Dell PowerSwitch S5212F-ON EMC SmartFabric OS10 User Guide Release 10.5.1 - Page 738

DRs and BDRs are configurable. If you do not define the DR or BDR, OS10 assigns them per the protocol. To determine which routers are the DR and BDR, OSPF looks at the priority of the routers on the segment. The default router priority is 1. The router with the highest priority is elected DR. If there is a tie, the router with the higher router ID takes precedence. After the DR is elected, the BDR is elected the same way. A router with a router priority set to zero cannot become a DR or BDR. Link-state advertisements A link-state advertisement (LSA) communicates the router's routing topology to all other routers in the network. Type 1-Router Router lists links to other routers or networks in the same area. Type 1 LSAs flood across their own area LSA only. The link-state ID of the Type 1 LSA is the originating router ID. Type 2-Network DR in an area lists which routers are joined within the area. Type 2 LSAs flood across their own area only. LSA The link-state ID of the Type 2 LSA is the IP interface address of the DR. Type 3- Summary LSA (OSPFv2), InterArea Prefix LSA (OSPFv3) ABR takes information it has learned on one of its attached areas and summarizes it before sending it out on other areas it connects to. The link-state ID of the Type 3 LSA is the destination network's IP address. Type 4-AS Border Router Summary LSA (OSPFv2), InterArea-Router LSA (OSPFv3) In some cases, Type 5 External LSAs flood to areas where the detailed next-hop information may not be available because it may be using a different routing protocol. The ABR floods the information for the router, the ASBR where the Type 5 originated. The link-state ID for Type 4 LSAs is the router ID of the described ASBR. Type 5-ASExternal LSA LSAs contain information imported into OSPF from other routing processes. Type 5 LSAs flood to all areas except stub areas. The link-state ID of the Type 5 LSA is the external network number. Type 7-NSSAExternal LSA (OSPFv2), LSA (OSPFv3) Routers in an NSSA do not receive external LSAs from ABRs but send external routing information for redistribution. They use Type 7 LSAs to tell the ABRs about these external routes, which the ABR then translates to Type 5 external LSAs and floods as normal to the rest of the OSPF network. Type 8-Link LSA Type 8 LSA carries the IPv6 address information of the local links. (OSPFv3) Type 9-LinkLocal Opaque LSA (OSPFv2), Intra-Area Prefix LSA (OSPFv3) Link-local opaque LSA as defined by RFC2370 for OSPFv2. Intra-Area-Prefix LSA carries the IPv6 prefixes of the router and network links for OSPFv3. Type 11-Grace Link-local opaque LSA for OSPFv3 only is sent during a graceful restart by an OSPFv3 router. LSA (OSPFv3) The LSA header is common to LSA types. Its size is 20 bytes. One of the fields of the LSA header is the link-state ID. Each router link is defined as one of four types-type 1, 2, 3, or 4. The LSA includes a link ID field that identifies the object this link connects to, by the network number and mask. Depending on the type, the link ID has different meanings. 1 Point-to-point connection to another router or neighboring router 2 Connection to a transit network IP address of the DR 3 Connection to a stub network IP network or subnet number 4 Virtual link neighboring router ID Router priority Router priority determines the designated router for the network. The default router priority is 1. When two routers attach to a network, both attempt to become the DR. The router with the higher router priority takes precedence. If there is a tie, the router with the higher router ID takes precedence. A router with a router priority set to zero cannot become the DR or BDR. If not assigned, the system selects the router with the highest priority as the DR. The second highest priority is the BDR. Priority rates from 0 to 255, with 255 as the highest number with the highest priority. 738 Layer 3