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Routing

OSPF cost calculation pitfall with high bandwidth interfaces

Cisco is using the (Reference bandwidth/interface Bandwidth) formula to calculate OSPF cost values of different interfaces; where the reference bandwidth is 10^8 by default. Using this given formula the cost of FastEthernet links would be 1 and the cost of Ethernet links is 10. This default reference bandwidth value is o¬≠nly suitable for links with bandwidth value less than or equal to 100Mbit.However interfaces with higher bandwidth values will have cost values of less than 1. This leads to routing problems because OSPF will assign all interfaces with bandwidth above 100Mbit the same cost of 1 (OSPF is rounding up …

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How-to:Configure BGP aggregation Part1

In this series we will explore how route aggregation is accomplished in BGP, We am going to split this topic into multiple posts in order to keep things simple and manageable. Our Network is shown in the digram below. R1 is directly connected to R2 and BGP is basically configured as shown: Router1 Configuration router bgp 1 no synchronization bgp log-neighbor-changes network 1.0.0.0 neighbor 192.168.12.2 remote-as 2 no auto-summary Router2 configuration router bgp 2 no synchronization bgp log-neighbor-changes !– loopbacks are advertised into BGP using network commands network 2.2.0.0 mask 255.255.255.0 network 2.2.1.0 mask 255.255.255.0 network 2.2.2.0 mask 255.255.255.0 network …

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BGP fast-external-fallover

This command is enabled by default on CISCO IOS. The command terminates external BGP sessions of any directly adjacent peer if the link used to reach the peer goes down; without waiting for the hold-down timer to expire. Although this feature improves the BGP conversion time, it may lead to great instability in your BGP table due to a flapping interface. The following example shows R1 & R2 with an EBGP session between them configured as follows: R1(config-if)#router bgp 100 R1(config-router)#nei 131.108.2.2 remot 200 R2(config)#router bgp 200 R2(config-router)#nei 131.108.2.1 remot 100 bgp fast-external-fallover enabled: By shutting down the interface connecting …

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