INE R&S ATC081 ~ ATC086 - Route Redistribution

081 - Route Redistribution Overview
082 - Route Redistribution Configuration & Verification, Connected Redistribution
083 - OSPF External Path Selection, TCL PING Scripting
084 - Routing Loops Overview, EIGRP Route Loop Prevention
085 - Metric Based Routing Loops, Route Tagging
086 - Administrative Distance Based Routing Loops, Debug IP Routing, IP Route Profile


Redistribution Lesson Objectives

• What routes are redistributed
• How connected redistribution works
• How IOS chooses routes to use
• Why routingloopscan occur
• How to identify routing loops
• How to prevent routing loops
Route Redistribution Overview

• Redistribution occurs from the routingtable not the routing database
• When redistributingprotocol X into Y, take…
– Routesin the routing table via protocol X
– Connected interfaces running protocol X
• Route advertisement rules
– RIP vs. EIGRP vs. OSPF vs. BGP

Connected Redistribution

• Implicitly occurs for connected links running the redistributed protocol
• Additional connected links can explicitly included or excluded
– redistribute connected [metric] [route-map]
– Overrides implicit redistribution
How IOS Chooses Paths

• Routing databasechooses one or more candidate paths
– EIGRP viaDUAL, OSPF via SPF, etc.
– Load balancingpossible via maximum-paths
• If multiple equal matches between protocols…
– Choose the lower Administrative Distance
• Install results in RIB and/or FIB
AdministrativeDistance Values

• 0 -Connected
• 1 -Static
• 5 -EIGRP Summary
• 20 -External BGP
• 90 -Internal EIGRP
• 110 -OSPF
• 115 -IS-IS
• 120 -RIP
• 160 -ODR
• 170 -External EIGRP
• 200 -Internal BGP
• 255 -Infinite
RIP Redistribution

• Doesn’t differentiate between internal and external routes
– Administrative distance of 120 for all routes
• No default seedmetric
– redistribute [protocol] metric [hops]
– default-metric [hops]
EIGRP Redistribution

• AD of 170 for externalEIGRP
– Helps to automatically prevent route feedback
• Uses Router-ID for loop prevention
• No default seed metric unless EIGRP to EIGRP
– redistribute [protocol] metric [bandwidth] [delay] [load] [reliability] [MTU]
– default-metric [bandwidth] [delay] [load] [reliability] [MTU]
OSPF Redistribution

• AD of 110 for all OSPF routes
• Uses Router-ID for flooding loop prevention
• Default seed metric 20 and metric-type E2/N2
• OSPF path selectionpreference
– E1 > E2 > N1 > N2
– E1 & N1 vs. E2 & N2 metrics
BGP Redistribution

• Uses ORIGIN code Incomplete (?)
• Normal EBGP and iBGP loop prevention
• IGPto BGP
– Denies OSPF external routes by default
• redistribute ospf [pid] match internal external
• BGP to IGP
– EBGP routes allowed, iBGProutes denied by default
• bgp redistribute-internal
• Legacy synchronization rule
• Can cause a routingloop
Why Routing Loops Occur

• Redistribution causes loss of end-to-end route calculation and loop prevention
– E.g. DUAL not compatible with SPF
• Looping cannot occur with single redistribution point
– Internal routing process prevents this
• Loops can occur if multiple redistributionpoints and wrong path is selected due to…
– Wrong path has lower metric than correct path
– Wrong path has lower AD than correct path
Identifying Routing Loops

• Visually trace the route advertisement path
– Ideally you should be able to predict 100% of loops before they occur
• CLI tools
– Connectivity testing with ICMP via TCL
– debug ip routing
– ip route profile
– traceroute
TCL Scripting

• Tool Command Language (TCL)
– Open source scripting language
– http://www.tcl.tk
• IOS supports TCL 8.3.4
– 99% of programming is outside our scope
– Useful in redistribution for automating PING scripts
TCL PING Script Example

R1# tclsh
R1(tcl)# for each X {
+>(tcl)# 1.2.3.4
+>(tcl)# 5.6.7.8
+>(tcl)# 9.10.11.12
+>(tcl)# } { ping $X }
TCL Considerations

• No good IOS documentation references available
– Syntax must be exact
– Cisco assumes you would reference www.tcl.tkdocs
• TCL parser is processed before IOS parser
– Potential problems with overlapping commands
– E.g. route-map “set” command
Preventing Route Feedback

• How to prevent loopingdepends on why loop occurred
• Metric based loops
– Fixed with ACL,prefix-list, or tag filtering and AD
• Administrative Distance based loops
– Generally fixed only with AD