IS-IS explained Part2 (Routing Levels)

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: Level1 Router: Level1 router is non-backbone router; builds only adjacency with neighbors in the same area. Level1 routers know only about intra-area routes or topology information and all they know about other areas (inter-area) is a default route pointing to the closest L1/L2 routers (similar …

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What is Integrated ISIS?

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 working group of the Internet Engineering Task Force (IETF) developed the specification for Integrated IS-IS (RFC 1195).

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IS-IS explained (Part1)

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 are link-state routing protocols. Both are using the SPF algorithm to calculate the best paths. Both are classless routing protocols. Both build adjacencies with their direct connected neighbors. Both support the concept of areas to allow hierarchical network topology. Both elect a DR on for …

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OSPF Virtual-links vs GRE tunnels

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 connection to the backbone to temporarily solve the problem. Virtual-links and GRE tunnels are used to provide this logical connection. Below is a brief comparison between both methods: Virtual-link: It is considered part of (Area 0) by default, without any additional configuration. It dose not …

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Connected Routes Advertisment

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 192.168.12.0 255.255.255.0 Serial1/1 ip route 192.168.21.0 255.255.255.0 F0/0 How are these connected routes advertised ? We have multiple options for advertising a connected route, listed as follows: A network command configured that covers any connected network whether defined under the interface or in a static …

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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 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 !-- R1's Loopback0 network 10.10.10.1 0.0.0.0 area 0 !-- R1's Loopback1 !-- R2 OSPF configuration router ospf 1 router-id 2.2.2.2 log-adjacency-changes network 172.16.12.2 0.0.0.0 area 0 network 172.16.102.1 0.0.0.0 area 0 !-- R2's Loopback0 !-- Routing tables R1(config-router)#do sh ip route ospf 172.16.0.0/16 is variably …

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