Comprehensive and Detailed Explanation:
OSPF DR/BDR election criteria:
Highest Priority (default is 1, 0 means ineligible).
Highest Router ID (if priorities are equal).
Given the routers in the figure:
Router A (RID 10.0.1.1, Priority 255) → Elected DR.
Router B (RID 10.0.2.2, Priority 0) → Not eligible for DR/BDR.
Router C (RID 10.0.3.3, Priority 0) → Not eligible for DR/BDR.
Since no other router has a nonzero priority, no BDR is elected.
✅ Reference: HCIA-Datacom Routing Guide – OSPF DR/BDR Election Process
In the given network diagram, we have three routers (Router A, Router B, and Router C) connected in a multi-access network (likely an Ethernet segment, as indicated by the shared line connecting all three routers). The election of the Backup Designated Router (BDR) in an OSPF (Open Shortest Path First) network follows strict rules defined in RFC 2328 and reinforced in the HCIA Datacom documentation. Let’s break it down step by step:
OSPF Roles in a Multi-Access Network:In OSPF, a multi-access network (e.g., Ethernet) elects a Designated Router (DR) and a Backup Designated Router (BDR) to reduce the number of adjacencies and optimize network traffic. The DR and BDR are responsible for maintaining the link-state database and flooding LSAs (Link State Advertisements) on behalf of other routers in the segment.
Election Process for DR and BDR (Strict OSPF Rules – RFC 2328):The election process is based on the following criteria (in order of precedence):
Router Priority: A higher priority value (configurable, default is 1, range 0–255) wins. A router with a priority of 0 is ineligible to become a DR or BDR.
Router ID (RID): If priorities are equal (and non-zero), the router with the highest RID wins. The RID is a unique 32-bit identifier, typically the highest IP address on the router’s loopback interface or the highest active physical interface IP address.
The router with the highest priority (or highest RID if priorities are tied) becomes the DR, and the router with the second-highest priority (or RID) becomes the BDR. A router with a priority of 0 is explicitly excluded from the DR/BDR election and cannot participate, even if it has the highest RID.
Analyzing the Given Data:
Router A: Router ID = 10.0.1.1, Priority = 255
Applying Strict OSPF Election Rules:
First, we evaluate the priorities:
Router A has a priority of 255, which is the highest possible value, ensuring it is elected as the DR.
Since Router A is the DR (due to its priority of 255), we need to elect a BDR from the remaining routers (Router B and Router C). However, both Router B and Router C have priorities of 0, which, per RFC 2328 and HCIA Datacom, explicitly prevents them from being elected as DR or BDR.
Resolving Potential Misinterpretations:
In my previous explanation, I suggested Router C could be elected as BDR based on its higher RID (10.0.3.3) compared to Router B (10.0.2.2), even though both had priority 0. This was incorrect and inconsistent with strict OSPF rules. RFC 2328 (Section 9.4) and HCIA Datacom V3.0 (Chapter 5: OSPF Protocol) clearly state that a router with a priority of 0 cannot participate in the DR/BDR election, and its RID is irrelevant for this purpose.
The HCIA Datacom documentation emphasizes adherence to OSPF standards, and in practice, if all routers except the DR have a priority of 0, the network operates with only a DR and no BDR. This is a valid OSPF configuration, though not ideal for redundancy, as the BDR role remains unassigned until a router with a non-zero priority joins the network.
References from HCIA Datacom Documents and RFC:
HCIA Datacom V3.0, Chapter 5: OSPF Protocol – DR/BDR Election Process
RFC 2328 (OSPF Version 2) – Section 9.4: DR and BDR Election (specifically, the rule that a router with priority 0 is ineligible for DR/BDR roles)
Huawei OSPF Configuration Guide (HCIA Datacom Certification Material) – Priority and Router ID in Multi-Access Networks
