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Link state protocols have introduced the concept of multiple routing areas withing the same routing domain. Link state protocols depend on the fact that all routers must have an identical link state database and then each router will start calculating its very own routing table from this information. However, this rule sometimes introduce scalability limitations [...]

IS-IS is the most selected protocol by service providers and large carriers all over the world; this is what makes the understanding  of this protocol important. We have been discussing fast convergence and high availability in the latest few posts and this post is not different, I am just going to give you an overview [...]

Shortest path first (SPF) is the algorithm used by IS-IS and OSPF routing protocols to calculate the topological information from the received link state updates. You can find more information about SPF calculation follow the link Dijkstra’s algorithm. Partial SPF is an efficient shortcut used by the routers to speed up the process of route [...]

For a network to converge, all routers in the network must collect and agree on all the topology information from each other. This information must be consistent, reflecting the current state of the network and free of routing loops or any other kinds of corruption. What is meant by fast convergence? when we use the [...]

The concept of Areas was introduced in OSPF or link state protocols in general and routers had different roles based on their location in the topology. We have backbone routers, Area border routers, ASBRs and so on. IS-IS is implementing the same concept of areas named as routing domains and introduced the following routers roles: [...]

The IS-IS Routing Protocol may be used as an IGP to support IP as well as OSI. This allows a single routing protocol to be used to support pure IP environments, pure OSI environments, and dual environments. Integrated IS-IS is deployed extensively in an IP-only environment in the top-tier Internet service provider (ISP) networks. The [...]

IS-IS was developed for OSI routing then extended to support IP by Integrated IS-IS. IS-IS is an IGP used for routing within a single administrative domain. IS-IS is a Link state routing protocol and uses the SPF algorithm for computing best paths just like OSPF. Below is a comparison between IS-IS and OSPF: Similarities: Both [...]

Everyone who works in networking knows that every area in the OSPF domain must be connected to the backbone area (Area0). The reason behind this constrain is explained here. However it may be difficult for some reason to physically connect an area to the backbone; in such cases you will have to provide a logical [...]

What is considered as a connected route in the routing table ? An interface is configured with an IP address and mask,the configured subnet is installed as connected in the routing table. A static route is configured with only an outgoing interface, and not an IP next-hop. !– These routes are considered connected ip route [...]

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 1.1.1.1 log-adjacency-changes network 172.16.12.1 0.0.0.0 area 0 network 172.16.101.1 0.0.0.0 area 0 [...]