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March 2012

OSPF Mechanics

1. OSPF Router ID Each router selects an OSPF Router ID when the OSPF process starts. The Router ID is a 32 bit number, usually written in dotted decimal format. The selection process is: Manually configured Router ID R(config-router)# router-id ROUTER-ID Highest Loopback IP address Highest non-Loopback “up/up” IP address …

OSPF LSAs

1. LSA components Sequence number InitialSequenceNumber = 0x80000001 MaxSequenceNumber= 0x7FFFFFFF When MaxSequnceNumber is reached, and a new update must be sent, the LSA is flushed by setting the Age to MaxAge and reflooding it over all adjacencies. Then, a new version of the LSA is sent with sequence number set …

OSPF Areas

Areas are identified by a 32 bit Area ID. This can be represented as a number in decimal or in dotted decimal format. Area 0 (0.0.0.0) is reserved for backbone. The backbone is responsible for summarizing the topologies of each area to every other area => all inter-area traffic must …

OSPF 101

1. Starting the routing process OSPF can be configured inside the routing process or on the interface. Even a combination of those will be a valid option. The interface configuration will override the routing process configuration. 1.1 Inside the routing process R(config)# router ospf PROCESS R(config-router)# network NETWORK-ADDR WILDCARD area …

More EIGRP Features

1. Router ID The router ID is a 32 bit number, usually represented as a dotted decimal (like an IP address). The Router ID is determined when the routing process is started by the following algorithm: Use the configured value R(config-router)# eigrp router-id ROUTER-ID Use the highest up/up loopback ip …

EIGRP Metric

[pmath size=12]K5!=0, doubleright Metric_EIGRP = [(K1B+K2B/{256-L}+K3D)*{K5/{R+K4}}]*256[/pmath] [pmath size=12]K5=0, doubleright Metric_EIGRP = (K1B+K2B/{256-L}+K3D)*256[/pmath] you will have to round down to the nearest integer default K valus: (K1,K2,K3,K4,K5) = (1,0,1,0,0) default EIGRP metric: B+D remember it as BuiLDeR 20 The EIGRP metric is 256 times larger than IGRP metric. 1. Metric Components …

EIGRP 101

1. Starting the routing process R(config)# router eigrp AS-NUMBER ! AS Number mast match between neighbors R(config-router)# network NETWORK-ADDR [WILDCARD] !If no wildcard is specified, the network is considered classful EIGRP will advertise routes learned by the EIGRP process and all routes that appear directly connected on the interfaces that …

RIP 101

1. Starting the routing process R(config)# router rip R(config-router)# network NETWORK-ADDR In RIP there is no wildcard option when configuring the network command. The NETWORK-ADDR will always be considered a classful address and will match all interfaces that are in the same classful network: R(config-router)#network 172.16.23.45 R1(config-router)#do sh run | …

How the routing table is built

1. The Routing Process The routing mechanism involves three different processes: the routing protocols that build the routing table, the routing process that looks up the routing table and the forwarding process that inquires the routing process what to do with a packet. Several routing protocols can run on one …

GRE Tunnels

1. Configuring GRE GRE tunnels appear as directly connected logical interfaces to the router even though the traffic that goes into the tunnel will actually be carried over other physical interfaces to the destination. To configure a GRE tunnel you have to set the source and the destination of the …