marcosystems.dev

Tag: cisco

  • Road to CCNP: Day 107 (OSPFv2, OSPFv3)

    3.2.b Configure simple OSPFv2/v3 environments, including multiple normal areas, summarization, and filtering (neighbor adjacency, point-to-point, and broadcast network types, and passive-interface)

    OSPF sends neighbor routers a Link-state advertisement (LSA). Inside the LSA is the link state and metric. Received LSAs are stored in a local database called the LSDB, and a router that receives an LSA floods it out through other links, just as it was received. This process continues until all routers have the same LSDB and picture of the entire network. Then, each router runs Dijkstra’s algorithm (shortest path first/SPF) with itself as the top of the tree.

    This gives the illusion to the router that there is no redundancy. But if a link goes down, the SPF will be recalculated, with the redundancy now in effect.

    Scalability = multiple areas are allowed.

    Should not exceed 50 routers per area. Must have area 0, or a backbone. All non-backbone areas must have an area border router (ABR) with the backbone. Non-backbone ABRs advertise/inject routes into the backbone, which backbone routers flood amongst themselves until they have the same LSDB, and then backbone ABRs then inject non-backbone routes into other non-backbone areas.

    Don’t Interrupt 2 Engineers Exchanging Large Files

    The DR/BDR process distributes LSAs in the following manner, assuming that all OSPF routers (DR, BDR, and DROTHER

    1. As an OSPF router learns of a new route, it sends the updated LSA to the AllDRouters (224.0.0.6) address, which only the DR and BDR accept and process
    2. The DR sends a unicast acknowledgment to the router that sent the initial LSA update
    3. The DR floods the LSA to all the routers on the segment via the AllSPFRouters

    OSPFv3 configuration

    1. First enable IPv6 unicast routing as OSPFv3 messages communicate over IPv6 links
      1. Command ipv6 unicast-routing
      1. Router ospfv3 [process-id]
    2. Define the router ID.
      1. If the router is IPv6 only, then a router-id must be manually assigned. In addition, if the router has no interfaces with IPv4 addresses, a router-id cannot be automatically assigned
      1. Use a manually configured router-id if one exists.
      1. Otherwise, choose the highest IPv4 address on a loopback interface.
      1. If there is no loopback, choose the highest IPv4 address on an active non-loopback interface.
      1. If there are no IPv4 addresses at all, OSPFv3 cannot dynamically pick a RID and effectively has 0.0.0.0; adjacencies won’t form.
      1. Command router-id [router-id]
    3. Enable OSPFv3 on an interface
      1. Command ospfv3 [process-id] ipv6 area [area #]
    4. OSPFv3 does not use the network statement for initializing interfaces.

    Passive interfaces

    • Command passive-interface [interface-id]
    • Passive-interface default
      • Enable per interface with no passive-interface [interface]

    Show commands

    show ip ospfOSPF process, RID, areas, SPF info, reference bandwidth
    show ip ospf neighborNeighbor adjacencies and states
    show ip ospf neighbor detail Detailed neighbor information
    show ip ospf interface brief  Quick view of OSPF-enabled interfaces
    show ip ospf interface  Full interface OSPF parameters
    show ip ospf interface g0/0          OSPF information for one interface
    show ip ospf databaseContents of the LSDB
    show ip route ospf OSPF routes installed in the routing table
    show ip protocols    Routing protocol configuration/parameters

    Summarization to sum up multiple addresses in one routing table entry

    Area [area-id] range [prefix/prefix-length]

    Network types

    OSPFv3 supports the same network types as v2, Broadcast, point-to-point, loopback

    To configure per interface, ospfv3 network [point-to-point/broadcast]

    IPv4 support in OSPFv3

    1. Ensure the IPv4 interface has an IPv6 address because OSPFv3 communication occurs over IPv6 and the router needs a link-local address to talk to neighbors
    2. Enable OSPFv3 on the IPv4 interface with command ospfv3 [process-id] ipv4 area [area-id]

  • Road to CCNP: Day 106

    3.2     Layer 3

                                  3.2.a          Compare routing concepts of EIGRP and OSPF (advanced distance vector vs. link

    state, load balancing, path selection, path operations, metrics, and area types)

    EIGRP diffusing update algorithm (DUAL)

    • Successor route = the best EIGRP path to a destination
    • Successor = the next-first hop on the best path
    • Feasible distance = The lowest EIGRP distance metric of the best route (“it is feasible that the best route is x”)
    • Reported distance = the neighbor’s own metric to a destination
    • Feasibility condition = Neighbor RD < current FD
      • Passes → path is guaranteed loop-free and can be a feasible successor.
      • Fails → EIGRP cannot guarantee from the FC alone that it is loop-free. It does not mean a loop definitely exists.
    • Feasible successor = Route that satisfies the feasibility condition (guaranteed loop-free) and therefore can be hot-swapped to in case the successor route goes down

    Topology table = contains all the network prefixes advertises in an AS

    • Network prefix
    • Neighbors that have that prefix
    • Metric from each neighbor
    • Values used to calculate the FD

    Neighbors advertise the entire routing table when forming an adjacency, and only advertise changes

    Message types

    • 1. Update = respond to requests, transmit routing and reachability info to neighbors
    • 2. Request = ask neighbors to get specific info
    • 3. Query = sent to search other paths during convergence
    • 4. Reply = sent in response to query
    • 5. Hello = discovery of EIGRP neighbors (absence of hello also detects unavailability)

    Metric Calculation

    • Bandwidth and delay
    • Interface load and reliability

    Load balancing

    Multiple paths to the same network prefix is ECMP = MULTI PATH

    Successor route and feasible successor can be installed at the same time and used to forward traffic, this is unequal cost load balancing

    Variance multiplier is a value

    Variance value = multiplier * feasible distance

    Any feasible successor whose FD is lower than this variance value can also be installed in the routing information base and be used to route traffic, up to a max amount of routes

    Traffic share count is ratio of traffic sent across each path

    Hello packets, heartbeat to neighbors, hello timer, interval of heartbeat, 5 second default, 60 on T1 interf.

    Hold time, time for EIGRP to hold out for for hello packets from neighbor

    Default hold time is 3 x hello timer, eg. 5 hello timer = 15 sec hold time, 60×3 = 180 sec on T1 interf or lower

    Receiving hello packet restarts hold time, when hold time reaches 0 the neighbor is deemed unreachable and notifies DUAL of topology change

    Convergence

    When an EIGRP node goes down, every link attached to that node goes down too. Anywhere where that node was part of the route (ie a successor or upstream router), path recomputation must occur

    Summarization

    When enabled, component routes are not advertised, and only the summary route is advertised.

    It is only advertised when a request for a component network is requested

    Configured on a per-interface basis