stable, reversing the shut down to SFP1 no E1 routes were advertised and visible on the UTM.

Analyzing the situation we found out, that by forwarding different LSAs over the both OSPF enabled interfaces we violated the basic OSPF rule, stating that LSAs have to be consistent on all nodes within an area.

Therefore in standard OSPF implementations filtering of external routes (E1 or E2) is only possible on ASBR, filtering of OSPF internal IA routes only on ABR.

But unfortunately inbound and outbound filter for OSPF on the EDGE are implemented on the VCO per interface and not per area.

My recommendation is:

MAKE Sure to have equal filter defined when using more than one OSPF enabled interface within an area, unless you want to have similar surprises.

 



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