Routing Within An Routing Within An Autonomous SystemAutonomous System

(RIP, OSPF)(RIP, OSPF)

Chapter 15Chapter 15

Static vs Dynamic Interior RoutesStatic vs Dynamic Interior Routes

Interior RoutersInterior Routers Two routers within an autonomous systemTwo routers within an autonomous system Are considered to be interior to one anotherAre considered to be interior to one another

How do they learn about their own networks?How do they learn about their own networks? Small, slowly changing systemsSmall, slowly changing systems

Establish and modify routes by handEstablish and modify routes by hand

Update table when new network added or deletedUpdate table when new network added or deleted

Figure…routing is trivialFigure…routing is trivial

Figure 15.1Figure 15.1

Disadvantages of manual systemDisadvantages of manual system Cannot accommodate rapid growthCannot accommodate rapid growth Cannot accommodate rapid changeCannot accommodate rapid change

Need automated methodsNeed automated methods Respond to change more easilyRespond to change more easily Improve reliabilityImprove reliability Better response to failuresBetter response to failures

Figure 15.2Figure 15.2

Multiple physical pathsMultiple physical paths Usually pick one to be primaryUsually pick one to be primary Router(s) fail along primary, must changeRouter(s) fail along primary, must change Manually: time-consuming & error proneManually: time-consuming & error prone Even in small internets need automated systemEven in small internets need automated system

For automationFor automation Interior routers must communicateInterior routers must communicate Exchange routing informationExchange routing information Once data established, advertiseOnce data established, advertise

One interior router advertises to other autonomous One interior router advertises to other autonomous systems via Exterior Gateway Protocolsystems via Exterior Gateway Protocol

No single interior protocol has emergedNo single interior protocol has emerged Varied topologies and technologiesVaried topologies and technologies Tradeoffs between simplicity & functionalityTradeoffs between simplicity & functionality

Easy to install & configure; less functionalityEasy to install & configure; less functionality Multiple protocols have become popularMultiple protocols have become popular

Small ASSmall AS Choose a single one; use exclusively internallyChoose a single one; use exclusively internally

Larger ASLarger AS Often choose a small setOften choose a small set

Interior Gateway ProtocolInterior Gateway Protocol Used as generic descriptionUsed as generic description Refers to any algorithm used by interior routersRefers to any algorithm used by interior routers Routers may run BGP to advertise reachabilityRouters may run BGP to advertise reachability

Need an IGP to obtain information within the ASNeed an IGP to obtain information within the AS

Figure 15.3Figure 15.3

Routing Information Protocol (RIP)Routing Information Protocol (RIP)

One of most widely used IGPsOne of most widely used IGPs Also know as route-dAlso know as route-d Came from Univ of CA – BerkeleyCame from Univ of CA – Berkeley

Developed for machines on their LANsDeveloped for machines on their LANs Relies on physical network broadcast to make Relies on physical network broadcast to make

routing exchanges quicklyrouting exchanges quickly Not designed to use on large WANsNot designed to use on large WANs

Versions of RIP adapted for WANs are soldVersions of RIP adapted for WANs are sold

Popularity not only due to technical meritsPopularity not only due to technical meritsWas distributed with popular 4BSD UNIX systemsWas distributed with popular 4BSD UNIX systems

Lots of TCP/IP sites used without even considering Lots of TCP/IP sites used without even considering technical aspectstechnical aspects

Once installed it became the basis for local routingOnce installed it became the basis for local routing RIP was built and adopted without a standardRIP was built and adopted without a standard

Most implementations derived from Berkeley codeMost implementations derived from Berkeley code

Interoperability was limitedInteroperability was limited Many undocumented details and subtletiesMany undocumented details and subtleties

New versions led to more problemsNew versions led to more problems

Standard appeared in June 1988Standard appeared in June 1988

RIP OperationRIP Operation Straightforward distance-vector routingStraightforward distance-vector routing Partitions participants into two categoriesPartitions participants into two categories

Active: Active: Advertise their routesAdvertise their routes Only routersOnly routers

Passive:Passive: Do not advertiseDo not advertise Host must use passive modeHost must use passive mode

Active RIP routers broadcast every 30 secondsActive RIP routers broadcast every 30 secondsSends routing update messageSends routing update message

Takes information from current routing databaseTakes information from current routing database Update is a set of pairsUpdate is a set of pairs

(IP address, integer distance to that network)(IP address, integer distance to that network)

Hop count is used as the distance metricHop count is used as the distance metric

One hop: directly connectedOne hop: directly connected

Two hops: reachable through one other routerTwo hops: reachable through one other router

Hops = number of networks datagram will encounterHops = number of networks datagram will encounter

Hop count for shortest path not always optimalHop count for shortest path not always optimal 3 Ethernets faster than 2 satellites3 Ethernets faster than 2 satellites Some RIP implementations allow assignment of artificially Some RIP implementations allow assignment of artificially

high hop counts when advertising slow networkshigh hop counts when advertising slow networks

Active & passive routers listen to broadcastsActive & passive routers listen to broadcastsAll update tables according to DV algorithmAll update tables according to DV algorithm

May take some time for advertisements to propagateMay take some time for advertisements to propagate Routers use Routers use hysteresishysteresis to improve performance to improve performance

Does not replace a route with an equal cost routeDoes not replace a route with an equal cost route

Prevents oscillation among equal pathsPrevents oscillation among equal paths Timers are used on all routesTimers are used on all routes

Solves problem of routes through crashed routersSolves problem of routes through crashed routers

Start timer when install route in tableStart timer when install route in table

Restart whenever receive msg advertising that routeRestart whenever receive msg advertising that route

Route is invalid if 180 seconds pass without another Route is invalid if 180 seconds pass without another advertisementadvertisement

RIP must handle three types of errorsRIP must handle three types of errorsRouting loopsRouting loops

Algorithm does not explicitly detect routing loopsAlgorithm does not explicitly detect routing loops Must either assume participants can be trusted or take Must either assume participants can be trusted or take

precautions to preventprecautions to prevent

InstabilitiesInstabilities Must limit hop count to preventMust limit hop count to prevent Maximum possible distance value is 16Maximum possible distance value is 16 If legitimate hop count is higher, must divide the internet into If legitimate hop count is higher, must divide the internet into

sections or use an alternate protocolsections or use an alternate protocol

Slow convergenceSlow convergence Routing messages propagate slowly across the networkRouting messages propagate slowly across the network Can lead to inconsistenciesCan lead to inconsistencies

Not unique to RIP; fundamental problem in any DV Not unique to RIP; fundamental problem in any DV protocolprotocol

Hop count limit helps but does not eliminateHop count limit helps but does not eliminate

Figure 15.4Figure 15.4

Solving slow convergenceSolving slow convergence ““Good news travel quickly; bad news travel Good news travel quickly; bad news travel

slowly”slowly”Quick to install good routeQuick to install good routeUnreachable only after timeout; then learn and Unreachable only after timeout; then learn and propagate new routepropagate new route

Split horizon updateSplit horizon updateRouter does not propagate information over the Router does not propagate information over the interface from where the route arrivedinterface from where the route arrived

RR22 would not advertise route to network would not advertise route to network 11 to to RR11

If If RR11 loses connectivity, it must quit advertising loses connectivity, it must quit advertising

After a few rounds of routing updates, all routers After a few rounds of routing updates, all routers agree that the network is unreachableagree that the network is unreachableDoes not prevent all routing loopsDoes not prevent all routing loops

Hold downHold downRouter ignores information for a period of timeRouter ignores information for a period of time

Typical time is 60 secondsTypical time is 60 seconds

Done after receiving msg that network is unreachableDone after receiving msg that network is unreachable

Wait so all machines can get bad news; keeps from Wait so all machines can get bad news; keeps from mistakenly accepting an out-of-date messagemistakenly accepting an out-of-date message

All machines must have same idea of hold downAll machines must have same idea of hold down Otherwise, get routing loopsOtherwise, get routing loops

Disadvantages:Disadvantages: If routing loop occurs, will be preserved for the hold downIf routing loop occurs, will be preserved for the hold down Also preserves incorrect routes during the hold down timeAlso preserves incorrect routes during the hold down time

Even when alternatives existEven when alternatives exist

Poison reversePoison reverseWhen connection disappearsWhen connection disappears

Advertising router keeps entry for few update periodsAdvertising router keeps entry for few update periods Puts infinite cost in the broadcastsPuts infinite cost in the broadcasts

Combine with triggered updatesCombine with triggered updates Router sends immediate broadcast when get bad newsRouter sends immediate broadcast when get bad news Not wait for next broadcast timeNot wait for next broadcast time Minimizes time it is vulnerable to believing good newsMinimizes time it is vulnerable to believing good news

These techniques solve some problems; These techniques solve some problems; introduce othersintroduce others

Triggered updatesTriggered updates Suppose many routers share common networkSuppose many routers share common network Single broadcast changes all tables; triggering moreSingle broadcast changes all tables; triggering more Broadcast avalancheBroadcast avalanche

BroadcastsBroadcasts Take substantial bandwidth themselvesTake substantial bandwidth themselves

Loops prevent stopping loopsLoops prevent stopping loops Looping messages may prevent routing msgs to break loopsLooping messages may prevent routing msgs to break loops

Hold down in WANsHold down in WANs Period so long, higher level protocol timers may expirePeriod so long, higher level protocol timers may expire Breaks the connectionsBreaks the connections

RIP1 message formatRIP1 message format Messages are of two typesMessages are of two types

Routing information messagesRouting information messages Periodic broadcast of unsolicited response messagesPeriodic broadcast of unsolicited response messages

Messages to request informationMessages to request information Routers or hosts can ask for info with request commandRouters or hosts can ask for info with request command Routers reply using a response commandRouters reply using a response command

Both use same formatBoth use same format

CommandCommand MeaningMeaning

11 Request for partial or full routing informationRequest for partial or full routing information

22 Response (network-distance pairs from sender’s routing table)Response (network-distance pairs from sender’s routing table)

99 Update Request (used with demand circuits)Update Request (used with demand circuits)

1010 Update Response (used with demand circuits)Update Response (used with demand circuits)

1111 Update Acknowledge (used with demand circuits)Update Acknowledge (used with demand circuits)

Figure 15.5Figure 15.5

RIP2 Address ConventionsRIP2 Address Conventions Skip…..Skip…..

RIP route interpretation and aggregationRIP route interpretation and aggregation Version 1 contains no provision for subnet maskVersion 1 contains no provision for subnet mask

Originally designed for classful addressingOriginally designed for classful addressing

Extended to allow subnettingExtended to allow subnetting

Important restriction:Important restriction: Subnet routes can only go in updates sent across networks Subnet routes can only go in updates sent across networks

that are part of the subnetted prefixthat are part of the subnetted prefix Cannot use with variable-length subnet addresses or Cannot use with variable-length subnet addresses or

classlessclassless Due to not having explicit subnet mask informationDue to not having explicit subnet mask information

May have updates for networks in & out of prefixMay have updates for networks in & out of prefix Router must prepare different update messagesRouter must prepare different update messages

RIP2 extensionsRIP2 extensions Contains provisions for explicit subnet maskContains provisions for explicit subnet mask Also include explicit next-hop informationAlso include explicit next-hop information

Prevents routing loopsPrevents routing loops

Prevents slow conversionPrevents slow conversion

RIP2 message formatRIP2 message format Puts new info in unused octets of address fieldPuts new info in unused octets of address field

Router can use both versions simultaneouslyRouter can use both versions simultaneously

Version number in same octet; inspect before processVersion number in same octet; inspect before process Adds 16-bit ROUTE TAGAdds 16-bit ROUTE TAG

Identify the origin of the routeIdentify the origin of the route

Figure 15.6Figure 15.6

Transmitting RIP messagesTransmitting RIP messages Messages do not have explicit length fieldMessages do not have explicit length field

Nor any explicit count of entriesNor any explicit count of entries Rely on delivery mechanism to tell lengthRely on delivery mechanism to tell length

With TCP/IPWith TCP/IP Rely on UDP to tell receiver the message lengthRely on UDP to tell receiver the message length RIP operates on UDP port 520RIP operates on UDP port 520

Disadvantage of RIP hop countsDisadvantage of RIP hop counts RIP restricts routing to a hop-count metricRIP restricts routing to a hop-count metric RIP restricts size of any internet using itRIP restricts size of any internet using it

Has small hop count value for infinity (16)Has small hop count value for infinity (16)

Limits span to at most 15 routers between hostsLimits span to at most 15 routers between hosts

Is not a limit on total number or density of routersIs not a limit on total number or density of routers In any case, hop count is a crude measureIn any case, hop count is a crude measure

Not always get least delay or highest capacity routesNot always get least delay or highest capacity routes

Makes routes static; cannot change due to loadMakes routes static; cannot change due to load

The Hello ProtocolThe Hello Protocol

IGP that uses routing metric other than hopsIGP that uses routing metric other than hops Now obsoleteNow obsolete Historically, used in original NSFNET backbone Historically, used in original NSFNET backbone

“fuzzball” routers“fuzzball” routers Uses metric of delayUses metric of delay Provides two functionsProvides two functions

Synchronizes clocks among a set of machinesSynchronizes clocks among a set of machines

Allows each machine to compute shortest delay pathsAllows each machine to compute shortest delay paths

Messages carry timestamp as well as routing infoMessages carry timestamp as well as routing info

Each participating machine maintains tableEach participating machine maintains table

Contains best estimate of neighboring machine clocksContains best estimate of neighboring machine clocks

Transmit timestamp with each packetTransmit timestamp with each packet Receiver computes estimate of delay on the link by using the Receiver computes estimate of delay on the link by using the

timestamp and its estimate of the sender’s clocktimestamp and its estimate of the sender’s clock

Periodically poll neighbors to update clock estimatesPeriodically poll neighbors to update clock estimates Standard D-V approach for updateStandard D-V approach for update

Send table of destinations & estimated delaysSend table of destinations & estimated delays

Receiver’s update tables if cheaper route advertisedReceiver’s update tables if cheaper route advertised

Delay Metrics & OscillationDelay Metrics & OscillationIs delay a good routing metric?Is delay a good routing metric? Would seem soWould seem so Worked well in the early Internet backboneWorked well in the early Internet backbone InstabilityInstability is the reason most protocols do not is the reason most protocols do not

use delayuse delayAny protocol that changes routes quickly can become Any protocol that changes routes quickly can become unstableunstable

Hop counts fixed; delay is notHop counts fixed; delay is not Minor variations in delay measurements occurMinor variations in delay measurements occur

Hardware clock driftHardware clock drift

CPU load during measurementCPU load during measurement

Delays by link-level synchronizationDelays by link-level synchronization If react quickly to slight variations, get two-If react quickly to slight variations, get two-

stage oscillationstage oscillationSwitch back and forth between alternate pathsSwitch back and forth between alternate paths

Heuristics to help avoid oscillationHeuristics to help avoid oscillation Hold downHold down

Slows down changingSlows down changing Round off measurements or use thresholdRound off measurements or use threshold

Ignore differences less than the thresholdIgnore differences less than the threshold Use average measurementUse average measurement

Keep average of recent measurementsKeep average of recent measurements

Use Use K-out-of-NK-out-of-N rule rule KK of the most recent of the most recent NN measurements must be less than the measurements must be less than the

current delay before route can be changedcurrent delay before route can be changed

Can still have instabilityCan still have instability Due to comparing delays on paths with Due to comparing delays on paths with

different characteristicsdifferent characteristicsTraffic has dramatic effect on delayTraffic has dramatic effect on delay

As load increases, delay grows rapidlyAs load increases, delay grows rapidly Fall into Fall into positive feedback cyclepositive feedback cycle

Burst of traffic at one place increases delayBurst of traffic at one place increases delay

Protocol changes routeProtocol changes route

New traffic may cause another change in delayNew traffic may cause another change in delay Another route change occursAnother route change occurs

Must have mechanism to dampen oscillationMust have mechanism to dampen oscillation

Previous heuristics may not helpPrevious heuristics may not help They help in simple case for paths with same They help in simple case for paths with same

throughput characteristicsthroughput characteristics Not good when paths have different delay and Not good when paths have different delay and

throughput characteristicsthroughput characteristicsCompare serial line and satellite linkCompare serial line and satellite link

First, both paths idle; serial line have much less delayFirst, both paths idle; serial line have much less delay Then, traffic quickly overloads low capacity lineThen, traffic quickly overloads low capacity line Satellite delay will be less; change to itSatellite delay will be less; change to it

High capacity; load not significantly change delayHigh capacity; load not significantly change delay But, unloaded serial line will now become attractiveBut, unloaded serial line will now become attractive Routing will change again and the cycle will continueRouting will change again and the cycle will continue

Oscillations do occur in practiceOscillations do occur in practiceDifficult to manageDifficult to manage

Combining RIP, Hello, and BGPCombining RIP, Hello, and BGP

Single router may use multiple protocolsSingle router may use multiple protocols Interior Gateway ProtocolInterior Gateway Protocol

Gather routing information within ASGather routing information within AS Exterior Gateway ProtocolExterior Gateway Protocol

Advertise routes to other ASsAdvertise routes to other ASs Should be easy to combine the twoShould be easy to combine the two

Technical and political obstacles existTechnical and political obstacles exist

IGP protocols are routing protocolsIGP protocols are routing protocols RIP and HELLO used to update routing tablesRIP and HELLO used to update routing tables Get info from other routers inside ASGet info from other routers inside AS routedrouted implements RIP implements RIP

Advertises information from local routing tableAdvertises information from local routing table

Updates local table when it receives updatesUpdates local table when it receives updates

RIP RIP truststrusts routers within the AS to send correct data routers within the AS to send correct data

Exterior protocols (BGP) do not trust routersExterior protocols (BGP) do not trust routers Do not advertise all possible routes in local tableDo not advertise all possible routes in local table

Keep database of Keep database of reachability reachability

Apply policy constraints when sending/receiving infoApply policy constraints when sending/receiving info

Ignoring policy constraints can make some Ignoring policy constraints can make some parts of the internet unreachableparts of the internet unreachable

Example:Example:Suppose router running RIPSuppose router running RIP

Propagates route to Purdue; actually has no routePropagates route to Purdue; actually has no route

Other RIP routers will accept and updateOther RIP routers will accept and update

Will pass Purdue traffic to the erroneous routerWill pass Purdue traffic to the erroneous router

Problem if EGP protocol not have policy constraintsProblem if EGP protocol not have policy constraints Border router pass illegal route to other ASsBorder router pass illegal route to other ASs Purdue may become unreachable for parts of the internetPurdue may become unreachable for parts of the internet

Gated: Inter-AS CommunicationGated: Inter-AS Communicationgatedgated Interface between autonomous systemsInterface between autonomous systems Understands multiple protocolsUnderstands multiple protocols

Both IGP’s and BGPBoth IGP’s and BGP Ensures policy constraints are honoredEnsures policy constraints are honored

Can accept RIP msgs and modify local table (Can accept RIP msgs and modify local table (routedrouted))

Can advertise routes from within AS using BGPCan advertise routes from within AS using BGP Has rules on which networks it may & may not advertiseHas rules on which networks it may & may not advertise Also has rules on how to report distances to those networksAlso has rules on how to report distances to those networks

Links IGP with BGPLinks IGP with BGP

Open SPF Protocol (OSPF)Open SPF Protocol (OSPF)

Chapter 13: link state algorithmChapter 13: link state algorithm Uses SPF to compute shortest pathsUses SPF to compute shortest paths Scales better than distance-vector algorithmsScales better than distance-vector algorithms

OSPF is an IGP using link state algorithmOSPF is an IGP using link state algorithm Designed by Internet Engineering Task ForceDesigned by Internet Engineering Task Force To encourage adoption of link state technologyTo encourage adoption of link state technology Tackles several ambitious goalsTackles several ambitious goals

Open standardOpen standard Anyone can implement without license feesAnyone can implement without license fees

Includes Includes type of service routingtype of service routing Have multiple routes for a given destinationHave multiple routes for a given destination Choose by TOS field in IP headerChoose by TOS field in IP header OSPF first among TCP/IP protocols to have thisOSPF first among TCP/IP protocols to have this

Provides Provides load balancingload balancing Distributes traffic over multiple, same cost routesDistributes traffic over multiple, same cost routes

Can partition routers and networks into Can partition routers and networks into areasareas Permits growth; makes management easierPermits growth; makes management easier

Allows exchanges to be Allows exchanges to be authenticatedauthenticated Variety of authentication schemesVariety of authentication schemes

Supports host-specific, subnet-specific, and Supports host-specific, subnet-specific, and classless routesclassless routes

Accommodates multi-access nets (Ethernet)Accommodates multi-access nets (Ethernet)

Can describe network via virtual networkCan describe network via virtual network Abstracts away from details of physical Abstracts away from details of physical

connectionsconnections Provides flexibility for managersProvides flexibility for managers

Allows routers to exchange routing info Allows routers to exchange routing info learned from external siteslearned from external sites Distinguishes where information came fromDistinguishes where information came from

TypeType MeaningMeaning

11 Hello (used to test reachability)Hello (used to test reachability)

22 Database description (topology)Database description (topology)

33 Link status requestLink status request

44 Link status updateLink status update

55 Link status acknowledgementLink status acknowledgement

Figure 15.7Figure 15.7

Figure 15.8Figure 15.8

Routers exchange database description msgsRouters exchange database description msgs Used to initialize network topology databaseUsed to initialize network topology database During exchange:During exchange:

One router is master; other is slaveOne router is master; other is slave

Slave acknowledges each description with a responseSlave acknowledges each description with a response Topology database may be largeTopology database may be large

Can divide into several messages using Can divide into several messages using II and and MM bits bits

II = 1: is initial message = 1: is initial message

MM = 1: additional messages follow = 1: additional messages follow

BitBit S S indicates if sent by master (1) or slave (0) indicates if sent by master (1) or slave (0)

Sequence numbers used to make sure all receivedSequence numbers used to make sure all received

Link TypeLink Type MeaningMeaning

11 Router linkRouter link

22 Network linkNetwork link

33 Summary link (IP network)Summary link (IP network)

44 Summary link (link to border router)Summary link (link to border router)

55 External link (link to another site)External link (link to another site)

Figure 15.9Figure 15.9

Link status request messageLink status request message After exchanging DB descriptions, router may After exchanging DB descriptions, router may

discover parts of its DB are out of datediscover parts of its DB are out of dateRequests neighbor to send updateRequests neighbor to send update

Lists specific links it wants info aboutLists specific links it wants info about

Neighbor responds with most current information Neighbor responds with most current information about those linksabout those links

Figure 15.10Figure 15.10

Links from router to:Links from router to:

- given area- given area

- specific network- specific network

- single, subnetted IP network- single, subnetted IP network

- networks at other sites- networks at other sites

Figure 15.11Figure 15.11

Figure 15.12Figure 15.12

Routing with Partial InformationRouting with Partial Information

Hosts can have partial informationHosts can have partial information Rely on routersRely on routers

Not all routers have complete informationNot all routers have complete information Usually single router in AS connects to othersUsually single router in AS connects to others Suppose site connects to global InternetSuppose site connects to global Internet

At least one router must have connection to an ISPAt least one router must have connection to an ISP

Routers inside AS know all destinations withinRouters inside AS know all destinations within Have default route to send all traffic to the ISPHave default route to send all traffic to the ISP

Examine routing tablesExamine routing tables Routers at center of Internet know allRouters at center of Internet know all

Have complete set of routes to all destinationsHave complete set of routes to all destinations

Learn from the routing arbiter systemLearn from the routing arbiter system

They do They do notnot use default routing use default routing

If destination address not in their table:If destination address not in their table: Either address is not valid, orEither address is not valid, or Address is valid but currently unreachableAddress is valid but currently unreachable

Routers beyond those at center do not know allRouters beyond those at center do not know allHave an incomplete set of routesHave an incomplete set of routes

Rely on default routes for addresses they do not haveRely on default routes for addresses they do not have

Using default routes for most routers has Using default routes for most routers has consequencesconsequences Local routing errors can go undetectedLocal routing errors can go undetected

Machine in AS routes packet to external ASMachine in AS routes packet to external AS

Suppose should have gone to local routerSuppose should have gone to local router

External system will route it backExternal system will route it back

Preserves connectivity even if routing incorrectPreserves connectivity even if routing incorrect

Very bad for a WANVery bad for a WAN Routing update messages will be smallerRouting update messages will be smaller

A good thing!A good thing!

SummarySummaryMust have automated routing proceduresMust have automated routing proceduresIGPsIGPs Used by 2 routers under control of single mgrUsed by 2 routers under control of single mgr Uses either DV or link state algorithm (SPF)Uses either DV or link state algorithm (SPF)

RIP: DV implemented by RIP: DV implemented by routedrouted Uses split horizon, hold-down and poison reverseUses split horizon, hold-down and poison reverse

Hello: obsolete; uses delay versus hop countHello: obsolete; uses delay versus hop countOSPF: uses link status algorithmOSPF: uses link status algorithm

gatedgated Interface between IGP (RIP) and EGP (BGP)Interface between IGP (RIP) and EGP (BGP)