1 © 2000, Cisco Systems, Inc. Integrated-ISIS Route Leaking.

1© 2000, Cisco Systems, Inc.

Integrated-ISISRoute LeakingIntegrated-ISISRoute Leaking


ISIS routing LevelsISIS routing Levels

L1

L1L2

L1

L1L2L12. Level-1 LSP withIP prefix: 10.1.0.0/16

2. Level-1 LSP withIP prefix: 10.10.0.0/16

L1

1. Level-1 LSP withAttached-bit (used as a default route by all level-1routers

3. Level-2 LSP withIP prefix: 10.1.0.0/16IP prefix: 10.10.0.0/16

Routers within an area do not have any routing information about prefixes originated outside the area. Default routing (ATT-bit or explicit 0.0.0.0/0) is used to reach destinations outside the area.


ISIS routing LevelsISIS routing Levels

L1L2

L1

L2L2

L1L2L1

L1L2

L11. Level-1 LSP withIP prefix: 10.1.0.0/16

4. At this point the prefix 10.1.0.0/16 will NOT be inserted in the L1 LSP (no route leaking by default)

2. Level-2 LSP withIP prefix: 10.1.0.0/16 2. Level-2 LSP with

IP prefix: 10.1.0.0/16



2. At this point prefix 10.1.0.0/16 will be inserted in L2 LSP

2. At this point prefix 10.1.0.0/16 is inserted in L2 LSP

0. Level-1 LSP with ATT bit set




Route LeakingRoute Leaking

• New ISIS feature/capability described in draft-ietf-isis-domain-wide

• Allows L1L2 routers to insert in their L1 LSP IP prefixes learned from L2 database if also present in the routing table

• ISIS areas are not stubby anymore



L1L2

L1

L1L2L1L2

L1L2L1

L1L2

L1


4. At this point prefix 10.14.0.0/16 will NOT be inserted in L2 LSP since it has the Down-bit set

3. Level-1 LSP with IP prefix: 10.14.0.0/16 Up/Down-bit set

3. At this point prefix 10.14.0.0/16 will be inserted in L1 LSP since route leaking is configured AND the prefix is present in the routing table as a L2 route




L1L2

L1

L1L2


L1L2

L1L2L1

L1L2

L11. Level-1 LSP withIP prefix: 10.1.0.0/16

2. Level-2 LSP withIP prefix: 10.1.0.0/16 2. Level-2 LSP with

IP prefix: 10.1.0.0/16

5. At this point the prefix 10.1.0.0/16 will NOT be inserted in the L1 LSP since a L1 route is preferred in the routing table

5. At this point the prefix 10.1.0.0/16 will NOT be inserted in the L1 LSP since a L1 route is preferred in the routing table

3. Level-1 LSP withIP prefix: 10.1.0.0/16Up/Down-Bit set




• For IP only

• Prefixes MUST be present in the routing table as ISIS level-2 routes

Otherwise no leaking occurs

Same criteria as L1 ---> L2

Inter-area routing is done through the routing table



• Solution for several issues:

• optimal inter-area routing

• BGP shortest path to AS exit point

• MPLS-VPN (PEs loopback reachability)



• When leaking routes from L2 backbone into L1 areas a loop protection mechanism need to be used in order to prevent leaked routes to be re-injected into the backbone



• UP/Down bit Extended IP Reachability TLV (135) contains Up/Down bit

Described in draft-ietf-isis-traffic

• UP/Down bit is set each time a prefix is leaked into a lower level

Allows multi-level hierarchy route leaking

• Prefixes with Up/Down bit set are NEVER propagated to a upper level



• Route leaking works also with IP Internal Reachability Information (TLV 128)

IP External Reachability Information (TLV 130)

Also known as: Old-style or Narrow-metric TLVs



• TVLs 128 and 130 have a metric field that consists of 4 TOS metrics

The first metric, the so-called "default metric", has the high-order bit reserved (bit 8) Routers must set this bit to zero on transmission, and ignore it on receipt

• The high-order bit in the default metric field in TLVs 128 and 130 becomes the Up/Down bit



• Route leaking can be used with both:

IP reachability TLVs: 128 and 130

Default Metric Bit-8 is used as Up/Down bit

Extended IP Reachability TLV: 135

Up/Down bit



• Route leaking is implemented in both 12.0S and 12.1

Cisco IOS 12.0S command

advertise ip L2-into-L1 <100-199>

Cisco IOS 12.1 command

redistribute isis ip level-2 into level-1 distribute-list <100-199>

• Both commands are supported

• 12.0S command will be converted into 12.1 syntax


Route Leaking A bit of bits history


• TVLs 128 and 130 have a metric field that consists of 4 TOS metrics

The first metric, the so-called "default metric", has the high-order bit reserved (bit 8) Routers must set this bit to zero on transmission, and ignore it on receipt

Bit 7 indicates the metric type (internal or external), and may be set to zero indicating internal metrics, or may be set to 1 indicating external metrics (only in TLV130)




• However…………….

• Cisco IOS software BEFORE 12.1 uses bit 8 as I/E bit !!!!

Since VERY early stage of ISIS implementation

• When doing (under “router isis”)redistribute …. metric-type external

!!!!! bit 8 is set instead of bit 7 !!!!!!!



R2

R3-12.1

2. Level-2 LSP with IP prefix: 10.1.0.0/16

2. At this point R3 (12.1) will NOT insert 10.4.0.0/16 in its L2 LSP since it considered the Up/Down bit set

R1-12.0S

1. R1 (12.0S) redistributes into ISIS with “metric-type external”Therefore bit-8 is (wrongly) set

1. Level-1 LSP withIP prefix: 10.4.0.0/16Bit-8 is set


• L1L2 router using Cisco IOS from 12.1 will NOT put in L2 LSP any prefix with Up/Down bit set

Even if route leaking is not configured

Up/Down bit for TLVs 128, 130 and 135



R2

R3

6. LOOP !!!4. R1 (12.0S) Advertises the prefix back to level-2 since itdoesn’t understand the Down-bit


• L1L2 router using Cisco IOS 12.0S will put in L2 LSP any TLV128 or TLV130 prefix since Up/Down bit is not recognized

Even if route leaking is not configured

R1-12.0S

3. Level-1 LSP withIP prefix: 10.4.0.0/16Down-bit Set (bit-8)




Always use TLV-135 Always use TLV-135 Configure routers with:Configure routers with:

Router isisRouter isis metric-style wide metric-style wide

Route Leaking Recommendation

Route Leaking Recommendation

Don’t use Don’t use ““metric-type external”metric-type external”

when redistributing into ISISwhen redistributing into ISIS

1 © 2000, Cisco Systems, Inc. Integrated-ISIS Route Leaking.

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Transcript of 1 © 2000, Cisco Systems, Inc. Integrated-ISIS Route Leaking.