How to: use IS-IS overload bit

Overload bit is special bit in the IS-IS LSP used to inform the network that the advertising router is not yet ready to forward transit traffic.  The overload bit was first intended for signaling overload or resource shortage on specific router for the rest of the network. You can use the command set-overload-bit intentionally on specific router to signal other routers not to use it as a transit hop in their SPF calculations. Typically this is done for a temporary situation like an overloaded router due to memory or processing shortage and released when the router recovers from the problematic …

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VRF Lite

The word VRF stands for Virtual Routing and Forwarding, this feature is used to create multiple instances of the routing table on the same routing device. VRFs are usally used in conjunction with MPLS VPN to separate the traffic of multiple MPLS VPN customers. VRF Lite feature is part of Cisco’s network virtualization portfolio. VRF Lite means VRF without  the need to run MPLS in the network.  VRF Lite allows the network administrator to create multiple routing instances on the same routing device within the enterprise. VRF Lite can be useful when you need to isolate traffic between two networks …

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BGP Route Reflector Basics

Everyone who ever studied BGP knows that BGP has strong rules to prevent routing and updates loops. In this post I will focus on iBGP loop prevention and specifically route reflectors as a result. The rule states: that any route received from an iBGP neighbor must not be advertised to any other iBGP neighbor. This loop prevention mechanism induces a requirement that all iBGP routers in the network should be connected in logical full mesh topology to allow for routing information propagation through out the network. However, full mesh connectivity has never been a scalable solution specially in large networks. Two mechanisms …

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distribute-list gateway with OSPF

The following example is showing how to use distribute-list with the gateway option for inbound route filtering  in OSPF. The diagram below shows R1 directly connected to R2 with OSPF configured between them. Initial Configuration: !-- R1 OSPF configuration router ospf 1 router-id log-adjacency-changes network area 0 network area 0 !-- R1's Loopback0 network area 0 !-- R1's Loopback1 !-- R2 OSPF configuration router ospf 1 router-id log-adjacency-changes network area 0 network area 0 !-- R2's Loopback0 !-- Routing tables R1(config-router)#do sh ip route ospf is variably …

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BGP Default-Originate insights

BGP has multiple ways of advertising a default route to its neighbors;  the neighbor default-originate command is one of these methods and used to advertise a default route to a specific neighbor. Below are some characteristics of the default route advertised by this way. Originator router: Default route does not need to be existing in the routing or BGP table before advertising. The default route will not be installed in the BGP table as local. The default does not appear in the output of the command show of the advertised routes. The route can not be filtered outbound by the …

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How-to: Avoid DNS lookup when mistyping a command

If domain lookup is enabled (default) the router treats each mistyped command as a hostname and tries to resolve its IP address by querying the DNS server. This behavior is undesirable in most cases because it wastes network administrators time waiting for routers trying to resolve mistyped commands. R2#xyz Translating "xyz"...domain server ( Translating "xyz"...domain server ( ( Translating "xyz"...domain server ( % Unknown command or computer name, or unable to find computer address This waste of time can be avoided with one of the following solutions: Solution1:Disabling domain lookup: This solutions is suitable to LAB environments when your routers …

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