Redistribution Topic Notes

Redistribution Overview

• Takes one protocol and propagates it into another
  • Original protocol information lost
  • Can be done from any routing protocol to any other routing protocol
• Redistribution is done on information in the RIB and not the routing protocol database
  • e.g. OSPF database or EIGRP Topology table
• Redistribution implicitly occurs for connected links running original protocol
  • Doesn’t apply for IPv6 interfaces

• Selecting routes for the Global RIB
  • If multiple equal prefixes between the same protocol
    • EIGRP chooses lowest AS number
    • OSPF chooses lowest Process ID number

RIP Redistribution

• Doesn’t differentiate between internal and external prefixes
• All prefixes have AD of 120
• No default seed metric of redistributing into RIP
  • Can be configured manually
    • Command:
      • (config-router)#default-metric <hops>
• Command:
  • (config-router)#redistribute <protocol> metric <hops>

EIGRP Redistribution

• Internal EIGRP has AD of 90
• External EIGRP has AD of 170
  • Helps prevent loops

• No default seed metric unless redistributing:
  • EIGRP to EIGRP
  • CONNECTED to EIGRP
    • Takes the metrics from the interface
  • Can be manually configured with a default seed metric
    • Command:
      • (config-router)#default-metric <bandwidth> <delay> <load> <reliability> <mtu>
    • Default seed metric can also be set on route-map when redistributing into EIGRP
• Command:
  • (config-router)#redistribute <protocol> metric <bandwidth> <delay> <load> <reliability> <mtu>

• EIGRP uses Router-ID for loop prevention
  • Router-ID included on internal routes as well in EIGRP version 5.0 and greater
  • Route will be dropped if own Router-ID is include in prefix update
• Determine EIGRP version
  • #show eigrp plugins

OSPF Redistribution

• Sets AD of 110 for all prefixes
  • Can be manually changed
    • Commands:
      • (config-router)#distance ospf intra-area <AD>
      • (config-router)#distance ospf inter-area <AD>
      • (config-router)#distance ospf external <AD>
• Uses Router-ID for loop prevention
  • Drops LSAs with own Router-ID
• Default seed metric is 20 and default metric-type is E2/N2
• Command:
  • (config-router)#redistribute <protocol> <process ID> subnets

BGP Redistribution

• IGP into BGP
  • Uses Origin Code Incomplete (?)*
  • Denies OSPF External routes by default
  • Can be configured manually
    • Command:
      • (config-router)redistribute ospf <process> match internal external
  • Command:
    • (config-router)#redistribute <protocol> <process ID>

• BGP to IGP
  • eBGP routes allowed redistribution
  • iBGP routes denied by default
    • Can be configured manually
      • Command:
        • (config-router)#bgp redistribute-internal
  • BGP routes automatically tagged with AS number

Redistribution Loops

• Can not occur on with a single redistribution point
• Can only occur with multiple redistribution points in topology
  • Wrong path selected due to
    • Lower Metric
    • Lower AD
•  Example 1 Redistribution:
  • Figure 1 is more likely to no have issues due to the loop prevention mechanisms built in to each routing protocol

Figure 1 – Mutual Redistribution at multiple gateways

• Example 2 Redistribution:
  • Figure 2 is more likely to have redistribution loops due to unequal distribution between the 2 protocols

Figure 2 – One-way Redistribution at multiple gateways

Control Plane loops

• Routes advertised and withdrawn constantly from routing table
• RIB in a constant state of flux
• Results in partial reachability
• More difficult to troubleshoot

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Data Plane loops

• Wrong path selected as best path
• RIB stable
• Loss of connectivity

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Troubleshooting Loops

• Can use the following tools to trace redistribution loops
  • TCL Ping Scripts
    • No good IOS documentation available
    • www.tcl.tk only source of general documentation
  • Routing table output
  • Traceroute
• TCP Script Ping example:
  • tclsh
    • foreach X {
    • 1.1.1.1
    • 2.2.2.2
    • 3.3.3.3
    • } {ping $X ]
  • tclquit

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Preventing Loops

• Higher AD routes more likely to be re-received from lower AD protocol

• IP Route Profile
  • Takes periodic snapshots of the routing table
  • Collects statistics of what is changing over time
  • Doesn't explain what has changed, only something has changed
  • Command:
    • (config)#ip route profile
  • Display the statistics of how much has changed over what period
    • Command:
      • #show ip route profile

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