ccna 2 week 7 distance vector routing. copyright © 2005 university of bolton topics distance vector...
TRANSCRIPT
CCNA 2 Week 7
Distance Vector Routing
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Topics
Distance Vector Protocol Issues
RIP
IGRP
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Routing Updates
Routing tables are learned dynamically from neighbours
Updates occur periodically and when topology changes
Each router sends its entire routing table to its neighbours
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Routing Loops
Net1 Direct FA0/0Net1 3hop via Rtr B Net1 1Hop via Rtr E
Net1 2Hop FA0/0
Net1 2Hop via Rtr A
Net1 unreachable
X
Net1 unreachable
Net1 unreachable
Net1 unreachable
Net1 4hop via Rtr C
Net1 5hop via Rtr D
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Maximum Hop Count
In a routing loop, traffic continuously circulates despite target network being down
Routers count to infinity while packets loop
DV protocols define infinity as specific maximum number to prevent endless looping
If max is 15, networks become unreachable once count is exceeded.
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Split Horizon Rule
Split horizon is used to avoid loop situation.
If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A.
Split horizon reduces incorrect routing information and routing overhead.
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Route Poisoning
If a network goes down, router can set a distance vector one greater than maximum to poison route
Once poisoned update propagates other routers are unaffected by incorrect updates
Router C sends poison reverse back to E so that it can't learn an incorrect route either
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Triggered Updates
Protocols like RIP send updates periodically every 30 seconds
Triggered updates are sent immediately when the routing table changes, with neighbours also forwarding the changes immediately
When used with route poisoning, triggered updates help to ensure unreachable networks are correctly propagated
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Holddown Timers
When a router receives an update marking a route as down the router marks the route as inaccessible and starts a holddown timer
While timer running:– An update from the same neighbour saying route is back then
route restored, timer cancelled– An update from a different neighbour with a better metric then the
router marks the network as accessible and removes the timer. – If an update is received from a different router with a higher
metric before the holddown timer expires, the update is ignored
RIP Protocol
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
RIP
Widely implemented standard
Uses a maximum hop count (15) to prevent routing loops
Implements features such as holddown timers and split horizon to prevent loops
RIP v2 supports extra features
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Basic Setup
Updates sent every 30 seconds unless triggered
Received hop count increased by 1
Routers use best path (lowest hop count)
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Classless routing
Sometimes a router receives packets destined for an unknown subnet of a network that has directly connected subnets – won't use default
Rtr(glaboal)# ip classless forwards packets to best supernet route.
A supernet route covers a range of subnets in one entry.
e.g. 10.10.0.0 /16, is a supernet route for 10.10.10.0 /24
The ip classless command is enabled by default since IOS 11.3. To disable use the no ip classless
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Further configuration
Router(config-if)# no ip split-horizon
Router(config-router)#timers basicupdate invalid holddown flush [sleeptime ]
RIP timer defaults 30s, 180s, 180s, 240s [N/A]
Holddown should be just greater than max time to propagate updates around network
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Further configuration
Turn off broadcasts on unwanted interfaces– BHM(config-router)# passive-interface FA0/0
On networks that don't support broadcast– BHM(config-router)# neighbor ip address
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Verify RIP
show ip protocols– shows how RIP is configured– details of interfaces– networks being advertised
show ip route– contents of routing table– direct and learned routes
Additional information:– show interfaceinterface – show ip interfaceinterface – show running-config
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Troubleshooting
debug ip rip– displays routing updates as received
Other debug commands– show ip rip database – show ip protocols {summary} – show ip route – debug ip rip {events} – show ip interface brief
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Preventing Updates
Route filtering regulates information advertised or received into routing table
Only apply to Distance Vector protocols
Router(config-router)# passive interface s0/0– Prevents route table being sent out via interface– Will still accept incoming routing tables
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Load Balancing
RIP will load balance up to 6 equal cost routes
Cost refers to the routing metric– Hop count for RIP– Takes no account of link speed
Round-robin balancing 1-2-3-4-1-2-3-4
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Load Balancing in detail
Where several protocols in use, router adds route with lowest administrative distance in table
– Effectively a priority list for routing protocols
Two routes with the same AD – lowest metric used
Same AD and metric – load balancing occurs
Limits vary by protocol, can set using:Router(config-router)# maximum-paths [number ]
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Static routes
Administrator can configure routes manually if none learned – often used for gateway route
Allow static routes to be overridden by learned routes by setting a high AD
Static routes pointing out an interface are redistributed via RIP
Static routes not associated with a network command only sent if redistribute static command used
IGRP
Interior Gateway Routing Protocol
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Routing metric
Uses composite metric– Bandwidth – The lowest bandwidth value in the path – Delay – The cumulative interface delay along the path – Reliability – The reliability on the link toward the destination as
determined by the exchange of keepalives – Load – The load on a link toward the destination based on bps
show ip route displays five weightings K1-K5
By default bandwidth and delay are used
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
IGRP routes
InteriorBetween subnets of a route attached to an interface
If not subnetted, IGRP does not advertise
SystemOther networks within Autonomous System
Derived from direct connections and other routers
ExteriorBetween Autonomous Systems
Gateway chosen from list of exterior routes in IGRP routing table
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Avoiding Loops
IGRP supports– Split horizon– Holddown timers– Poison Reverse Updates
Increasing metric by factor of 1.1 triggers poison reverse update to place route in holddown
Timers as before include Update, Holddown, Invalid and Flush – default update time is 90s
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Configure IGRP
BHM(config)# router igrp 101– 101 is the AS number and does vary!
BHM(config-router)# network 192.233.99.0
Cancel either command using the no prefix
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Debug IGRP
show ip route– IGRP routes prefixed with 'I'
Useful commands– show interfaceinterface – show running-config – show running-config interfaceinterface – show running-config | begin interfaceinterface – show running-config | begin igrp – show ip protocols
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton
Debug commands
show ip protocols
show ip route
debug ip igrp events
debug ip igrp transactions
ping
traceroute
Copyright © 2005 University of BoltonCopyright © 2005 University of Bolton