Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.100.0 - Page 578

Autonomous System Border Router (ASBR) The autonomous system border area router (ASBR) connects to more than one AS and exchanges information with the routers in other ASs. Generally, the ASBR connects to a non-interior gate protocol (IGP) such as BGP or uses static routes. Internal Router (IR) The internal router (IR) has adjacencies with ONLY routers in the same area, as Router E, M, and I shown in the example in the Router Types. Designated and Backup Designated Routers OSPF elects a designated router (DR) and a backup designated router (BDR). Among other things, the DR is responsible for generating LSAs for the entire multiaccess network. Designated routers allow a reduction in network traffic and in the size of the topological database. • The DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in an area form a slave/master relationship with the DR. Every time a router sends an update, the router sends it to the DR and BDR. The DR sends the update out to all other routers in the area. • The BDR is the router that takes over if the DR fails. Each router exchanges information with the DR and BDR. The DR and BDR relay the information to the other routers. On broadcast network segments, the number of OSPF packets is further reduced by the DR and BDR sending such OSPF updates to a multicast IP address that all OSPF routers on the network segment are listening on. These router designations are not the same ad the router IDs described earlier. The DRs and BDRs are configurable in Dell Networking OS. If you do not define DR or BDR, the system assigns them. OSPF looks at the priority of the routers on the segment to determine which routers are the DR and BDR. The router with the highest priority is elected the 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 the DR or BDR. Link-State Advertisements (LSAs) A link-state advertisement (LSA) communicates the router's local routing topology to all other local routers in the same area. The LSA types supported by Dell Networking are defined as follows: • Type 1: Router LSA - The router lists links to other routers or networks in the same area. Type 1 LSAs are flooded across their own area only. The link-state ID of the Type 1 LSA is the originating router ID. • Type 2: Network LSA - The DR in an area lists which routers are joined within the area. Type 2 LSAs are flooded across their own area only. The link-state ID of the Type 2 LSA is the IP interface address of the DR. • Type 3: Summary LSA (OSPFv2), Inter-Area-Prefix LSA (OSPFv3) - An ABR takes information it has learned on one of its attached areas and can summarize it before sending it out on other areas it is connected to. The link-state ID of the Type 3 LSA is the destination network number. • Type 4: AS Border Router Summary LSA (OSPFv2), Inter-Area-Router LSA (OSPFv3) - In some cases, Type 5 External LSAs are flooded to areas where the detailed next-hop information may not be available. An ABR floods the information for the router (for example, the ASBR where the Type 5 advertisement originated. The link-state ID for Type 4 LSAs is the router ID of the described ASBR). • Type 5: LSA - These LSAs contain information imported into OSPF from other routing processes. They are flooded to all areas, except stub areas. The link-state ID of the Type 5 LSA is the external network number. • Type 7: External LSA - Routers in an NSSA do not receive external LSAs from ABRs, but are allowed to 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. 578 Open Shortest Path First (OSPFv2 and OSPFv3)