© 2011 Juniper Networks, Inc. All rights reserved. | www.juniper.net

Multilevel IS-IS Networks

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IS-IS Level 2 Network

Area 49.1111 Area 49.2222 Area 49.3333

L2

L2

L2

L2

L2

L2L2

L2

L2

L2

L2

L2

L2

Area 49.1111L2 PDU

Area 49.2222L2 PDU

Area 49.3333L2 PDU

Area 49.1111L2 PDU

Area 49.2222L2 PDU

Area 49.3333L2 PDU Area 49.1111

L2 PDUArea 49.2222

L2 PDU

Area 49.3333L2 PDU

L2 = Interface configuration

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IS-IS Level 1 Network

Area 49.4444

L1

L1

L1 L1

L1

L1L1

L1

L1

L1

L1

L1

Area 49.4444L1 PDU

L1 = Interface configuration

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L1 or L2 = Interface configuration

IS-IS Level 1 and Level 2 Network

Area 49.5555 Area 49.6666 Area 49.7777

L2

L1

L2 L2L1

L2

L2

L2

L2

L2

L2

L2

L1

L1

L1

L2

L2

L1

L1

L1

Area 49.5555L2 PDU

Area 49.6666L2 PDU

Area 49.7777L2 PDU

Area 49.5555L1 PDU

Area 49.7777L1 PDU

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Multilevel Operation An L1/L2 IS-IS network operates in a similar fashion

to an OSPF NSSA with no summaries• Local L1 routes are advertised into Level 2• External routes can be advertised into an L1 area

The L1/L2 border is a natural route boundary• L2 routes are not advertised into L1 areas by default• External L1 routes are not advertised to Level 2 by default

• Route leaking policies are used to modify this default behavior• Using only wide metrics eliminates internal/external distinction

L1/L2 attached routers set the attached bit in their L1 LSPs• L1 routers install a locally generated 0/0 default route to

the closest L2 attached router• Disable with ignore-attached-bit command

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Ignoring the Attached Bit

Level 2 to Level 1 route leaking policy in place• Limited LSP flooding scope provides some

protection from software or network faults• Default route unnecessary

Area 49.6666 Area 49.7777

L2L2

L1L2

L2L2

L2

L1

L1

ignore-attached-bit

L2

L1L2L2

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IS-IS Multilevel Configuration

Each IS-IS interface operates at both Level 1 and Level 2, by default• Disable a specific level to not have the interface operate at

that level•lo0 interface will be passive at both levels in this example

• Disable at a particular level to prevent lo0 address advertisement in that level

protocols { isis { interface so-0/0/0.0 { level 1 disable; } interface ge-0/1/0.0 { level 2 disable; } interface lo0.0; }}

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Case Study: Routing Leaking

Area 49.0001

L2

L1

R5R3

R1

L1

Level 2 routes are to be advertised into Area 49.0001• Requires routing policy on L1/L2 area border

router R3• Use the from level 2/to level 1 syntax

Area 49.0002

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Case Study: Route Leaking Policy

Use a policy to advertise (leak) routes across an L1/L2 area border

Routes advertised from an L2 area into an L1 area have the up/down bit set to down• Ensures that another L2 router will not re-advertise the route back

into an L2 area to avoid routing loops

[edit policy-options]user@router# show policy-statement route-leak { term L2-to-L1 { from { protocol isis; level 2; route-filter 192.168.16.0/20 orlonger; } to level 1; then accept; }}

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Route Summarization The L1/L2 area border is a natural place to

summarize routing information• Override the default route flooding between the areas

with a routing policy Create aggregate routes in local routing table

• Policy required to advertise aggregate routes into another level—use from /to level for maximum control

[edit policy-options]user@router# show policy-statement external-L1-summary-route term on-the-L1L2-router { from { protocol aggregate; route-filter 172.16.20.0/22 exact; } to level 2; then accept;}

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Case Study: Internal Level 1 Route Summarization

Area 49.0001

L2

L1

R5R3

R1

L1

10.0.4.0/22

Internal Level 1 routes can be summarized• Requires routing policy and a local aggregate

route• For example, suppress specific routes in the

10.0.4.0/22 block and advertise a single 10.0.4.0/22 summary route

Area 49.0002

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Case Study: Level 1 Route Summarization Policy

Sample policy for summarizing internal Level 1 routes•Requires local 10.0.4.0/22 aggregate definition on

R3•Use of to ensures that summary route is not

injected into the Level 1 area[edit]user@R3# show policy-options policy-statement internal-L1-summary-route term local-summary-route { from { protocol aggregate; route-filter 10.0.4.0/22 exact; } to level 2; then accept;}term suppress-specifics { from { route-filter 10.0.4.0/22 longer; } to level 2; then reject;}

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[edit protocols isis]

user@R3# show

export [ external-L1-summary-route internal-L1-summary-route ];

interface at-0/2/0.0 {

level 1 disable;

}

interface at-0/2/1.0 {

level 2 disable;

}

interface lo0.0;

Applying IS-IS Policies

Apply IS-IS policies at the global level of the isis stanza• Multiple export polices can be applied, or a single

policy with multiple terms can be used