MULTI-AREA OSPF ADJACENCY ON IOS-XR

By : Genie

CCIE R&S, SP, Security

CoderGenie Technologies

Agenda

• Define the problem

• All about Multi-Area Adjacency

• Configuration Example

• Demo

Background

• OSPF always prefers intra-area routes over inter-area routes

• This may causes suboptimal path depending on which area the link is

configured

• The virtual link is not the feasible in reality to solve the suboptimal

routing issue, while deploying the parallel physical or logical links for

each area should not scale from the aspect of cost, capacity, and

operational impact

Solution

• A feature such that a single physical link can belongs to the multiple

area without creating sub-interface would be effective for this kind of

OSPF suboptimal routing issue

OSPF Area

• OSPF topology can be segmented into multiple areas to control the

size of topology, and to set the effective summarization and

aggregation

• Determining the optimal point to divide into areas (where to set ABRs

and their links) is one of the keys to design the OSPF network

• ABR hides the topology information from one area to another, and

creates individual flooding domains per area

• OSPF link can belong to one area uniquely per RFC 2328

• It may be the case that the packets hops several routers to reach the

neighboring router because OSPF always prefers the intra-area route

than inter-area

Suboptimal path at Area Boundary

• If Link X is in area 0, R3 will

choose an path through R5, R6,

and R4 to reach 192.168.2.0/24

rather than Link X

• This is because OSPF always

prefers intra-area routes over inter-

area routes

R1 R2

R3 R4

R5 R6

19

2.1

68.1

.0/2

4

19

2.1

68.2

.0/2

4

Area 0

Link X

Area 1

Possible solution options

• 1) Having multiple secondary address and establish the adjacencies

over them through different areas

• Needs additional IP address per area

• 2) Configuring sub-interfaces and assign them to each area

• Needs additional IP address per area

• 3) Having multiple logical OSPF internal interface constructs which

are unnumbered p2p and belong to each area (OSPF Multi-Area

Adjacency specified in RFC 5185)

• Only link type 1 (p2p) but no link type 3 (stub network) is advertised for

multi-area adjacency to advertise only topological information

Summary

• ABRs will simply establish multiple adjacencies belong to the different

areas over a single interface

• Each multi-area adjacency is announced as a p2p link in the

configured area

• Link type 3 (stub network) is not advertised for multi-area adjacency

• It means the interface address is not considered as a reachable leaf but

only topological path in multi-area adjacency

• Primary adjacency using the link will operate and advertise the link in

a usual manner consistent with RFC 2328

Advertising Multi-Area Adjacencies

• Multi-area adjacencies are announced as p2p links

• Once the the router’s multi-area adjacency reaches the Full

state, it will be added as a link type 1 (p2p) to Router LSA

• Link ID = Remote’s Router ID

• Link Data = Neighbor’s IP Address or IfIndex (if underlying

interface is unnumbered)

• Unlike numbered p2p links, no type 3 (stub network) link is

advertised for multi-area adjacency

Configuration Example

P1

P1

PE1

Bundle-ether 1

Bundle-ether 1

lo0: 10.255.255.100/32

Area 0

Area 1Multi-area adj

(Area 0)Multi-area adj

(Area 1)

Gi0/4/0/0Gi0/4/0/4

Gi0/1/0/2

Gi0/1/0/4

router ospf 1

router-id 10.255.255.1

area 0

interface Bundle-Ether1

cost 5

network point-to-point

!

interface Loopback0

!

interface GigabitEthernet0/4/0/0

cost 10

network point-to-point

!

!

area 1

multi-area-interface Bundle-Ether1

cost 10

!

interface GigabitEthernet0/4/0/4

cost 10

network point-to-point

Multi-Area Adjacency link must be configured as point-to-point in primary area

Multi-Area Adjacency NeighborsRP/0/5/CPU0:GSR-01#show ospf neighbor

* Indicates MADJ interface

Neighbors for OSPF 1

Neighbor ID Pri State Dead Time Address Interface

10.255.255.2 1 FULL/BDR 00:00:38 10.0.12.2 Bundle-Ether1

Neighbor is up for 4d16h

10.255.255.3 1 FULL/ - 00:00:39 10.0.13.2 GigabitEthernet0/4/0/0

Neighbor is up for 00:08:49

10.255.255.2 1 FULL/ - 00:00:32 10.0.12.2 Bundle-Ether1*

Neighbor is up for 4d16h

10.255.255.100 1 FULL/ - 00:00:36 10.1.10.2 GigabitEthernet0/4/0/4

Neighbor is up for 3d15h

DEMO

References

• RFC 5185 OSPF Multi-Area Adjacency

• http://blog.codergenie.com/blog/post/2012/07/20/OSPF-Multi-Area-

Adjacency.aspx

CODERGENIE TECHNOLOGIES

http://www.codergenie.com/trainings

http://blog.codergenie.com

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