Ip Routing On Cisco Ios Ios Xe Ios Xr

Ch 1

Ch 1

Ch 3 - How are router works

  • In IOS-XR the IP address repository manager (IPARM) enforces IP address uniqueness. If there is a conflict by default it will assign an IP to the lowest rack/slot/interface. This can be changed with the command ipv4 conflict-policy (static | highest-ip | longest-prefix).
  • In IOS ip_input process is used for process switching.
  • In IOS-XR NetIO process is used for process switching.
  • Each router hop by hop has to recompute the IP checksum, since the IP TTL is decremented.
  • IP Packet switching consist of Process Switching, Software CEF, Hardware CEF. Software CEF (line card CPU) is used to program the Hardware CEF (ASIC or NPU)

Ch 4 - Static Routing

  • What's the difference between Local and Connected routes? Connected are networks of directly connected interfaces for example, while local are host routes of these interfaces such as
  • Three type of Static routes:
    • Directly attached - static route only points to the next-hop interfaces. Best for p2p interfaces. If used on an Ethernet interface will have to ARP for each next-hop address, very bad for forwarding performance. Exp ip route serial1/0.
    • Recursive - Next hop is another IP address that has to be lookedup. The AD shows as 1 vs directly connected.
    • Fully specified

Chapter 5 - EIGRP

  • In IOS to advertise secondary connected networks into the topology table, they have to be redistributed. This is not the case in IOS-XR.
  • IOS-XR 5.1.0 supports multiple EIGRP instances.
  • On IOS-XR to check what routing protocols are operating, use the command show protocols ipv4 or show protocols ipv6. For more specific information on a protocol, use show protocols ospf or show protocols ipv4 bgp.
  • Router ID selection in IOS and IOS-XR is different.
    • IOS - highest IPv4 address of up lookback interface, then highest IPv4 address of any active up physical interfaces.
    • IOS-XR - lowest IPv4 address of up lookback interface, then lowest IPv4 address of any active up physical interfaces. Prevents the problem to configuring new interfaces with higher IP and changing the RID.


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