CCIE R&S Written Overview: MPLS

MPLS Layer 3 VPNs

MPLS Overview

• Multiprotocol Label Switching

• Open standard per RFC 3031

“Multiprotocol Label Switching Architecture”

• Previously Cisco proprietary Tag Switching

MPLS Overview – Multiprotocol

• Can transport different payloads

• Layer 2

– Ethernet, HDLC, PPP, Frame Relay, & ATM

• Layer 3

– IPv4 & IPv6

MPLS Overview – Label Switching

• Traffic is switched between interfaces based on locally significant label values

• Similar to how a Frame Relay or ATM switch uses input/output DLCIs and VPI/VCIs

MPLS Label Format

• 4 byte header used to switch packets

• RFC 3032 – “MPLS Label Stack Encoding”

– 20 bit Label = Locally significant to router

– 3 bit EXP = Class of Service

– S bit = Bottom of Stack

• If 1, label is last in the stack

– 8 bit TTL = Time to Live

How Labels Work

• MPLS Labels are bound to FECs

Forwarding Equivalency Class

– Mainly IPv4 prefix for our purposes

– Could also be IPv6 prefix or layer 2 circuit

• Router uses MPLS LFIB to switch traffic

– Essentially CEF table + Label

• Switching logic

– If traffic comes in if1 with label X send it out if2 with label Y

MPLS Device Roles

• PE / LER Provider Edge Router / Label Edge Routers

• Connects to Customer Edge (CE) devices

• Receives unlabeled packets and adds label

– AKA “label push” or “label imposition”

• In L3VPN performs both IP routing & MPLS lookups

• P / LSR devices Provider Router / Label Switch Routers

• Connects to PEs and/or other P routers

• Switches traffic based only on MPLS label

Label Push / Pop / Swap

• PE and P routers perform three major operations

Label push

– Add a label to an incoming packet

– AKA label imposition

• Label swap

– Replace a label on an incoming packet

• Label pop

– Remove a label from an outgoing packet

– AKA label disposition

Label Distribution

• Adjacent P/PEs must agree on label per FEC

• Label binding can be dynamic through…

Tag Distribution Protocol (TDP)

• Cisco proprietary and legacy

Label Distribution Protocol (LDP)

Resource Reservation Protocol (RSVP)

• Used for MPLS Traffic Engineering (MPLS TE)

Multiprotocol BGP (MP-BGP)

Label Distribution Protocol (LDP)

• Standard per RFC 3036 “LDP Specification”

• Neighbor discovery

– UDP port 646 to 224.0.0.2

• Neighbor adjacency

– TCP port 646 to remote LDP Router-ID

• Label advertisement

– Advertise FEC for connected IGP interfaces

– Advertise FEC for IGP learned routes

Penultimate Hop Popping (PHP)

• Penultimate means next to last

• Normally last hop must…

– Lookup MPLS Label

– Pop MPLS Label

– Lookup IPv4 destination

• PHP avoids extra lookup on last hop

• Accomplished through Implicit NULL label advertisement for connected prefixes

MPLS Layer 3 VPNs

• RFC 4364 – BGP/MPLS IP Virtual Private Networks (VPNs)

• AKA MPLS L3VPN

• Combines logic of MPLS Tunnels with separation of layer 3 routing information

– PEs learns customer routes from CEs

– PEs advertises CEs routes to other PEs via BGP

– BGP next-hops point to MPLS tunnels

• E.g. Loopbacks of PE routers

How MPLS L3VPNs Work

• MPLS L3VPNs have two basic components

• Separation of customer routing information

– Virtual Routing and Forwarding (VRF) Instance

– Customers have different “virtual” routing tables

• Exchange of customer routing information

– MP-BGP over the MPLS network

– Traffic is label switched towards BGP next-hops

Virtual Routing and Forwarding

• Each VRF has its own routing table

show ip route vrf [name | * ]

• Interfaces not in a VRF are in the global table

show ip route

• VRF and global routes are separate

– Implies addressing can overlap in different VRFs

– Implies VRFs can’t talk to each other because they have no routes to each other

• VRFs without MPLS is considered “VRF Lite”

VRF Aware Routing

• Routing inside a VRF can be through…

– VRF aware static routes

– VRF aware IGPs

• RIP, EIGRP, OSPF, or IS-IS

– MP-BGP

– Policy Routing

VRF Lite vs. MPLS VPNs

• In VRF Lite all devices in transit must carry all routes

– Same as normal IP routing logic

• In MPLS VPNs only PE routers need customer routes

• Accomplished through…

VPNv4 Route

• RD + Prefix makes VPN routes globally unique

MPLS VPN Label

• PE routers exchange label for each customer route via VPNv4

Transport Label

• Label towards PE’s BGP next-hop

Multiprotocol BGP

• RFC 4364 “BGP/MPLS IP Virtual Private Networks (VPNs)”

– MP-BGP defines AFI 1 & SAFI 128 as VPN-IPv4 or “VPNv4”

• 8 byte Route Distinguisher (RD)

– Unique per VPN or per VPN site

– ASN:nn or IP-address:nn

• 4 byte IPv4 address

– Unique per VPN

– Implies globally unique routes

Controlling VPNv4 Routes

• Route distinguisher used solely to make route unique

• New BGP extended community “route-target” used to control what enters/exits VRF table

• “export” route-target

– What routes will be go from VRF into BGP

• “import” route-target

– What routes will go from BGP into VRF

• Allows granular control over what sites have what routes

– “import map” and “export map” allow control on a per prefix basis

VPNv4 Route Target

• 8 byte field per RFC 4360 “BGP Extended Communities Attribute“

• Format similar to route distinguisher

– ASN:nn or IP-address:nn

• VPNv4 speakers only accept VPNv4 routes with a route-target matching a local VRF

– Route reflection exception

– no bgp default route-target filter

• VPNv4 routes can have more than one route target

• Allows for complex VPN topologies

– Full mesh

• Import and export same everywhere

– Hub and Spoke

• Spokes import only hub’s routes

– Central services

• Multiple VPNs can import routes from a central site or from a central server

– Management VPNs

• Management Loopback on CE routers can be exported into special management VPN

 

Some screencaps from the MPLS Layer 3 VPN Overview videos:

IMG_0441 IMG_0443 IMG_0445 IMG_0446 IMG_0448 IMG_0449 IMG_0451 IMG_0463

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CCIE R&S Written Overview: IPv6

IPv6 Overview

• Main motivation for IPv6 is lack of IPv4 address space

• IPv4 uses 32-bits (4 bytes)

– 2^32 = 4,294,967,296 max addresses

• IPv6 uses 128-bits (16 bytes)

– 2^128 = 34,028,236,692,938,463,463,374,607,431,770,00 0,000+

IPv4 vs. IPv6 Addressing Format

• IPv4 Dotted Decimal

– 1.2.3.4

– Each place denotes 1 byte

• IPv6 Hexadecimal

– XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX

– Two characters = one byte

IPv6 Address Space

• Four main address types

– Global Unicast

• 2000… – 3FFF…

– Unique Local

• FC00…

• Deprecates Site Local (FEC0)

– Link Local

• FE80…

– Multicast

• FF…

Modified EUI-64 Addressing

• IPv6 host addresses are generated from interface MAC address

• MAC address is 48-bits

• IPv6 host address is 64-bits

• Extra 16 bits derived as follows:

– MAC 1234.5678.9012

– Invert 7th most significant bit

• 1034.5678.9012

– Insert “FFFE” in middle

• 1034:56FF:FE78:9012

IPv6 Address Resolution

• Ethernet

– ICMPv6 ND replaces ARP

• NBMA

– Static resolution on multipoint interfaces

– Inverse Neighbor Discover not implemented

ICMPv6 Neighbor Discovery

• ICMPv6 ND

• Replaces IPv4 ARP

• NS – Neighbor Solicitation

– Ask for information about neighbor

• NA – Neighbor Advertisement

– Advertise yourself to other neighbors

• RS – Router Solicitation

– Ask for information about local routers

• RA – Router Advertisement

– Advertise yourself as an active router

• Send neighbor solicitation to solicited node multicast

– FF02:0:0:0:0:1:FF00::/104 + 24 low-order bits

• If no reply address is unique

– Duplicate Address Detection (DAD)

• Send unsolicited neighbor advertisement to announce yourself

– Sent to all hosts multicast

• FF02::1

• Essentially the same as 255.255.255.255

IPv6 Routing Overview

• IPv6 unicast routing off by default

– ipv6 unicast-routing

• Dynamic routing through

– RIPng

– OSPFv3

– EIGRPv6

– IS-IS

– BGP

• Dynamic information recurses to remote link-local address

– Layer 3 to layer 2 resolution on multipoint NBMA medias

IPv6 Static Routing

• Same static routing implications as IPv4

– To next-hop

• Resolve next-hop

– To multipoint interface

• Resolve final destination

– To point-to-point interface

• No resolution required

IPv6 Routing

• RIPng, OSPFv3, & EIGRPv6

– Use separate processes

• BGP & IS-IS

– Use the same process

– Different Address families

RIPng Overview

• RFC 2080 – RIPng

• Similar in operation to RIPv1 / RIPv2

• UDP port 521 multicast to FF02::9

• Configuration

– Interface level ipv6 rip [process] enable

– Automatically enables global process

• Split-horizon enabled globally

– no split-horizon on multipoint NBMA

EIGRPv6 Overview

• Similar in operation to IPv4 EIGRP

• IP protocol 88 multicast to FF02::A

• Configuration

– Interface level ipv6 eigrp [ASN]

– Process level no shutdown

OSPFv3 Overview

• RFC 2740 – OSPFv3

• Similar in operation to OSPFv2

• Router-id is IPv4 address

– Use router-id command if no IPv4 configured

• Configuration

– Interface level ipv6 ospf [process-id] area [area-id]

– Automatically enables global process

OSPFv3 LSAs

• Most LSAs are the same as in OSPFv2

– LSA 1 – Router LSA

– LSA 2 – Network LSA

– LSA 3 – Inter-Area-Prefix-LSA

• Same as OSPFv2 Summary LSA

– LSA 4 – Inter-Area-Router-LSA

• Same as OSPFv2 ASBR Summary LSA

– LSA 5 – AS-External-LSA

– LSA 7 – Type-7-LSA

• Two new LSAs

– LSA 8 – Link-LSA

• Link-Local scope

• Used for link-local next-hop calculation

– LSA 9 – Intra-Area-Prefix-LSA

• Area scope

• Used to advertise global addresses of connected links

• LSA 1 & 2 are still used to build the graph of the network, but are now decoupled from the actual addresses on the links

OSPFv3 Network Types

• Same network types as OSPFv2

– Broadcast

• DR/BDR Election

– Non-broadcast

• DR/BDR Election

• Unicast updates to link-local address

– Point-to-point

– Point-to-multipoint

– Point-to-multipoint non-broadcast

• Unicast updates to link-local address

BGP for IPv6 Overview

• Same process for IPv4 and IPv6

– Uses address-family configuration

• Normal BGP rules apply

– Requires underlying IGP transport

– iBGP loop prevention

• Don’t advertise iBGP learned routes to other iBGP neighbors

• Exception through route-reflection / confederation

– EBGP loop prevention

• Don’t accept routes with your own AS in the path

– Same best-path selection process

Tunneling IPv6 over IPv4

• Static tunnels

– GRE

• Default tunnel mode

– IPv6IP

• Less overhead, but no CLNS transport

• Automatic tunnels

– IPv4 Compatible Tunnel

• IPv6 next-hop is IPv4 address, e.g. ::192.168.1.1

– Automatic 6to4

• Imbeds IPv4 address into IPv6 prefix to provide automatic tunnel endpoint determination

– ISATAP

• Automatic host to router and host to host tunneling

CCIE R&S Written Overview: BGP

BGP Overview

• Border Gateway Protocol

– Standards based Exterior Gateway Protocol (EGP)

– RFC 4271 A Border Gateway Protocol 4 (BGP-4)

• Path Vector Protocol

– Uses multiple “attributes” for inter-domain routing between Autonomous Systems

BGP Features

• “Classless” Protocol

– Supports VLSM and summarization

• Highly Scalable

– IGPs can scale to thousands of routes

– BGP can scale to hundreds of thousands of routes

– Current Global (Internet) BGP table ~ 400,000 routes

• Highly Stable

– Internet routing table never converges

– BGP stable enough to handle routing and decision making at the same time

• Used to Enforce Routing Policy

– IGP uses link cost for routing decision

• Effective traffic engineering nearly impossible with IGP

– BGP uses attributes of the route itself

• Traffic engineering feasible and simple to implement

• Uses Autonomous System Number (ASN) to identify process

– BGP ASNs originally 2-byte field

• Values 0-65535

– RFC 4893 defines 4-byte ASNs

• 65535.65535 “AS Dot” notation

• 0.[0-65535] denote original 2-byte ASNs

• Doesn’t use its own transport

– Uses unicast TCP at port 179

• BGP peers are not discovered

– Manually configured via neighbor statement

• BGP neighbors do not have to be connected

– IGP is always on a link-by-link basis

– BGP is a logical peering over TCP

– Implies that BGP always needs IGP underneath

• BGP has different types of neighbors

– External BGP vs. Internal BGP

• Path vector attributes

– Choose BGP bestpaths to build routing table

• Control Plane Security

– Supports TCP MD5 Signature Option

• Extensible

– Multiprotocol BGP extensions beyond normal IPv4 Unicast routing

Establishing BGP Peerings

• Like IGP, first step in BGP is to find neighbors to exchange information with

• Peering establishment and maintenance uses four types of packets

– OPEN

– KEEPALIVE

– UPDATE

– NOTIFICATION

BGP OPEN Message

• Used to negotiate parameters for peering

• Includes…

– BGP version

• Should be 4

– Local ASN

– Local Router-ID

– Hold time

• Negotiated to lowest requested value

– Options

• AKA “capabilities”

BGP KEEPALIVE Message

• Used for dead neighbor detection

• If hold time = 0, keepalives disabled

BGP UPDATE Message

• Used to advertise or withdraw a prefix

• Includes..

– Withdrawn routes

• List of routes that should be discarded

– NLRI

• Route being advertised

– Path vector attributes

• Attributes of route being advertised

• Used for bestpath selection

BGP NOTIFICATION Message

• Used to convey error messages

• After notification sent, BGP session closed

• Examples

– Unsupported Version Number

– Unsupported Optional Parameter

– Unacceptable Hold Time

– Hold Timer Expired

BGP Peering Types

• External BGP (EBGP) Peers

– Neighbors outside my Autonomous System

• Internal BGP (iBGP) Peers

– Neighbors inside my Autonomous System

• Update and path selection rules change depending on what type of peer a route is being sent to/received from

EBGP Peerings

• Peers in different ASes

• Usually directly connected neighbors

– e.g. DS3 Frame Relay link to ISP

• Can be “multihop”, but TTL defaults to 1

• Uses AS-Path attribute for loop prevention

– If I receive an update from an EBGP peer with my own ASN in the AS-Path, discard it

iBGP Peerings

• Peers in the same AS

• Many times not directly connected

– Implies IGP needed to provide TCP transport

• Loop prevention via route suppression

– Routes learned from an iBGP peer cannot be advertised on to another iBGP peer

– Implies that all routers running BGP within the AS must peer with each other

• i.e. “iBGP full mesh” of n*(n-1)/2 peerings

iBGP Full Mesh

• Can be fixed with two exceptions

– Route Reflectors

• Same logic as OSPF DR/IS-IS DIS

– Confederation

• Split the AS into smaller Sub-ASes

BGP Peering Redundancy

• BGP peering is based on TCP reachability to peer address

• If peer address is unreachable, peering goes down – e.g. if IP address of Serial link is used for peering and Serial link is down, peer goes down

• Using Loopback addresses for peerings allows rerouting around link failures and adds redundancy – e.g. as long as any link is up, Loopback can be reached

• Can also be used for load balancing

Building the BGP Table

• Once peerings are established, UPDATE messages are exchanged to advertise NLRI and build the BGP table

• NLRI can be originated by…

– Network statement

– Redistribution

– Aggregation

– Conditional Route Injection

• Unlike IGP, networks do not have to be directly connected to be advertised, they only have to be in the routing table – e.g. prefixes in local routing table learned via OSPF can be advertised with BGP network statement

BGP Path Vector Attributes

• UPDATE includes path vector attributes for a route

• Attributes fall into different categories…

– Well-known vs. optional

• Well-known must be implemented

• Optional may or may not be implemented

– Mandatory vs. discretionary

• Mandatory must be present in update

• Discretionary may or may not be present

– Transitive vs. non-transitive

• Transitive passes between EBGP and iBGP neighbors

• Non-transitive passes only between iBGP neighbors

• Well-known mandatory

– Next-hop

– AS-Path

– Origin

• Well-known discretionary

– Local Preference

– Atomic Aggregate

• Optional transitive

– Aggregator

• Optional non-transitive

– MED

BGP Bestpath Selection

• Once updates are exchanged, path selection begins

– Bestpath selection algorithm compares path vector attributes and elects one route as “best” for each prefix

– Only best route is sent to the routing table

– Only best route can be advertised to other BGP peers

– Multipath can occur, but in very strict circumstances

BGP Bestpath Selection Order

• Algorithm runs top down until a deciding match occurs

• Cisco IOS selection order is…

– Weight (highest)

– Locally significant Cisco proprietary attribute

– Local Preference (highest)

– Locally originated routes

– AS-Path (shortest)

– Origin (lowest)

– MED (lowest)

– EBGP learned routes over iBGP learned routes

– Smallest IGP metric to next-hop value

• Other tie-breaking checks occur if no bestpath

– Oldest route, lowest Router-ID, lowest interface IP address, etc.

Manipulating BGP Bestpath Selection

• Vector attributes can be manually modified to define different routing policy for different routes

– E.g. control inbound/outbound traffic flow on a per-prefix basis

• Attributes typically modified are…

– Weight

– Local-Preference

– AS-Path

– MED

• Inbound routing policy affects outbound traffic

– Change weight or local-pref in to affect traffic out

• Outbound routing policy affects incoming traffic

– Change AS-Path or MED to affect traffic in

CCIE R&S Written Overview: OSPF

OSPF Overview

• Open Shortest Path First

– Open Standards Based Interior Gateway Routing Protocol (IGP)

– RFC 2328 “OSPF Version 2”

• Link-State Protocol

– Uses Dijkstra’s SPF Algorithm

OSPF Features

• “Classless” Protocol

– Supports VLSM And Summarization

• Guarantees Loop-Free Topology

– All routers agree on overall topology

– Uses Dijkstra’s SPF Algorithm to calculate SPT

• Standards Based

– Inter-operability between vendors

• Uses its own transport protocol

– IP protocol 89 (OSPF)

– Uses unicast or multicasts to 224.0.0.5 and 224.0.0.6

• Large Scalability

– Hierarchy through “areas”

– Topology summarization

• Fast Convergence

– Actively Tracks Neighbor Adjacencies

– Event Driven Incremental Updates

• Efficient Updating

– Uses reliable multicast and unicast updates

– Non-OSPF devices do not need to process updates

• Bandwidth Based Cost Metric

– More flexible than static hop count

• Control Plane Security

– Supports clear-text and MD5 based authentication

• Extensible

– Future application support through “opaque” LSAs, e.g. MPLS Traffic Engineering

Forming OSPF Adjacencies

• Like EIGRP, OSPF uses “hello” packets to discover neighbors on OSPF enabled attached links

• Hello packets contain attributes that neighbors must agree on to form “adjacency”

– Not all OSPF neighbors actually form adjacency

• To form adjacency neighbors must agree on…

– Unique Router-ID

– Unique IP Address

– Interface Area-ID

– Hello interval & dead interval

– Interface network address

– Interface MTU

– Network Type

– Authentication

– Stub Flags

– Other optional capabilities

OSPF Network Types

• Network type used to deal with different media characteristics

• OSPF network types control…

– How updates are sent

– Who forms adjacency

– How next-hop is calculated

• OSPF Network Types are…

– Broadcast

– Non-Broadcast

– Point-to-Point

– Point-to-Multipoint

– Point-to-Multipoint Non-Broadcast

– Loopback

OSPF DR & BDR

• Designated Router (DR) used on broadcast and non-broadcast network types to…

– Minimize adjacencies

– Minimize LSA replication

• Backup Designated Router (BDR)

– Used for redundancy of DR

• DROthers

– All other routers on link

– Form full adjacency with DR & BDR

– Stop at 2-Way adjacency with each other

OSPF DR & BDR Election

• Election based on interface priority and Router-ID

– Priority

• 0 – 255

• Higher better

• 0 = never

– Router-ID

• Highest loopback / interface IP

• Can be statically set

• Higher better

• No preemption unlike IS-IS’s DIS

Sending OSPF Updates

• OSPF “flooding procedure” is used to synchronize the database between routers

– Routers in the same area share the same database

– Database is used as an input to SPF algorithm to calculate SPT

• How flooding occurs depends on LSA type

• Different LSAs used to describe different types of routes

– Intra Area

– Inter Area

– External

– NSSA External

OSPF LSA Types

• Type 1 – Router LSA

• Type 2 – Network LSA

• Type 3 – Network Summary LSA

• Type 4 – ASBR Summary LSA

• Type 5 – External LSA

• Type 7 – NSSA External LSA

• Others outside our scope…

– Type 6 – Multicast LSA

– Type 8 – Inter-AS OSPF

– Types 9, 10, & 11 – Opaque LSA

OSPF Path Selection

• OSPF path selection order is fixed as follows…

– (O) Intra Area

– (O IA) Inter Area

– (E1) External Type 1

– (E2) External Type 2

– (N1) NSSA External Type 1

OSPF Route Filtering

• Routers in the same OSPF area must have the same database

– Limits filtering capabilities of routing advertisements

• Filtering can be accomplished…

– Locally from the database to the routing table

• Distribute-list in

– On the ABR

• Summarization

• Stub Areas

• Inter Area (LSA Type 3) filter

OSPF Stub Areas

• Used to filter routes on ABR based on LSA type

– Reduces database size without impacting reachability

• Four types of stub areas…

– Stub

– Totally Stubby

– NSSA

– Totally NSSA

OSPF Virtual Links

• OSPF area 0 must be contiguous

– Breaks in area 0 result in failure of SPF calculation

– Virtual Links can be used to fix these breaks

• Virtual Links are…

– Used to connect area 0 over a non-transit area

– A virtual area 0 adjacency between two ABRs over a non-transit area

• Requirements…

– Non-transit area must have full routing information

– Cannot be a stub area and should not have filtering

OSPF Reconvergence Tuning

• OSPF database calculation & lookup times a function of hardware

– e.g. faster CPU, more memory, faster lookups

• Resource needs can be lowered through…

– Areas for flooding domain segmentation

– Summarization

– Stub areas

• Further optimization through timers

– Hello & dead timers

– Faster neighbor down detection

– Pacing timers

• How long do I wait between updates, retransmits, etc.

– Throttling timers

• How often do I generate LSAs, run SPF, etc.

CCIE R&S Written Overview: EIGRP

EIGRP Overview

• Enhanced Interior Gateway Routing Protocol

– Successor to Interior Gateway Routing Protocol (IGRP)

• Cisco proprietary “hybrid” protocol

– Both Distance Vector and Link State Behavior

– Really “Advanced Distance Vector”

EIGRP Features

• “Classless” protocol

– Supports VLSM and summarization

• Multiple routed protocol support

– IPv4, IPX, & Appletalk

• Uses its own transport protocol

– IP protocol 88 (EIGRP)

• Reliable Transport Protocol (RTP)

– Uses multicast to 224.0.0.10 and unicast

• Forms active neighbor adjacencies

– Guarantees packet delivery and supports partial updates

• Guarantees loop-free topology

– Diffusing Update Algorithm (DUAL)

• Fast convergence

– Fastest of all IGP in certain designs

• Granular Metric

– Hybrid metric derived from multiple factors

• Unequal Cost Load Balancing

– Only IGP that supports true load distribution

• Summarization

– Like RIPv2 supports auto-summary and manual summaries

• Control Plane Security

– Supports MD5 based authentication

Forming EIGRP Adjacencies

• Neighbors are discovered with HELLO packets

– Sent to 224.0.0.10 from primary IP address

• Neighbors must agree on…

– IPv4 subnet

– Autonomous System Number

– Authentication

– Metric Weightings (K values)

• Neighbors do not need to agree on timers

– Opposite of OSPF timer logic

Sending EIGRP Updates

• Once neighbors are found, EIGRP UPDATE messages used to exchange routes

– Sent as multicast to 224.0.0.10 or as unicast

• Update messages describe attributes of a route

– Prefix + Length

– Next-Hop

– Bandwidth

– Delay

– Load

– Reliability

– MTU

– Hop Count

– External Attributes

Calculating the EIGRP Topology

• All routes learned from all neighbors make up the EIGRP “topology table”

• Once topology is learned, DUAL runs to choose loop-free best path to each destination

– Best path has the lowest “composite metric”

• Composite metric calculated from…

– Administrative Weighting (K values)

– Bandwidth

– Delay

– Load

– Reliability

• Path with lowest composite metric is considered best and installed in IP routing table

• Only best route is advertised to other EIGRP neighbors

• One or more backup routes can also be precalculated per destination

EIGRP Loop Prevention

• EIGRP guarantees loop-free topology through usage of…

– Split Horizon

• Don’t advertise routes out the link they came in on

– DUAL Feasibility Condition

• If your metric is lower than mine, you are loop-free

EIGRP Reconvergence

• Active EIGRP neighbor adjacency reduces convergence time

– Adjacent neighbors’ hello packets contain “hold time”

– If no hello is received within hold time, neighbor declared unreachable

• When neighbor is lost…

– Paths via that neighbor are removed from topology and routing table

– If backup routes exist, they become new best paths and are inserted in routing table

• In this case EIGRP can have sub-second convergence

– If no backup routes exist, DUAL must run again

• When best path is lost and no backup routes exist, route goes into

“active” state and “active timer” starts

– Stable routes not in active state are considered “passive”

• EIGRP “QUERY” message is reliably sent to remaining neighbors asking if there is an alternate route

– QUERY is propagated to all neighbors within EIGRP “QUERY domain” or “flooding domain”

• Summarization and EIGRP Stub feature limits the QUERY domain

– Neighbors respond with EIGRP “REPLY” packet indicating if alternate route is available

• If alternate route exists, DUAL recalculates new best path

• If no alternate route, prefix removed from topology table

• If active timer expires and no REPLY received, route is declared “Stuck-In-Active” (SIA) and removed from topology table

CCIE R&S Written Overview: RIP

RIP Overview

• Standards Based Distance Vector IGP

– Uses split-horizon, poison reverse, count to infinity

– UDP port 520 for transport

• Two versions

– RIPv1

• Classful

• Updates as broadcast

– RIPv2

• Classless

• Updates as multicast to 224.0.0.9

Enabling RIP

• Enable the global process

– router rip

• Enable the interface process

– network [address]

– Matches major network only

RIP Features

• RIP Versions

– Supports both v1 and v2 concurrently

• Summarization

– RIPv2 is classless but does automatic classful summarization by default

– Manual summaries can be configured per interface

• Split-Horizon

– Updates received in an interface will not be sent back out the same interface

• Update Types

– Configurable as broadcast, multicast, or unicast

• Metric Calculation

– 1 hop per device

– Maximum of 16 hops

– Metric can be changed with offset list

• Convergence Timers

– Four timers of update, invalid, holddown, and flush

• Authentication

– Clear text and MD5 update authentication

• Filtering Updates

– Passive Interfaces

– Distribute Lists

– Offset Lists

– Administrative Distance

RIP Command Reference

• Very little functionality in RIP compared to other IGPs

Ciscos website/documentation contains all commands used in the RIP process.

CCIE R&S Written Overview: IPv4 Overview

IPv4 Routing Protocols Overview

• Static Routing

• RIPv2

• EIGRP

• OSPF

• BGP

• Policy Routing

• IP Tunneling

IP Routing Overview

• Longest Match Routing (Highest number of bits matching destination route/Most specific subnet)

• Metric vs. Distance

– Same protocol vs. different protocols

Administrative Distance Reference

Connected 0
Static 1
EIGRP Summary 5
External BGP 20
Internal EIGRP 90
IGRP* 100
OSPF 110
IS-IS 115
RIP 120
EGP* 140
ODR 160
External EIGRP 170
Internal BGP 200
Infinite 255 *Deprecated