IP Forwarding

IP Forwarding:

  • before creating a new frame, the router updates the IP header TTL filed requiring a recomputation of the IP header checksum.
  • Frame Relay Inverse Arp:
    • With InArp, routers already know the DLCI and need to learn the corresponding IP address.
    • unlike LAN, packet does not need to arrive at the router to trigger the InARP protocol; instead an LMI status message triggers InARP.
    • After receiving an LMI PVC Up message, each router announces its own IP address over the VC. If you disable LMI message, InARP does not work cuz nothing triggers it.
    • when “dynamic” keyword is uses in “show frame-relay map”, it means learnt by InARP (the ip address). For P2P interfaces, entry does not use InARP and you would not see any L3 address or “dynamic” keyword in the output.
    • point to point interfaces “ignore” the received InARP information.
    • “broadcast” keyword says to send copies of broadcasts over the VC.
    • on multipoint or physical spoke:“frame-relay map ip <ipaddresswannareach> <dlci_of_hub> broadcast
    • formula for Full Mesh links: N (N – 1) / 2, with N equaling the number of host
    • Disabling InARP:
      • no frame-relay inverse-arp (can be disabled for all VCs, or all VCs for a particular L3 protocol and even for a particular L3 protocol per DLCI.). Tells router to ignore received InARP messages. e.g “no frame-re inverse-arp ip 400
  • Classless Routing: when default route exists and no specific match is made, the default route is used
  • Classful Routing: when default route exists AND the class A,B or C network for the IP address does not exist, default route is used. Otherwise router discards the packet.
  • MLS:
    • When do you not need a VLAN interface? when MLS switch connects to a router using a cable from a switch interface to a routers LAN interface, and the only two devices are the router and the one physical port on the MLS switch, you can use “routed switch port”.
    • VLAN interface: vlan must exist. Otherwise shows as “up and down”. VLAN interface uses L2 logic and L2 MAC address table. “ip routing” global command must be configured.
  • Policy Routing:
    • allows policy decisions other than destination IP address.
    • routemaps match on referring ACL (numbered or named IP ACL using the “match ip address” command) or based on packet length ( match length) command. Can set TOS and Precedence.
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