INE R&S ATC033 - IP Routing Overview, Switching Paths, Static Routing

IP Routing Overview

• Three main steps
  – Routing
    • Find the outgoing interface

  – Switching
    • Move the packet between interfaces

  – Encapsulation
    • Build the layer 2 header


The Routing Process

• Find the longest match
  – show ip route 1.2.3.4
    • 1.0.0.0/8
    • 1.2.0.0/16
    • 1.2.3.0/24
• Recurse to the outgoing interface
  – 1.2.3.4 via 5.6.7.8
  – 5.6.7.8 via 9.0.1.2
  – 9.0.1.2 via 3.4.5.6
  – 3.4.5.6 directly connected, FastEthernet0/0


Metric vs. Distance

• What if there are multiple longest matches?

• Same protocol
  – Choose lowest metric
• Different protocols
  – Choose lowest Administrative Distance


The Switching Process

• Moves the packet between interfaces
  – Process, Fast, CEF, etc.
  – Where load balancing occurs

• Configuration
  – ip cef
  – [no] ip route-cache
• Verification
  – show ip interface
  – show ip cef [address] [detail]
  – show ip cef exact-route [source] [destination]


The Encapsulation Process

• Build the layer 2 header

• Multipoint interfaces require layer 3 to layer 2 resolution
  – Ethernet
    • show arp
  – Frame Relay
    • show frame-relay map


 Routing to a Next-Hop

• Recurse to the interface

• If multipoint, resolve layer 2 address of next-hop

• Example
  – ip route 10.0.0.0 255.0.0.0 1.2.3.4
  – Find interface for 1.2.3.4
  – If multipoint, find layer 2 address of 1.2.3.4


Routing to a Multipoint Interface

• Recursion not required

• Resolve layer 2 address for final destination
  – Ethernet Proxy-ARP
  – NBMA Mappings
• Example
  – ip route 10.0.0.0 255.0.0.0 FastEthernet0/0
  – Find layer 2 address of 10.0.0.1


Routing to a Point-to-Point Interface

• Recursion not required

• Layer 2 resolution not required

• Example
  – ip route 10.0.0.0 255.0.0.0 Serial0/0.1


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Administrative Distance

000  Connected
001  Static
005  EIGRP Summary
020  EBGP
090  EIGRP
110  OSPF
120  RIP
170  EIGRP External
200  iBGP

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