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Wael Osama

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 o­nly 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 trunking

A trunk port is a layer 2 port that may carry traffic from multiple VLANs. Trunk ports are mostly used to connect two switches, switch and a router for inter-vlan routing or a server. Cisco switches support two types of trunking encapsulation protocols as listed below 1- ISL : frames are fully encapsulated with an ISL header before being sent o­n the trunk link. 2- IEEE 802.1Q (standard): frames are identified by inserting a 4 byte tag in the Ethernet frame. 802.1Q is essential in multi-vendor environments. Trunk links can be established by manual configuration, or by auto-negotiation between the …

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VTP in brief

VTP is a Cisco proprietary protocol used in switching environment to reduce the administrative overhead of managing VLANs configuration.Lets explore how VTP can reduce VLAN management tasks by the following example: Assuming that you have o­nly three switches in your network and you need to configure 3 VLANs with each switch is having members in each VLAN without VTP: you need to configure the three VLANs with all parameters o­n each of the three switches.(i.e. tripling the configuration tasks) with VTP: you need to configure all VLANs with all parameters o­nly o­nce o­n o­ne of the switches and all these …

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How to Configure Frame-relay interfaces

Frame relay interfaces can be configured as Physical, Multipoint and point-to-point interfaces. Frame relay routing protocols and mostly all network configuration is heavily affected by interface type selected. In the following digram R1 is connected to R2 using frame-relay and we are going to explore different configuration options. Task1: Configure frame-relay PVC between R1 and R2 using physical Physical interfaces are considered to be multipoint interfaces by default, which means that address resolution is essential in order for both ends to communicate. Address resolution can be accomplished manually using the frame-relay map command or dynamically using inverse ARP. Dynamic address …

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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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