Daily Archives: April 9, 2012

Implementing EIGRP

Hybrid Routing Protocol

  • Rapid convergence (Diffusing Update Algorithm) DUAL
  • Stores all available backup routes
  • No periodic updates, sends partial updates when path or metric changes
  • DUAL sends path updates about a link rather than about the entire route table
  • Multi protocol support (appletalk, ipv4, ipv6, IPX)
  • Classless
  • Uses multicast and unicast
  • Support unequal metric load-balancing
  • Supports summarisation anywhere within the network rather than at major network boundaries only

Has three tables: Neighbour Table, Topology Table and Routing Table

Topology Table

Shows the route entries for each destination learned.  Best routes are chosen and put into the routing table.

Neighbour Table

Shows the adjacent eigrp routers

Successor Route = Best Route

Feasible Successor = Backup Route

Advertised distance – metric for an eigrp neighbour to reach a network

Feasible distance – advertised distance for a network learned from a neighbour plus the metric to reach that neighbour.

The router compares all the feasible distances to reach a network and selects the lowest feasible router and puts it into the route table.

Topology databse contains all the routes that are known to each eigrp neighbour.

If a route becomes invalid DUAL checks for a feasible successor to the destination.  If none is found then a re computation must occur

EIGRP requires an AS, similar to the process ID for OSPF.

EIGRP automatically summarises  on the classful boundary.

EIGRP Load Balancing

EIGRP uses a metric to compute the costs to a given network, by default EIGRP uses two criteria to determine a metric:

  • Bandwidth
  • Delay

You can use the following but they are not recommended because they usually result in frequent recalculation of the topology table:

  • Reliability
  • Load

For IP, IOS load balances across 4 equal-cost paths by default.  I can be configured for up to 16 equal-cost paths.

Setting maximum-path to 1 disables load balancing.

Unequal cost load balancing is controlled with the variance command.  Values from 1 to 128.

Eg: Variance will determine a feasible successor if the metric is within the variance.

Variance  = 2 best path metric = 20 Backup Route = 10, then 10×2 = 20, therefor there is equal cost on both links.

EIGRP Authentication

MD5 Authentication

Each key has an ID, the combination of the key ID and the interface associated with the message identifies the MD5 key in use.

Manage Keys Using Key chains

Troubleshooting EIGRP

  • Troubleshoot Neighbour Relationships
  • Troubleshoot Route Table Issues
  • Troubleshoot Authentication

Troubleshoot EIGRP Neighbour Relationships

  • All Interfaces up/up?
  • Getting, not a common subnet error?
  • All EIGRP neighbors configured?
  • Does Hello info match neighbour?

Both router and neighbour must share a directly connected subnet.  EIGRP Hello packets must patch the AS and K values to neighbor up.  Debug EIGRP packets to troubleshoot hello information.

Troubleshooting EIGRP Routing Tables

  • Advertising all Required Networks
  • Route Filters blocking Networks?
  • Duplicate EIGRP Router ID’s?
  • Is there a discontigious network?

Route table Key:

D = intra-AS Route, D EX = External AS Route

Route ID is formed from the highest IP address assigned to a loopback interface, if no loopback interface, the active interface with the highest IP is used.

Troubleshoot EIGRP Authentication

Use debug eigrp packets to verify authentication information.


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Single Area OSPF

  • A link is an interface on a router
  • Router sends a Link state advertisement (LSA) packet every 30 minutes and immediately when a router changes state.
  • A link state database (topology database) provides an overall picture of the network.
  • AS – Autonomous System
  • Area – Grouping of contiguous networks
  • All areas communicate through the backbone area (area 0)
  • Neighbor OSPF routers must recognise each other on the network before they can share information
  • The Hello protocol  establishes and maintains neighbor relationships

Hello Packets Contain

Router ID: 32 bit unique number, the highest IP address on an active interface by default eg: is higher than

Hello and Dead Intervals: The frequency in seconds that the router sends hello packets, dead timer is the time a router waits before declaring a router dead the default is 4 times the hello interval and the hello interval is usually 10 seconds on a multiaccess network.

Neighbors: Communications is established when a router recognises itself in a hello packet from a neighbor router.

Area ID: Two routers must share a common segment and their interfaces must belong to the same area on that segment.

Router Priority: 8 bit number indicating a priority, used to select a designated router (DR) and a backup router (BDR).

Authentication Password: If auth is enabled 2 routers must exchange the same password.

Stub Area Flag: Router must agree on stub area flag (for stub areas)

OSPF Cost = Reference Bandwidth/Interface Bandwidth (in bps)

SPF Algorithm

Each router is the root of a tree and calculates the shortest path to each node, based on the cost to reach each destination.  LSA’s are flooded throughout an area to build the topology database on each router.

The cost or metric of an interface is an indication of the overhead required to send packets across an interface.

There is more overhead/cost/time delays sending over a T1 line than a 10Mpbs link.

Loopback Interface

  • Router ID uniquely identifies each router in the OSPF network
  • You can override the default IP being advertised by configuring an IP address on a loopback interface
  • OSPF is more reliable if a loopback interface is configured because the interface is always active and cannot be in a down state like a real interface
  • Advertised loopback address can be used for management an unadvertised loopback interface cannot

Troubleshooting OSPF

  • Troubleshoot Neighbour Adjacencies
  • Troubleshoot Routing Table Issues
  • Troubleshoot Authentication

Troubleshooting Neighbour Adjacencies

  • Are all interfaces up?
  • Do local and neighbor MTU sizes match?
  • Are all OSPF neighbors configured?
  • Does all Hello packet info match the neighbour?

A healthy neighbour state is FULL

The four items in a hello packet that must match are:

  1. Area ID
  2. Hello and Dead Intervals
  3. Authentication Password
  4. Stub Area Flag

Troubleshooting OSPF Routing Tables

  • Are all OSPF networks advertised?
  • Do route filters block OSPF networks?

Route Table Codes

O – OSPF Intra area route, within same area

OIA – OSPF Inter Area, from another router

OE1 or E2 – External OSPF route from another AS

Debugging IP OSPF Adjacencies will show any mismatched authentication types:

  • Null = Type 0
  • Simple Pass – Type 1
  • MD5 – Type 2
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Link State and Advanced Distance Vector

  • Link State Advertisements (LSA’s)
  • Topology Database
  • SPF Algorithm
  • SPF Tree
  • Routing Table (Paths + Ports)

Autonomous System – Collection of networks under a common administratio that share a common routing strategy.

Area – A logical subdivision of an AS.

All areas communicate through the backbone.

Link state routers exchange link state packets with other routers in the network.

Benefits of Link State Protocol

  • Uses metrics to choose the routing path
  • Uses triggered, flooded updates
  • Each router has a full picture of the network, hard for routing loops to occur
  • Link state packets are sequenced AND aged
  • Can be designed for smaller link state databases
  • Easier troubleshooting because each router “knows” the whole network topology
  • Link state will usually scale to a larger network size, pending the design

Limitations of Link State Protocol

  • Requires 3 databases AND a routing table
  1. Topology Database
  2. Adjacency Database
  3. Forwarding Database
  • Dijkstra’s algorithm is CPU intensive for large or complex networks
  • Hierarchical design is required to avoid excessive CPU or memory usage
  • For Complex network designs the protocol may need to be tuned
  • Initial discovery can flood LSP’s throughout the network, this is temporary but can be noticeable
  • Powerful routers are required because of the CPU and memory requirements

Advanced Distance Vector

  • Uses distance vector with more accurate metrics
  • Uses topology changes to trigger routing database updates
  • Converges more rapidly than distance vector
  • Emphasizes economic use of resources, bandwidth and processor overhead

VLSM– Variable Length Subnet Mask

  • Allows hierarchical levels within an address space
  • When sumarising start with the largest subnet (the one with the most hosts).  This allows most efficient use of addressing possible.

Router Summarisation – One subnet which contains all subnets reachable from that router

Route sumarisation is also known as route aggregation or supernetting.

Classful routing protocol’s (RIPv1, IGRP) can only summarise on the classful boundary /8, /16, /24

Classless routing protocols (OSPF, EIGRP, IS-IS, RIPv2) can summarise on VLSM.

Route summarisation allows smaller routing tables and the ability to isolate network changes.

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