Inter-domain routing and BGP BGP in JunOS Olof Hagsand KTH/CSC · OSPF accept all OSPF routes reject all (OSPF uses LSAs) MPLS accept all MPLS routes export all active MPLS routes.

DD2491, p1 2008

Inter-domain routing and BGP

BGP in JunOS

Olof Hagsand KTH/CSC

DD2491 p1 2008

DD2491, p1 2008

JunOS Routing modelNeighbours

Protocols

Neighbours

Protocols

RIB

FIB

ExportImport

Note: Export policies may be applied only to active routes!

Protocol Default import action Default export action

direct and static accept all N/A

RIP accept all RIP routes reject all

BGP accept all BGP routes export all active BGP routes

IS-IS accept all IS-IS routes reject all (IS-IS uses LSAs)

OSPF accept all OSPF routes reject all (OSPF uses LSAs)

MPLS accept all MPLS routes export all active MPLS routes

DD2491, p1 2008

BGP Routing Process Model

• Pool of routes received from peers

• Import policy for filtering and attribute manipulation

• Decision process to select best routes

• Pool of routes used by router

• Export policy for filtering and attribute manipulation

• Pool of routes that the router advertises

Peer

importpolicy

decisionprocess

RIBRIB exportpolicy

Peer

Peer Peer

DD2491, p1 2008

BGP Routing Information Bases (BGP RIBs)CISCO version

AdjRIBIn

AdjRIBIn

AdjRIBIn

AdjRIBIn

BGPdecisionprocess

LocRIB

AdjRIBOut

AdjRIBOut

AdjRIBOut

AdjRIBOut

Input Policy Engine Output Policy Engine

© 2001 Cisco Press

DD2491, p1 2008

BGP RIBs

BGP routing table consists of three parts

• Adj-RIB-In

– One per peer BGP speaker

– Stores routing information learned from peer

– Filtered/manipulated input policy engine

• Loc-RIB

– Selected best routes by decision process to each available destination

• Adj-RIB-Out

– One per peer BGP speaker

– Stores routing information selected for advertisement to peer

– Output policy applied to Loc-RIB before going into Adj-RIB-Out

– This is redistributed if REFRESH capability is used

DD2491, p1 2008

Import/Export Policy

• Import policy

– Affects routes received from peer BGP speakers

– Filtering based on IP prefixes, AS_PATH and other BGP attributes

– Manipulates path attributes to influence its own decision process

• Export policy

– Affects routes in Loc-RIB (candidates for advertisement)

• In JunoS: only active BGP routes

– Differentiates between internal and external peers

DD2491, p1 2008

BGP example policies

AS1

importpolicy

decisionprocess

RIBRIB exportpolicy

AS3

AS2 AS4

10.0.0.0/240/0

10.0.0.0/2410.2.0.0/240/0

•Deny 0/0 from AS1•Give 10.0.0.0/24 from AS1 better pref

•Use 10.0.0.0/24 from AS1•Use 0/0 and 10.2.0.0/24 from AS2

•Do not propagate 0/0•Do not announce 10.2.0.0/24 to AS3•Give 10.0.0.0/24 metric 10 toward AS4

10.0.0.0/24

10.0.0.0/2410.2.0.0/24

0/0 AS2 BGP10.0.0.0/24 AS1 BGP

10.2.0.0/24 AS2 BGP

DD2491, p1 2008

Configuring BGP in JunOS

• Many configurations can be made on global, group and peer level.

• More specific is preferred (peer before group before global)

protocol bgp {

mtu-discovery Global properties

group external-peers {

type external; Group properties

peer-as 42;

neighbor 192.168.200.13;

neighbor 192.168.200.14;

neighbor 192.168.200.14{

peer-as 93; Peer properties

}

}

}

See: http://www.juniper.net/techpubs/software/junos/junos90/

DD2491, p1 2008

BGP commands in JunOS (1) advertiseinactive Advertise inactive routes

advertisepeeras Advertise routes received from the same autonomous

system

authenticationalgorithm Authentication algorithm name

authenticationkey MD5 authentication key

authenticationkeychain Key chain name

cluster Cluster identifier

damping Enable route flap damping

description Text description

disable Disable BGP

+ export Export policy

> family Protocol family for NLRIs in updates

> gracefulrestart BGP graceful restart options

> group Define a peer group

holdtime Hold time used when negotiating with a peer

+ import Import policy

includempnexthop Include NEXTHOP attribute in multiprotocol updates

ipsecsa IPSec SA name

keep How to retain routes in the routing table

DD2491, p1 2008

BGP commands in JunOS (2)

localaddress Address of local end of BGP session

> localas Local autonomous system number

localpreference Value of LOCAL_PREF path attribute

logupdown Log a message for peer state transitions

> metricout Route metric sent in MED

mtudiscovery Enable TCP path MTU discovery

> multihop Configure an EBGP multihop session

noadvertisepeeras Don't advertise routes received from the same

autonomous system

noaggregatorid Set router ID in aggregator path attribute to 0

outdelay How long before exporting routes from routing table

passive Do not send open messages to a peer

> pathselection Configure path selection strategy

peeras Peer autonomous system number (1..65535)

preference Preference value

removeprivate Remove wellknown private AS numbers

tcpmss Maximum TCP segment size (1..4096)

> traceoptions Trace options for BGP

DD2491, p1 2008

Routing policy: syntax and flow

• Changing the default routing policy

• Syntax:

policy-options {

policy-statement name {

term term-name {

from {

match;

}

then {

action;

}

}

}

}term1 term2

defaultpolicy

term3

term1 term2 term3

Policy 1

Policy 2

term

accept

reject

nextroute

DD2491, p1 2008

Applying policies

• Export policy evaluation order: p4->p2->p0

• If verdict (accept, reject) policy chain is terminated

• Side-effects may still apply

protocol bgp {

export p0; Global properties

import p1;

group external-peers {

type external; Group properties

export p2;

import p3;

neighbor 192.168.200.14{

export p4; Peer properties

import p5;

}

}

}

DD2491, p1 2008

Policy-options statements

• as-path name reg-exp

– Create a named AS-PATH regular expression

– Example: as-path asp0 “65000{4}”

• as-path-group { [as-path] }

• community name members [ ids ]

– Example: community c0 members 701:555

• damping name [options]

• policy-statement

• prefix-list name { ip-addresses }

– Create a named list of prefixes

– Example: prefix-list p0 {10.0.0.1; 192.168.1.0/24;}

# set policy-options ?

DD2491, p1 2008

Policy-statement matches

– aspath– community– family– localpreference– metric– neighbor– nexthop– origin– preference– prefixlist– protocol– routefilter– ...

# set policy-options policy-statement <name> term <name> from ?

DD2491, p1 2008

Policy-statement actions

• accept

• reject

• next policy

• next term

• trace

Sideeffects with accept:– as-path-prepend– community– color– external– load-balance per-packet– local-preference– metric– next-hop– origin– preference

# set policy-options policy-statement <name> term <name> then ?

DD2491, p1 2008

Lab overview

RTX3

RTX2

RTX1

RTX4

Tier1

ASX ASX+1

10.X.8.0/2410.X.9.0/2410.X.10.0/2410.X.11.0/24

AS65500

0/0/0 0/0/0

0/0/0

1/0/1

1/0/1

1/0/1

1/0/1 1/0/0

1/0/0

1/0/0

1/0/0

172.16.10X.0/30

172.16.X+1.0/30

172.16.X.0/30

.1

.1.1 .2 .2

.2

ASX1

Inter-domain routing and BGP BGP in JunOS Olof Hagsand KTH/CSC · OSPF accept all OSPF routes reject all (OSPF uses LSAs) MPLS accept all MPLS routes export all active MPLS routes.

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