Exterior Routing 201 Exterior Routing 201 Howard C. Berkowitz Howard C. Berkowitz [email protected] [email protected] [email protected] [email protected] (703)998-5819 ESN 451-5819 (703)998-5819 ESN 451-5819
Dec 15, 2015
Exterior Routing 201Exterior Routing 201
Howard C. BerkowitzHoward C. [email protected] [email protected]
[email protected]@clark.net
(703)998-5819 ESN 451-5819(703)998-5819 ESN 451-5819
NANOG 21 Exterior Routing tutorial 2/17/20012
AgendaAgenda
• What's the problem?— Formal and informal clue
— ISP service offerings
• Quirks, Defnitions, and Issues
• ISP External Scenarios
• POP and other infrastructure
• Router requirements
• Playing in the Club
• Turning it On
If there's time...full employment for consultants: path selection
What is the Problem to be What is the Problem to be Solved?Solved?
NANOG 21 Exterior Routing tutorial 2/17/20014
Good little boys and girlsread RFC1771
and live happily ever after
NANOG 21 Exterior Routing tutorial 2/17/20015
• Noah.
• Noah.
• (yawn) MMMmmmmhp?
• Noah.
• Yeahh?
• Build an ISP.
NANOG 21 Exterior Routing tutorial 2/17/20016
ISPs Facing End UserISPs Facing End User
• Entry— Basic Internet Access
— Hosting
— Availability and QoS
— Dealing with specialized access providers (DSL, CATV, etc.)
— Dealing with content providers
— Voice services?
• Improvement for Users– Improving capacity– Improving availability– Adding services– Perceptions of end-to-end
SLA
NANOG 21 Exterior Routing tutorial 2/17/20017
Before the AnimalsBefore the Animals
Uplinks
Routers
User Hosts
Downlinks
Management
Facilities
HVAC Staff
NANOG 21 Exterior Routing tutorial 2/17/20018
Load the ArkLoad the Ark
Policies
Traffic
From Downstreams
Policies
Traffic
From Upstreams
From Users
Traffic AAA
Traffic
From Virtual Hosts
Quirks, Definitions and Quirks, Definitions and IssuesIssues
NANOG 21 Exterior Routing tutorial 2/17/200110
I said "peer," not "peer"I said "peer," not "peer"
• Peer relationship 1— Basic BGP session
• Peer relationship 2— Mulual benefit customers reach one another
— No monetary exchange
— Each advertises customer routes
• Transit Provider relationship— Customer pays for service
— Full routes available to customer
NANOG 21 Exterior Routing tutorial 2/17/200111
(C) O'Leary Museum and Library Association Ltd. Inc.
NANOG 21 Exterior Routing tutorial 2/17/200112
Closest Exit RoutingClosest Exit RoutingHot potatoHot potato
• Paths are not optimized end-to-end
• Paths are optimized for each AS
src
dest
NANOG 21 Exterior Routing tutorial 2/17/200113
Asymmetrical RoutingAsymmetrical Routing
• No guarantee that traffic leaving your AS at one point
• Will return at the same point
• Remember
—Each AS in both directions makes decisions on its information
ISP ISP ScenariosScenarios
NANOG 21 Exterior Routing tutorial 2/17/200115
Basic Internet Access ISPBasic Internet Access ISP
POP2 POP3
Core
POP1HostedServers
InternalServers
ISP #2 ISP #1
/18 /18 /18
/16/16
8x/23
To 70-90% of customers Default routeTo 5-10% of customers Partial routesTo 10% of customers Full routes
From customersFew # public routes ??? VPN
NANOG 21 Exterior Routing tutorial 2/17/200116
Bilateral PeeringBilateral Peering
BigISP 1
BigISP 2
eBGP Relationship
Exchange of customer routes only Some aggregation No infrastructure routes
Highest bandwidth requirement
"Tier 1 Provider" Does not buy transit service from anyone Has default-free routers Gets all routes from bilateral/multilateral peering Total RIB size of 1.3-1.5 * DefaultFreeZone (D)
NANOG 21 Exterior Routing tutorial 2/17/200117
Large Content ProviderLarge Content Provider
10/100
Server
L4 distributionGE ports
Firewall,etc.
Provider
Server Server
L7 Distribution
L3 Path Determination• Sometimes bandwidth limited
• Provider may be default free
• Often high touch processing limited
• Possible SLA and VPN agreements
May participate in content distribution, caching
NANOG 21 Exterior Routing tutorial 2/17/200118
Layer2
Fabric
ISP 1
ISP 2
ISP 3
ISP 4
ISP 5
ISP 6RouteServer
Multilateral PeeringMultilateral PeeringeBGP Relationships
Depending on exchange rules Exchange of customer routes only Most common case Some aggregation No infrastructure routes Some ISPs buy transit services Can receive full routes Private peerings
Largest carriers tend to avoid due tocongestion
ISPs can peer with route server rather than a mesh of ISPs May be done to reduce BGP peers Or simply for statistics collection
NANOG 21 Exterior Routing tutorial 2/17/200119
Special Case: Local ExchangesSpecial Case: Local Exchanges
• Entry— Who's in charge?
— Connectivity
— Facilities
— Allow content providers?
— Allow end users?
— Peering model?
— Supplementary services?
• Improvements
Layer2? 3?Fabric
ISP 1
ISP 2
ISP 3
ISP 4
ISP 5
ISP 6RouteServer
POP and Other Internal POP and Other Internal DesignDesign
NANOG 21 Exterior Routing tutorial 2/17/200121
Typical Basic POP ImplementationTypical Basic POP Implementation
Gigabit Ethernet Frame Interfaces ATM Interfaces
PSTN
LAN Switch
ManagementServers
AccessServer
DialupCustomers
Router Fabric
ISP CoreRouter 1
ISP CoreRouter 2
DedicatedCustomers
CustomerSite Routers
FrameDS3
2x/25
32x/30
DedicatedCustomers
CustomerSite Router
FullDS3
1 per POP25 per POP
450 users per POP
/18 /18
NANOG 21 Exterior Routing tutorial 2/17/200122
Transit Provider POP, Transit Provider POP, Intra-POPIntra-POP
Design Alternatives 1. POP is a route reflector cluster Core is higher-level cluster
2. Each POP is a private or public AS Full mesh iBGP or route reflectors inside POP Confederation between POPs
3. IGP within POP Controlled redistribution inside POP to BGP Prefer intra-POP of same metric
AccessRouter
AccessRouter
POPRouter
POPRouter
NANOG 21 Exterior Routing tutorial 2/17/200123
Public AS
POP ConfederationsPOP Confederations
POP AS65000 POP AS65111 POP AS65222
NANOG 21 Exterior Routing tutorial 2/17/200124
Public AS
POP ReflectorsPOP Reflectors
POP AS65000 POP AS65111 POP AS65222
NANOG 21 Exterior Routing tutorial 2/17/200125
Open Access/Specialized AccessOpen Access/Specialized Access
Layer1/2
FabricSubscribers
ISP 1
ISP 2
ISP 3InternalRoutedNetwork
TunnelServer
ContentServers
NANOG 21 Exterior Routing tutorial 2/17/200126
InternalRouting
&Switching
Tunneled AddressingTunneled Addressing
CLE
Data Provider1
Voice Provider1
EnterpriseVPN NAS
CLE
CLE
AccessGateway
Data Provider2
L2TP, Differv High
L2TP, Differv High
VoIP
Access OAM address space
PPPoE or GRE
ISP address space
Data 1Data 2VPN
DHCPDNS
Router Router RequirementsRequirements
Big part of the solution...but not Big part of the solution...but not all.all.
NANOG 21 Exterior Routing tutorial 2/17/200128
Routing ParadigmsRouting Paradigms
Number of Routes
Forwarding Bandwidth
Hello Processing
Number of Interfaces
Policy Analysis
QoS Awareness
Low High Medium
Low High Medium
Low Medium High
Medium High Medium
End to End EtE & PHB PHB
Low High Low
L4/7 Processing Medium High Low
Enterprise Edge Core
NANOG 21 Exterior Routing tutorial 2/17/200129
Observations on Routing Table SizeObservations on Routing Table Size
• Global default-free table continues to grow exponentially— 96509 routes as of Tony Bates' CIDR report 2/11/2001
— Let the default routing table size be D
• Large provider often has 1.3 to 1.5 D active routes— additional routes are more-specific customer & internal
— may also have substantial numbers of inactive routes
NANOG 21 Exterior Routing tutorial 2/17/200130
Growth in Global Routing Table SizeGrowth in Global Routing Table Size
184K
368K
Sep 01
Sep 02
Sep 03
Sep 04
736K
85K public
NANOG 21 Exterior Routing tutorial 2/17/200131
Growth in Typical Tier 1 Routing Growth in Typical Tier 1 Routing Table SizeTable Size(external + customer, not infrastructure)(external + customer, not infrastructure)
276
552
Sep 01
Sep 02
Sep 03
Sep 04
1104
85K public42K internal
NANOG 21 Exterior Routing tutorial 2/17/200132
Observation: More than RoutesObservation: More than Routes
• Customer routes
• Paths per route
• Route validity
NANOG 21 Exterior Routing tutorial 2/17/200133
ConvergenceConvergence
• Global routing system
• Intra-AS
• Single Router
NANOG 21 Exterior Routing tutorial 2/17/200134
Single Router ConvergenceSingle Router Convergence
• Initialization— Time to add new route
— Time to add better route
— Time to withdraw route
— Time to withdraw and replace route
• Parameters— Matrix: number of peers
versus– Routes advertised– Routes accepted
• Performance Modifiers— Route filtering
— Route flapping
— Packet vs. route filtering
draft-berkowitz-bgpcon-0x.txt
NANOG 21 Exterior Routing tutorial 2/17/200135
Distinguish among casesDistinguish among cases
• Failover of link or router between customer and provider
• Rerouting to intranet/adjacent provider resources
• Rerouting to arbitrary internet destnation
More multihoming in next tutorial
S-T-R-E-T-C-HS-T-R-E-T-C-H
Joining the Joining the ClubClub
NANOG 21 Exterior Routing tutorial 2/17/200138
More than Just Addresses, Protocol...More than Just Addresses, Protocol...
Routing Registry
Routeobjects
ASobjects
Maintainerobjects
RoutingSystem
Configs
Customer
DNS
NAT
Hosts
SpecifyPolicy
Route Track Service
ISP with Prefixes
Allocate
Directories
SWIP
ReverseDNS
AddressRegistry
RouteRegistry
NANOG 21 Exterior Routing tutorial 2/17/200139
ComplexityComplexity
• BGP itself is fairly simple
• Additional attributes it carries are more complex
• Policy actions taken inside router (BGP sender or receiver) far more complex than the protocol itself
NANOG 21 Exterior Routing tutorial 2/17/200140
"BGP Transmits Policies""BGP Transmits Policies"
NANOG 21 Exterior Routing tutorial 2/17/200141
OOppeerraattiioonnaall RReellaattiioonnsshhiippss 11
AAddddrreesssseess aanndd DDeelleeggaattiioonn
Addressauthority
Addressdelegation
Prefixes
Hosts
DNS
ReverseDNS
NANOG 21 Exterior Routing tutorial 2/17/200142
Obtain routable address spaceObtain routable address space
• Apply to registry— RIPE, APNIC, ARIN
— If immediate need for /19 or /20*
• Obtain addresses from upstream ISP— If /19 or /20 cannot be justified
• Registry needs— Network design
— Justification for address space
NANOG 21 Exterior Routing tutorial 2/17/200143
Origination vs. AdvertisingOrigination vs. Advertising
128.0.0.0/20
/23POP Dialups
/23Internal
/23Customers
/23Customers
/25/25/25/2532*
/30
32*
/28/24 /24/25/25
AS 65000
192.0.0.0/16AS64444
an AS65000Customer
AS65000
128.0.0.0/19
AS65000
192.0.0.0/16
AS64444
NANOG 21 Exterior Routing tutorial 2/17/200144
Aggregating your Own TrafficAggregating your Own Traffic
128.0.0.0/20
/23POP Dialups
/23Internal
/23Customers
/23Customers
/25/25/25/2532*
/30
32*
/28/24 /24/25/25
AS65000
128.0.0.0/19
Suppress more specific routesunless required by multihoming
NANOG 21 Exterior Routing tutorial 2/17/200145
Advertising with NO-EXPORTAdvertising with NO-EXPORT
AS6333364.0.0.0/12
Assigns64.0.0.0/22
Assigns64.0.4.0/22
AS62222 AS61111
AS6100096.1.0.0/16
Advertises64.0.0.0/22 NO-EXPORT
Advertises64.0.4.0/22 NO-EXPORT96.1.0.0/16
NANOG 21 Exterior Routing tutorial 2/17/200146
Aggregation is better than Aggregation is better than AggravationAggravation
• Blackhole routes for your blocks— Avoid more-specifics— Use NO-EXPORT when controlling load to upstream
• Encourage customers to aggregate— Proxy aggregation hard to administer
• Understand which blocks you can advertise— And do ingress/egress filtering
NANOG 21 Exterior Routing tutorial 2/17/200147
Preparing for Address Request (1)Preparing for Address Request (1)
• Address requirements of services are you offering
• Dynamic addressing— Dialup
— Residential broadband
• Private addressing— Enterprises homed only to you
— Dialup/broadband not offering servers
• Globally addressable
NANOG 21 Exterior Routing tutorial 2/17/200148
Prepare for Address Request (2) Prepare for Address Request (2) An ISP TopologyAn ISP Topology
POP11 internal LAN
100Dial
Ports
8smallLANs
1med.LAN
POP21 internal LAN
100Dial
Ports
8smallLANs
1med.LAN
POP31 internal LAN
100Dial
Ports
8smallLANs
1med.LAN
POP41 internal LAN
100Dial
Ports
8smallLANs
1med.LAN
CoreRouter 1
CoreRouter 2
Hosting Farm 1 Hosting Farm 2InfrastructureServers
Switch
NANOG 21 Exterior Routing tutorial 2/17/200149
EEssttaabblliisshhiinngg aann AASS ((11))
AASS NNuummbbeerr RReeqquueesstt
• In request to AS number registry— Administrative and technical contacts
— Autonomous system name
— Router description
— Deployment schedule
— Networks (by name) connected by the router(s)
— Internet addresses of the routers
NANOG 21 Exterior Routing tutorial 2/17/200150
Establishing an AS (2)Establishing an AS (2)Registering in Routing RegistryRegistering in Routing Registry
• Minimum requirements— Maintainer object
— AS object
— Route object (s)
NANOG 21 Exterior Routing tutorial 2/17/200151
Establishing an AS (3)Establishing an AS (3)Operational deploymentOperational deployment
• Build configuration— Policy implementation
— Ingress/egress filtering
• Establish security procedures
• Start BGP connections
NANOG 21 Exterior Routing tutorial 2/17/200152
RRoouuttiinngg RReeggiissttrryy OObbjjeeccttss
• Basic— AS
— Route
— Maintainer
• Additional— Inter-AS Network
— Community
— Router
Refinements
NANOG 21 Exterior Routing tutorial 2/17/200153
OOppeerraattiioonnaall RReellaattiioonnsshhiippss 33::
RReeggiissttrriieess,, DDoommaaiinnss,, eettcc..
Addressauthority
Addressdelegation
Prefixes
Hosts
Routeobjects
ASDNS
ReverseDNS
NANOG 21 Exterior Routing tutorial 2/17/200154
Autonomous SystemAutonomous System
• Basis of exterior routing
• AS originate routes for some prefixes they want to be visible
• AS advertise routes to one another— Advertisement may not contain all addresses
— Not all advertisements need be accepted
NANOG 21 Exterior Routing tutorial 2/17/200155
Current AS DefinitionCurrent AS DefinitionRFC 1930RFC 1930
• Connected group of IP CIDR blocks
• Run by one or more network operators
• Single routing policy — announced to the general Internet
— announced with BGP-4
NANOG 21 Exterior Routing tutorial 2/17/200156
AASS NNuummbbeerr
• 16 bit number— 32 bit under discussion
• Numbers assigned by registries— Routing policy should be stored in registry
— ISPs can mirror routing registry -- place for sensitive data
• Private ASNs— 64512 through 65535
— Private AS stripping, confederations
NANOG 21 Exterior Routing tutorial 2/17/200157
OOppeerraattiioonnaall RReellaattiioonnsshhiippss 22::
AAddddeesssseess aanndd AAuuttoonnoommoouuss SSyysstteemmss
Addressauthority
Addressdelegation
Prefixes
Hosts
ASDNS
ReverseDNS
NANOG 21 Exterior Routing tutorial 2/17/200158
Full Employment for Consultants: Full Employment for Consultants: Policies are Policies are insideinside Routers Routers
• Advertising Policies— Outbound to other AS
— BGP advertisement sources
— Outbound route filters
— Route must be in internal routing table
• Acceptance Policies— Inbound AS filters
— Inbound route filters
NANOG 21 Exterior Routing tutorial 2/17/200159
Stop! Stop! What are you going to What are you going to
Advertise?Advertise?•Routes Assigned/Allocated to You
•Routes Assigned/Allocated to Customers
•Routes for which you provide Transit
NANOG 21 Exterior Routing tutorial 2/17/200160
Advertising AffectsAdvertising Affects
• The way the world sees you/sends to you
• Binary— Routes to which you provide routing
• Quantitative Preferences— Multi-Exit Discriminators to your Neighbors
— AS Path Manipulation to all
NANOG 21 Exterior Routing tutorial 2/17/200161
Routes Eligible to AdvertiseRoutes Eligible to Advertise
• Are reachable by your IGPor static routes
• Unless they are black holes— Which conceptiually are reachable
• Do not advertise— Spoofed source addresses
— Your internal addresses
— RFC1918 space
— Known rogues?– RBL?
NANOG 21 Exterior Routing tutorial 2/17/200162
Stop! Stop! What are you going to What are you going to
Accept?Accept?
•It depends
•Only those routes you will do something about
•Otherwise default
NANOG 21 Exterior Routing tutorial 2/17/200163
Do Not AcceptDo Not Accept
• RFC1918 source or destination
• Unexpected sources not assigned/allocated to peers
• Your internal addresses from peers
Turning it Turning it OnOn
NANOG 21 Exterior Routing tutorial 2/17/200165
BBGGPP CCoonnffiigguurraattiioonn OOvveerrvviieeww
• Plans and policies first!
• Define system of BGP speakers
• Specific BGP speaker configuration— Identifier
— BGP process
— Neighbors
— NLRI to advertise
— Filters and other policy mechanisms
Cisco commands used as examples
NANOG 21 Exterior Routing tutorial 2/17/200166
Policy Implementation FlowPolicy Implementation Flow
MainBGPRIB
AdjRIBOut
Outgo-ing
AdjRIBIn
Incom-ing
MainRIB/FIB
IGPs
Static&
HWInfo
NANOG 21 Exterior Routing tutorial 2/17/200167
AS1R1
AS21R1
AS1R1
AS21R1
AS1R1
AS21R1
AS1R2
AS21R2
All equivalent from a policy standpoint!
Policy vs. Protocol FlowPolicy vs. Protocol Flow
NANOG 21 Exterior Routing tutorial 2/17/200168
BGP ConfigurationsBGP Configurations
• Know global information (AS, policies, etc.)
• Establish router ID
• Create BGP process
• Identify internal and external peers
NANOG 21 Exterior Routing tutorial 2/17/200169
RRoouutteerr IIDD aanndd llooooppbbaacckk iinntteerrffaaccee
interface loopback 0ip address 192.168.0.1 255.255.255.0
RReeffiinniinngg tthhee CCoonnffiigguurraattiioonn
Single and Multiple Links Single and Multiple Links
to a Single Providerto a Single Provider
NANOG 21 Exterior Routing tutorial 2/17/200171
TThhee BBGGPP TTuunnnneell
Serial 0 Serial 0
Serial 1 Serial 1
Loop 0 Loop 0
ebgp-multihop needed whenneighbor is not on same subnet
NANOG 21 Exterior Routing tutorial 2/17/200172
LLooaadd BBaallaanncciinngg 11::
IIPP LLeevveell ttoo SSiinnggllee PPrroovviiddeerr RRoouutteerr
Serial 0 Serial 0
Serial 1 Serial 1
Loop 0 Loop 0
CustomerAS
ProviderAS
NANOG 21 Exterior Routing tutorial 2/17/200173
LLooaadd BBaallaanncciinngg 11::
MMuullttiippllee RRoouutteerrss
CustomerAS
ProviderAS
NANOG 21 Exterior Routing tutorial 2/17/200174
AAnnootthheerr NNoonn--BBGGPP AAlltteerrnnaattiivvee
OOSSPPFF RRoouuttiinngg DDoommaaiinn
Default Route (0.0.0.0/0) Metric Type 1 Equal Metrics
Static routesD1-A0ASBR1
D1-A0ASBR2
ISP 1
NANOG 21 Exterior Routing tutorial 2/17/200175
Multiple OSPF DefaultsMultiple OSPF Defaults
ISP 1POP
ISP 2POP
Default Route (0.0.0.0/0) Metric Type 2
Higher Metric to ISP 2 (Backup)
Static routesD1-A0ASBR1
D1-A0ASBR2
NANOG 21 Exterior Routing tutorial 2/17/200176
Blackhole RouteBlackhole Route
• Establish static route to your block(s) ip route 1.2.3.4 255.255.240.0 null0
• Redistribute/import into BGP
• Suppress more-specific prefix advertising
NANOG 21 Exterior Routing tutorial 2/17/200177
Effects of BlackholingEffects of Blackholing
• No route flapping outside your AS
—If your internal routes go up or down
• Incoming traffic for specific routes that are down
—Doesn’t match any internal route
—Automatically discarded without concerning anyone else
BGP Path BGP Path SelectionSelection
NANOG 21 Exterior Routing tutorial 2/17/200179
Next Hop AccessNext Hop Access
R2
R1X
Advertised routevia R1
Advertised routevia R2
NANOG 21 Exterior Routing tutorial 2/17/200180
SSccooppee::
MMEEDD vvss.. LLooccaall PPrreeffeerreennccee vvss.. WWeeiigghhtt
Weight
Local Preference
Weight
AS1 AS2
MED
NANOG 21 Exterior Routing tutorial 2/17/200181
AAddmmiinniissttrraattiivvee WWeeiigghhtt ((CCiissccoo eexxtteennssiioonn))
Advertised routevia R1
Advertised routevia R2
Rules in this router set R1 weight to 100, R2 weight to 500
R2
R1X
NANOG 21 Exterior Routing tutorial 2/17/200182
Weight exampleWeight examplefor load sharingfor load sharing
PrimaryISP
Default local preference 500All routes ^ AS_Backup + local preference 100
BackupISP
Default local preference 200
NANOG 21 Exterior Routing tutorial 2/17/200183
TTiieebbrreeaakkeerr ffoorr EEqquuaall WWeeiigghhtt::
LLooccaall PPrreeffeerreennccee
R2
R1
Advertised routevia R1, local preference 100
Advertised routevia R2, local preference 500
NANOG 21 Exterior Routing tutorial 2/17/200184
Local Preference Local Preference example for load sharingexample for load sharing
PrimaryISP
Default local preference 500All routes ^ AS_Backup + local preference 100
BackupISP
Default local preference 200
NANOG 21 Exterior Routing tutorial 2/17/200185
PPrreeffeerr llooccaallllyy oorriiggiinnaatteedd rroouutteess
R2
R1
Advertised routevia R1
Locally definedvia R2
NANOG 21 Exterior Routing tutorial 2/17/200186
AS PathAS Path
1
1
1 2
2 4
1 2 4
5
1 2 4 5
NANOG 21 Exterior Routing tutorial 2/17/200187
SShhoorrtteesstt AASS PPaatthh ((CCiissccoo eexxtteennssiioonn))
R2
R1
AS AS AS AS Route
AS AS Route
NANOG 21 Exterior Routing tutorial 2/17/200188
Full Employment For Consultants:Full Employment For Consultants:Interpreting AS PathInterpreting AS Path
• Default assumption: local preference set based on AS_PATH
• Cisco considers it as part of the algorithm
NANOG 21 Exterior Routing tutorial 2/17/200189
AS Path PrependingAS Path Prepending
• Applies to routes you advertise
• Makes them less attractive to others
• Increases AS_PATH length— your AS put in the path twice
NANOG 21 Exterior Routing tutorial 2/17/200190
Limitations of PrependingLimitations of Prepending
6
1
1 1
1
1 2
2
3
1 1 3
4
1 2 4
5
1 2 4 5
NANOG 21 Exterior Routing tutorial 2/17/200191
Route Learned from eBGP
Route Learned from iBGP
R2
R1
External Paths PreferredExternal Paths Preferred
NANOG 21 Exterior Routing tutorial 2/17/200192
RemoteAS
MED=100
MED=500
R2
R1
Lowest MEDLowest MED
NANOG 21 Exterior Routing tutorial 2/17/200193
Full Employment For Consultants:Full Employment For Consultants:Weight, Local Preference & MEDWeight, Local Preference & MED
• HIGHER value wins— Weight
— Local preference
• LOWER value wins— MED
— Cisco default: route with no MED preferred
— IETF: route with no MED least preferred
NANOG 21 Exterior Routing tutorial 2/17/200194
Full Employment For Consultants:Full Employment For Consultants:Scope of MEDScope of MED
• Default assumption: — MEDs only compared between exits to the same adjacent AS
• Alternate: always-compare-MED— Useful at exchange points, possibly private peerings
— Cisco knob
NANOG 21 Exterior Routing tutorial 2/17/200195
CClloosseesstt NNeeiigghhbboorr
IGP metric to R1=100
IGP metric to R1=500
R2
R1
NANOG 21 Exterior Routing tutorial 2/17/200196
LLoowweesstt BBGGPP rroouutteerr IIDD
R22.2.2.2
R11.1.1.1