Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

OSPF Question

Can anyone explain me the Options counter?

sh ip ospf nei 172.21.2.10

Neighbor 172.21.2.10, interface address 172.21.2.10

In the area 0.0.0.0 via interface Serial0.21

Neighbor priority is 1, State is FULL

Options 66

Dead timer due in 00:01:51

  • Other Network Infrastructure Subjects
2 REPLIES
New Member

Re: OSPF Question

Greetings:

This URL will give you some general information.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/np1_r/1rprt1/1rospf.htm#xtocid2729437

More specifically, the options field is associated to a neighbor, as defined in the RFC

67= 01000011

O ET

P

Therefore, we have the E-bit set => neighbor is not a stub area

The T bit is set = > the router supports TOS (not RFC's)

The OP bit is set => the originating router supports Opaque LSA's

From RFC 2328 (realize that not all the fields are defined yet)

A.2 The Options field

The OSPF Options field is present in OSPF Hello packets, Database

Description packets and all LSAs. The Options field enables OSPF

routers to support (or not support) optional capabilities, and to

communicate their capability level to other OSPF routers. Through

this mechanism routers of differing capabilities can be mixed within

an OSPF routing domain.

When used in Hello packets, the Options field allows a router to

reject a neighbor because of a capability mismatch. Alternatively,

when capabilities are exchanged in Database Description packets a

router can choose not to forward certain LSAs to a neighbor because

of its reduced functionality. Lastly, listing capabilities in LSAs

allows routers to forward traffic around reduced functionality

routers, by excluding them from parts of the routing table

calculation.

Five bits of the OSPF Options field have been assigned, although

only one (the E-bit) is described completely by this memo. Each bit

is described briefly below. Routers should reset (i.e. clear)

unrecognized bits in the Options field when sending Hello packets or

Database Description packets and when originating LSAs. Conversely,

routers encountering unrecognized Option bits in received Hello

Packets, Database Description packets or LSAs should ignore the

capability and process the packet/LSA normally.

+------------------------------------+

| * | * | DC | EA | N/P | MC | E | * |

+------------------------------------+

The Options field

E-bit

This bit describes the way AS-external-LSAs are flooded, as

described in Sections 3.6, 9.5, 10.8 and 12.1.2 of this memo.

MC-bit

This bit describes whether IP multicast datagrams are forwarded

according to the specifications in [Ref18].

Moy Standards Track [Page 187]

RFC 2328 OSPF Version 2 April 1998

N/P-bit

This bit describes the handling of Type-7 LSAs, as specified in

[Ref19].

EA-bit

This bit describes the router's willingness to receive and

forward External-Attributes-LSAs, as specified in [Ref20].

DC-bit

This bit describes the router's handling of demand circuits, as

specified in [Ref21].

Thanks...Steve

New Member

Re: OSPF Question

Great,

thank you.

Andre

190
Views
0
Helpful
2
Replies