Routing protocol basics, part 2

A look at the characteristics of the OSPF, IS-IS, and BGP routing protocols.

Managing routers requires a basic knowledge of routing protocols. The more understanding you have of basic protocols the easier it will be to troubleshoot your network's routing schemes later. This excerpt from Informit looks at the basic characteristics of some Interior and Exterior Gateway Protocols.


OSPF
Open Shortest Path First (OSPF) overcomes the disadvantages of RIP and is not proprietary in nature, but its openness supports only the IP routed protocol. The protocol is a link-state IGP based on the Dijkstra algorithm developed by the Internet Engineering Task Force (IETF) to support large heterogeneous networks. Lots of research was completed from 1987 until the current OSPFv2 specification in 1991. Link-state advertisements are sent to all, which causes an initial flood on the router; after that, however, OSPF is very efficient in operation. It uses three different databases (tables) for the neighbors, link states, and routes.

The following are OSPF characteristics:

  • Open protocol.
  • Good for small to large networks.
  • Not as easy to design and configure as other protocols.
  • Supports only the IP Layer 3 routed protocol stack.
  • Link-state routing protocol (doesn't just send to neighbors like distance vector).
  • IGP.
  • Multicast link-state advertisement (LSA) updates over 224.0.0.5 and 224.0.0.6.
  • IP protocol number 89.
  • Administrative distance is 110.
  • Metric is a cumulative cost (inversely proportional to bandwidth).
  • Supports only equal-cost load sharing, but some implementations can take advantage of type of service (TOS) requests.
  • Requires a routing hierarchy in that every area must touch the backbone area (otherwise temporary fixes such as virtual links are used). Various router types, LSA types, area types, and states, depending on your design and Layer 2 topology.
  • Uses Dijkstra algorithm to select loop-free paths and give it fast convergence. This uses LSAs and is based on the Shortest Path First (SPF) algorithm, where the protocol got its name.
  • Supports VLSM and summarization (classless).
  • Supports manual summarization only; this is not automatic like EIGRP. Must be performed on an ABR (area range) or ASBR (summary address) only.
  • Route tagging for policy-based routing.

IS-IS
The ISO was working on Intermediate System-to-Intermediate System (IS-IS) about the same time the IAB was working on OSPF. The late 1980s and early 1990s, a time in our history when everyone thought the OSI suite would overtake TCP/IP, was when Integrated IS-IS was proposed. Although originally designed for OSI routing, IS-IS was developed by ISO to support CLNS/CLNP. Integrated IS-IS, which supports IP, was a later development. The purpose was to provide a single routing protocol that could route Connectionless Network Protocol (CLNS) and IP. IS-IS is in use by ISPs today. OSPF and IS-IS have many common features. The following are IS-IS characteristics:

  • Open protocol.
  • Good for medium to very large networks.
  • ISO link-state routing protocol similar to OSPF.
  • IGP.
  • IS-IS Layer 2 PDUs rather than IP packets.
  • Uses Layer 2 multicast.
  • Administrative distance is 115.
  • Very limited metric dynamic range (0–63).
  • Equal-cost load sharing.
  • Two-level hierarchical topology.
  • Uses Dijkstra/SPF algorithm.
  • Supports VLSM and summarization.
  • Manual summarization.
  • Route tagging for policy-based routing.

BGP
Border Gateway Protocol (BGP) is an EGP that pretty much replaces the legacy EGP protocol itself. BGP performs routing between autonomous systems and is the standard routing protocol on the Internet. This is referred to as External BGP (EBGP), whereas when BGP is used to route within an AS it is referred to as Interior (IBGP). BGP is not a routing protocol for the fainthearted. It requires all manual configurations for a very good reason; you are not only affecting you, but you are also affecting me. For troubleshooting BGP, if routes are not in the BGP table, there is no way they will be in the routing table. Always make sure your neighbors are talking to you. One of the most useful commands in troubleshooting BGP is show ip bgp summary.

The following are BGP characteristics:

  • Open protocol.
  • Good for very large internetworks.
  • Not as easy to design and configure as other protocols. Everything is manual, including neighbors (peers).
  • Advanced distance vector or path vector routing protocol.
  • EGP.
  • TCP port 179.
  • Internal administrative distance is 200; external is 20.
  • Metrics include many attributes such as MED, Origin, AS-Path, Next-hop, and Community.
  • Does not demand a particular routing hierarchy; roll your own.
  • Automatic and manual summarization features.
  • Route tagging for policy-based routing.

Read more about network troubleshooting at Informit.


This was first published in May 2005

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