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Routing Facilities



Configuring NAT

    We will use the ip nat pool name start-ip end-ip {netmask netmask | prefix-length prefix-length} command to define a pool of IP addresses for NAT.

    The ip nat inside source {list {access-list-number | access-list-name} | route-map name} {interface type number | pool name} [overload] command. If static translation is preferred, the command is ip nat inside source {static {local-ip global-ip}.

  • list access-list-number - Number of a standard IP access list. Packets with source addresses that pass the access list are dynamically translated using global addresses from the named pool.
  • list access-list-name - Name of a standard IP access list. Packets with source addresses that pass the access list are dynamically translated using global addresses from the named pool.
  • route-map name - Specifies the named route map.
  • interface type number - Specifies the interface type and number for the global address.pool name Name of the pool from which global IP addresses are allocated dynamically.
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Implementing a BGP Solution for ISP Connectivity

BGP Characteristics

    BGP Is Carried Inside TCP Segments, Which Are Inside IP Packets.

    It uses TCP to handle the acknowledgment function. TCP uses a dynamic window, which allows for up to 65,576 bytes to be outstanding before it stops and waits for an acknowledgment. For example, if 1000-byte packets are being sent and the maximum window size is being used, BGP would have to stop and wait for an acknowledgment only when 65 packets had not been acknowledged.

    TCP is designed to use a sliding window, where the receiver sends an acknowledgment before the number of octets specified by the window have been received (such at the halfway point of the sending window). This method allows any TCP application, such as BGP, to continue streaming packets without having to stop and wait, as OSPF or EIGRP would require.


    The BGP synchronization rule states that a BGP router should not use, or advertise to an external neighbor, a route learned by IBGP, unless that route is local or is learned from the IGP. In other words, BGP and the IGP must be synchronized before networks learned from an ....

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Manipulating Routing Updates


Multiple IP Routing Protocols

    The key issues that arise when using redistribution are as follows:
  • Routing feedback (loops) - Depending on how you employ redistribution - for example, if more than one boundary router is performing route redistribution - routers might send routing information received from one autonomous system back into that same autonomous system. This feedback is similar to the routing loop problem that occurs with distance vector protocols.
  • Incompatible routing information - Because each routing protocol uses different metrics to determine the best path and because the metric information about a route cannot be translated exactly into a different  protocol, path selection using the redistributed route information might not be optimal.
  • Inconsistent convergence times - Different routing protocols converge at different rates. For example, RIP  converges more slowly than EIGRP, so if a link goes down, the EIGRP network learns about it before the RIP network.

    A metric of infinity tells the router that the route is unreachable and, therefore, should not be advertised. Therefore, when redistributing routes into RIP, IGRP, and EIGRP, you must specify a seed metric; otherwise, the redistributed routes will not be advertised.

  • RIP - 0, which is interpreted as ....
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Link-State Routing Protocol

  • Respond quickly to network changes.
  • Send triggered updates when a network change occurs.
  • Send periodic updates, known as link-state refresh, at long time intervals, such as every 30 minutes.

    When a link changes state, the device that detected the change creates a link-state advertisement (LSA).

    Link-state database (LSDB) is used to calculate the best paths through the network.

    Each router independently calculates its best paths to all destinations in the network, using Dijkstra’s (SPF) algorithm. For all the routers in the network to make consistent routing decisions, each link-state router must keep a record of the following information:

  • Its immediate neighbor routers - If the router loses contact with a neighbor router, within a few seconds it invalidates all paths through that router and recalculates its paths through the network. For OSPF, adjacency information about neighbors is stored in the OSPF neighbor table, also known as an adjacency database.
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