IP Routing

The Important

  • Significant built-in automation
  • Basis of the Internet and Enterprise Networking
  • Supports a large scope of applications/services
  • Statistical multiplexing

IP Routing (Simplified)

Networks are defined by two fundamental characteristics

  • How they choose what path is taken (routing)
  • The forwarding treatment at every node (per hop behaviors – PHB)

Both can influence the performance of IP networks/customer experience.

Control Plane / Routing Protocols

  • Routers exchange information about networks connected to them/they know about.
  • Routes from other routers, plus own/local & configured routes, are stored in a routing table.
Figure 1. Simplified routing protocol network reachability exchange

More information on IP Routing Protocols:

IP Forwarding

  • Information is extracted from routing table(s) to the forwarding table
  • Forwarding table has information on how to forward different destination addresses
  • Router moves a received packet, one router closer to the destination
Figure 2. Simplified illustration of forwarding in an IP routed network

For more on forwarding:

IP routing strengths

  • Routing automatically understands path changes without human intervention.
  • Routing automatically understands new destinations without human intervention.
  • Routing supports a large scope of concurrent applications/services.
  • Statistical multiplexing combined with load balancing lead to better link capacity utilization.

IP routing weaknesses

  • Automated path learning and forwarding table creation adds complexity
  • Packet looping can occur temporarily as a consistent view of the network is reestablished following a topology change

Beyond Best Effort IP

While IP routing started as a single approach to hop-by-hop, connectionless, best effort packet switching, it is evolved to be a toolset of approaches to networking. Saying “how” IP routing works, is no longer a simple affair.

  • IP can forward all packets without discrimination
  • IP can forward based on priority
  • IP can forward based on bandwidth allocation
  • IP can forward along a path that has the least number of routers
  • IP can forward along a path that has the most bandwidth
  • IP can forward along a path that has the lowest latency
  • IP can spread the forwarding load over many different paths
  • IP can forward along a virtual topology that is a subset of the physical topology
  • IP can be partitioned into many virtual private networks
  • IP can transport IP, Ethernet, & TDM
  • IP supports voice, video, email, websites, collaboration, and more
 IPv4IPv6IP/MPLSSR MPLSSRv6
Forwarding headerIPv4 headerIPv6 headerMPLS labelMPLS labelIPv6 header
Routing protocolsBGP4, OSPF, IS-ISBGP4, OSPF, IS-ISBGP4, OSPF, IS-ISBGP4, OSPF, IS-ISBGP4, OSPF, IS-IS
QoS markingDiffservDiffservDiffservDiffservDiffserv
Resource reservationRSVPRSVPRSVPTBDRSVP
Load balancingECMPECMPECMP, TEECMP, TEECMP, TE
Label mapping protocolsN/AN/ALDP,
RSVP-TE
OSPF, IS-ISOPSF-ISIS
Traffic steeringWeightsWeightsWeights, TEWeights TEWeights, TE
Table 1. Some of the Different flavors of IP routing
  • BGP: Border Gateway protocol
  • DIFFSERV: Differentiated Services
  • ECMP: Equal cost multipath, an approach to load balancing traffic over two or more routes.
  • IP: Internet Protocol
  • IPv4: IP Version 4
  • IPv6: IP Version 6
  • IS-IS: Intermediate System to Intermediate System
  • MPLS: Multprotocol Label Switching
  • OSPF: Open Shortest Path First
  • SR MPLS: Segment routing for MPLS
  • SRv6: Segment routing for IPv6
  • TE: Traffic Engineering

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