Nokia 4A0-112 - Nokia IS-IS Routing Protocol

Theadjacency databasecontains information about the state of the router’s adjacencies (i.e., the routers that it is directly connected to and has established a neighbor relationship with). It does not store information about all links in the domain; that information is stored in thelink-state database.

Therouting table manager (RTM)is responsible for selecting the best route when multiple routing protocols provide a route for the same destination prefix. It makes the decision on which route to add to therouting tablebased on the administrative distance, metric, and other criteria.

BFD (Bidirectional Forwarding Detection)is used to detect link failures quickly and helps improve convergence time in link-state routing protocols. The BFD session between routers R1 and R3 allows them to detect the failure of the link between them more quickly than the regular routing protocol hello timers. Thetransmit interval,receive interval, andmultiplier settingsdetermine how fast BFD detects a failure and triggers the routing protocol to converge, which directly impacts the convergence time.

The process of establishing an IS-IS adjacency on a broadcast link involves several steps, and the transition from theInitializingstate to theUPstate occurs when both routers exchange sufficient information to confirm they have a consistent view of the link-state database.

TheCSNP (Complete Sequence Number PDU)packet is used to advertise the list of LSPs in the router's link-state database. When a router receives the first CSNP from theDIS (Designated Intermediate System)that lists the LSPs in the DIS's database, it indicates that the router has enough information to establish the adjacency and synchronize its database.

From the route tables ofR3andR4, we can see thatremote routesare present, withR3andR4both having routes referencing routers in different areas (Area 49.01 and 49.02). These remote routes are characteristic ofroute leaking, which is the process of sharing routes between different IS-IS areas.

R3has routes forR1,R2, andR4, which are in Area 49.01, suggesting thatR1 and R2have advertised their routes toR3, possibly due toroute leaking.

R4has similar routes forR2andR3, indicating thatR2might have advertised its routes toR4.

This sharing of routes between areas is indicative ofroute leakingbeing configured on bothR1andR2, allowing these routes to be shared across the areas.

ARP for IPv4 and Neighbor Discovery Protocol (NDP) for IPv6 are both used to map a device's IP address to its MAC address. This is essential for communication within a local network.

ARP is used in IPv4 networks, and NDP performs a similar function in IPv6 networks, ensuring devices can communicate effectively on the same network segment.

Dividing a domain into areas typically does not reduce the number of routers needed to handle traffic. It is more focused on improving scalability, reducing the size of routing tables, and limiting the scope of SPF calculations, but it doesn't inherently reduce the number of routers required.

When a router receives a link-state advertisement (LSA), it doesnotupdate the age field before forwarding it. Theage fieldin an LSA is typically updated by the originating router or during the process of forwarding the LSA within the network. Routers do not modify the age field upon receiving and forwarding an LSA.

From the output, we can see that theusagecolumn indicates whether a router is operating as an L1 or L2 router:

RoutersR1andR2have a usage ofL2, meaning they are L2 routers.

RoutersR3andR4have a usage ofL1, meaning they are L1 routers.

TheL1/L2designations refer to whether the routers participate in both Level 1 and Level 2 of IS-IS:

L1 routersonly communicate within their own area.

L2 routerscommunicate between areas.

In this case,R1andR2are L2 routers, andR3andR4are L1 routers.