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Routing

OSPF RID Story

Understanding the need for OSPF RID and how to control it is indeed a very important aspect to take care of when dealing with OSPF. The RID is a dotted decimal value used by OSPF routers to identify the other OSPF routers. Beside being used in OSPF operations, and to identify the neighbors in the output of the show commands, OSPF depends on the RID in many configuration aspects such as virtual-links, manipulating distance per neighbor, and others. Cisco routers derive their Router IDs by the following means: 1. If the Router ID has been manually configured using the router-id …

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How OSPF selects best routes

It is known for all of us that when two routes are received from the same routing protocol, the route with the lowest metric will be selected and installed in the routing table. In OSPF this is only true when the two routes are of the same type. OSPF has the following route types: Intra-area routes: Intra-area routes are those which are originated and received from routers in the same area. Inter-area routers: Routes that are originated in different areas are considered inter-area routes. External routes: Routes that do not belong to the OSPF domain and were inserted from another …

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How-to:Configure BGP aggregation Part2

In the first post we explored the basic configuration of BGP route aggregation with and without the summary-only keyword. In todays post our task is to explore the suppress-map keyword using the same topology used in the first post. “please refer Part1 or the series for R1&R2 configuration and network diagram“. Task1: Configure route aggregation on R2 for its loopback networks and allow only networks 2.2.0.0/24 and 2.2.2.0/24 to be advertised to R1 with the summary. It is known that the aggregate-address command advertises the aggregate plus the more specific networks by default. The suppress map technique can be used …

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How routers select best routes ?

Routers select best routes based on the following criteria: Longest prefix match: Routers select routes with the longest match to the destination address in the forwarded packet. For example if a packet is destined to 192.168.12.1 and the router has 192.168.0.0/16 and 192.168.12.0/24 in its routing table, it will forward the packet using the 192.168.12.0/24 route. Administrative distance: If a router is receiving the same route from multiple routing protocols it will install the route with the lowest Administrative distance in the routing table. For example if the router is receiving 192.168.12.0/24 from both OSPF (AD:110) and RIP (AD:120) the …

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Passive-interface command behavior in RIP, EIGRP & OSPF

Passive-interface command is used in all routing protocols to disable sending updates out from a specific interface. However the command behavior varies from o­ne protocol to another. In RIP this command will disable sending multicast updates via a specific interface but will allow listening to incoming updates from other RIP speaking neighbors. This simply means that the router will still be able to receive updates o­n that passive interface and use them in the routing table. In EIGRP the passive-interface command stops sending outgoing hello packets, hence the router can not form any neighbor relationship via the passive interface. This …

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Automatic summarization in RIP and EIGRP

Summarization in RIP and EIGRP can be configured manually using the ip summary-address command or automatically using auto-summary command. when auto summarization is in effect routers behave as shown in the steps below: Subnets are summarized to their classful boundary before being sent to neighbors if they are not part of the major subnet of the exit interface. EIGRP automatically installs a discard route in the local routing table. RIP does not install a discard route by default. In the following example we will explore EIGRP with automatic summarization enabled: Router1 is connected to Router2 by a serial link and …

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