RFC 4915
Multi-Topology (MT) Routing in OSPF
RFC 4915: Multi-Topology (MT) Routing in OSPF 💖
okay bestie, this is the RFC that teaches OSPF to run multiple topologies at once 🦋 ✨
Title block facts: it's from the Network Working Group and is RFC 4915, titled Multi-Topology (MT) Routing in OSPF. It's a Standards Track document dated June 2007. The authors are P. Psenak (Cisco Systems), S. Mirtorabi (Force10 Networks), A. Roy (Cisco Systems), L. Nguyen (Cisco Systems), and P. Pillay-Esnault (Cisco Systems). Its header has no "Obsoletes" or "Updates" line.
Status of This Memo ✨
ok so this doc is an Internet standards track protocol for the Internet community 🌸 it wants discussion and suggestions for improvements, lowkey begging for feedback. for the standardization state and status, check the current edition of the "Internet Official Protocol Standards" (STD 1) 💅 and yes, distribution of this memo is unlimited, so share it with everyone 🫶
Copyright Notice 🎀
Copyright (C) The IETF Trust (2007).
Abstract 🦋
this RFC describes an extension to Open Shortest Path First (OSPF) that defines independent IP topologies called Multi-Topologies (MTs) 🌈 MTs can compute different paths for unicast traffic, multicast traffic, different classes of service based on flexible criteria, or an in-band network management topology. it's giving main character energy 👑
there's also an optional extension for excluding selected links from the default topology 🧚
Table of Contents 📖
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Differences between Multi-Topology and TOS-Based
Routing . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2.2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Base MT Functional Specifications . . . . . . . . . . . . . . 4
3.1. MT Area Boundary . . . . . . . . . . . . . . . . . . . . . 4
3.2. Adjacency for MTs . . . . . . . . . . . . . . . . . . . . 4
3.3. Sending OSPF Control Packets . . . . . . . . . . . . . . . 5
3.4. Advertising MT Adjacencies and the Corresponding IP
Prefixes . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4.1. Inter-Area and External Routing . . . . . . . . . . . 5
3.5. Flushing MT Information . . . . . . . . . . . . . . . . . 6
3.6. MT SPF Computation . . . . . . . . . . . . . . . . . . . . 6
3.7. MT-ID Values . . . . . . . . . . . . . . . . . . . . . . . 6
3.8. Forwarding in MT . . . . . . . . . . . . . . . . . . . . . 6
4. Default Topology Link Exclusion Functional Specifications . . 7
4.1. Exclusion of Links in the Default Topology . . . . . . . . 7
4.2. New Area Data Structure Parameter . . . . . . . . . . . . 7
4.3. Adjacency Formation with Link Exclusion Capability . . . . 8
4.4. OSPF Control Packets Transmission over Excluded Links . . 9
4.5. OSPF LSA Advertisement and SPF Computation for
Excluded Links . . . . . . . . . . . . . . . . . . . . . . 9
5. Interoperability between MT-Capable and Non-MT-Capable
Routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Demand Circuit Compatibility Considerations . . . . . . . 10
6. Migration from Non-MT-Area to MT-Area . . . . . . . . . . . . 10
7. MT Network Management Considerations . . . . . . . . . . . . . 11
7.1. Create Dedicated Management Topology to Include All
the Nodes . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2. Extend the Default Topology to All the Nodes . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 13
Appendix B. OSPF Data Formats . . . . . . . . . . . . . . . . . . 13
B.1. Router-LSAs . . . . . . . . . . . . . . . . . . . . . . . 13
B.2. Network-LSAs . . . . . . . . . . . . . . . . . . . . . . . 15
B.3. Summary-LSAs . . . . . . . . . . . . . . . . . . . . . . . 16
B.4. AS-external-LSAs . . . . . . . . . . . . . . . . . . . . . 17
B.5. Type-7 AS-external-LSAs . . . . . . . . . . . . . . . . . 181. Introduction 🌸
so OSPF uses a fixed packet format, which makes backward-compatible extensions tricky 😭 the OSPF specification [OSPF] brought in the Type of Service (TOS) metric through an earlier spec, [TOS-OSPF], so you could announce a different link cost per TOS 🍓 TOS-based routing as described in [TOS-OSPF] was never deployed and was later deprecated. [M-ISIS] describes a similar mechanism for ISIS.
the plan is to reuse those TOS-based metric fields 💅 they get redefined and used to advertise different topologies, with a separate metric for each one ✨
1.1. Differences between Multi-Topology and TOS-Based Routing 🦋
Multi-Topology routing is different from [TOS-OSPF] TOS-based routing in these ways:
- ok so with TOS routing [TOS-OSPF], the TOS or Diffserv Code Point (DSCP) in the IP header gets mapped straight to the matching OSPF SPF calculation and routing table 🧠 that limits the topologies to the 16 TOS values in Section 12.3 of [TOS-OSPF]. with Multi-Topology routing, deciding which traffic maps to which topology is out of scope for this document 🙌
- with TOS routing [TOS-OSPF], traffic that's unreachable in the routing table for its TOS falls back to the TOS 0 routing table 💀 with Multi-Topology routing, this fallback is optional 🌈
- with TOS routing [TOS-OSPF], you couldn't exclude individual links or prefixes from a topology 😭 if the LSA options T-bit was set, every link or prefix was either advertised explicitly or defaulted to the TOS 0 metric. with Multi-Topology routing, any link or prefix that isn't advertised for a specific topology just doesn't exist in that topology 🫶
2. Terminology 📖
2.1. Requirements Notation 💜
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC-KEYWORDS].
2.2. Terms 🎀
we're using this terminology in the doc:
- Non-MT router: routers that do not have the MT capability.
- MT router: routers that have MT capability as described in this document.
- MT-ID: the renamed TOS field in LSAs, used to represent a Multi-Topology ID.
- Default topology: the topology built using the TOS 0 metric (default metric).
- MT topology: the topology built using the corresponding MT-ID metric.
- MT: shorthand for MT topology.
- MT#0 topology: the representation of the TOS 0 metric in MT-ID format.
- Non-MT-Area: an area that contains only non-MT routers.
- MT-Area: an area that contains both non-MT routers and MT routers, or only MT routers.
3. Base MT Functional Specifications 🧚
3.1. MT Area Boundary 🌷
each OSPF interface belongs to one single area, and all MTs sharing that link need to belong to the same area 💖 so the area boundaries are the same for every MT, but each MT's attachment to the area is independent ✨
3.2. Adjacency for MTs 🦋
each interface can be configured to belong to a set of topologies 🌈 even when it's in multiple topologies, a single adjacency still forms with neighbors on the interface. adjacency formation doesn't depend on the topologies configured on the local interface or on the neighboring router, which is so clean 😭
3.3. Sending OSPF Control Packets 📮
sending OSPF control packets is unchanged from [OSPF] 💅 but for control packets sent to the far end of a virtual link, the transit area path MUST be made up of links in the default topology, and those control packets MUST be forwarded using the default topology 🔥
3.4. Advertising MT Adjacencies and the Corresponding IP Prefixes 🍓
the TOS metric field gets reused to advertise a topology-specific metric for links and prefixes in that topology ✨ in the payload of Router, Summary, and Type-5 and Type-7 AS-external-LSAs (see Appendix B), the TOS field is redefined as MT-ID 🫶
MT-ID metrics in LSAs SHOULD be in ascending order of MT-ID 🧠 if an MT-ID shows up in an LSA or router link more than once, the metric in the first MT-ID instance MUST be used 💅
when a router brings up a FULL adjacency over a link that belongs to a set of MTs, it advertises the matching cost for each MT-ID 🌈
by default, all links are in the default topology, and every advertised prefix in the default topology uses the TOS 0 metric, just like in [OSPF] 🎀
each MT has its own MT-ID metric field 🦋 when a link isn't part of a given MT, its MT-ID metric is left out of the LSA 🙈
the Network-LSA has no MT information, because the Designated Router (DR) is shared by all MTs 💜 so there's no change to the Network-LSA at all ✨
3.4.1. Inter-Area and External Routing 🌸
in Summary-LSAs and Type-5 and Type-7 AS-external-LSAs, the TOS metric fields are redefined as MT-ID metric fields, and they're used to advertise prefix and router reachability in the matching topology 🧚
when a router originates a Summary-LSA, or a Type-5 or Type-7 AS-external-LSA that belongs to a set of MTs, it includes the matching cost for each MT-ID 🔥 by default, the prefix participates in the default topology and uses the TOS 0 metric for it, similar to standard OSPF [OSPF] 💖
setting the P-bit in a Type-7 AS-external-LSA is topology independent and applies to all MT-ID advertised in the body of the LSA 🫶
3.5. Flushing MT Information 🧹
when a link or prefix that existed or was reachable in a topology isn't part of that topology anymore, or is unreachable there, a new version of the LSA MUST be originated, leaving out the metric info for that link or prefix in that topology 😭
the MT metric in the Router-LSA can also be set to the maximum possible metric, so the router can become a stub in a certain topology [STUB] 🌈
3.6. MT SPF Computation 💅
by looking at MT-ID metrics in the LSAs, OSPF computes multiple topologies and finds paths to IP prefixes for each MT independently ✨ a separate SPF runs for each MT-ID to find independent paths to IP prefixes 🦋
Network-LSAs are used by all topologies during the SPF computation 🫶 during the SPF for a given MT-ID, only the links and metrics for that MT-ID are considered. Entries in the Router Routing table are MT-ID specific too 👑
3.7. MT-ID Values 🎀
okay so AS-External-LSAs use the high-order bit in the MT-ID field (the E-bit) for the external metric-type, which means only MT-IDs in the 0 to 127 range are valid 💅 here's the breakdown of what's reserved:
0 - Reserved for advertising the metric associated
with the default topology (see Section 4.2)
1 - Reserved for advertising the metric associated
with the default multicast topology
2 - Reserved for IPv4 in-band management purposes
3-31 - Reserved for assignments by IANA
32-127 - Reserved for development, experimental and
proprietary features [RFC3692]
128-255 - Invalid and SHOULD be ignoredthe 128-255 range is basically a no-go zone 🙈 those values SHOULD be ignored!
3.8. Forwarding in MT 🧚
this part is literally not this doc's job 💀 it doesn't spell out how the info in the different topology-specific forwarding structures gets used during packet forwarding, or how incoming packets get matched up with the right topology. BUT for things to work correctly, the forwarding behavior AND the way incoming packets are matched to a topology must be applied consistently across the whole network. no mixing and matching, bestie 🫶
4. Default Topology Link Exclusion Functional Specifications ✨
okay so Multi-Topologies basically assume that every router is in the default topology 👑 but sometimes it's super useful to kick some links out of the default topology and save them for specific types of traffic. main character energy for links 🦋
the Multi-Topologies extension that handles excluding links or prefixes from the default topology is described in the subsections below 💖
4.1. Exclusion of Links in the Default Topology 🌸
ngl this one is a little tricky 🙈 OSPF doesn't have a concept of an unreachable link. instead, any link can have a maximum metric of 0xFFFF advertised in the Router-LSA. so for link exclusion, routers have to ignore the TOS 0 metrics in Router-LSAs for the default topology, and they alternately use the MT-ID#0 metric to advertise the metric for the default topology instead. which means all routers within an area MUST agree on how the default topology metric gets advertised, no exceptions 😭
the unused T-bit is repurposed as the MT-bit in the option field. this makes sure that a link-excluding capable MT router will only form an adjacency with another router that's set up the same way 🔥
+---+---+---+---+---+---+---+---+
|DN |O |DC |EA |NP |MC |E |MT |
+---+---+---+---+---+---+---+---+
Figure 1: OSPF Option Bits- MT-bit: If DefaultExclusionCapability is enabled, the bit MUST be set in Hello packets and SHOULD be set in Database Description packet (see Section 4.2).
4.2. New Area Data Structure Parameter 💅
we're adding a brand new parameter to the Area Data Structure, and it's called:
DefaultExclusionCapability: this configurable parameter makes sure that every router in an area has this capability turned on before the default topology can be disabled on a router link in that area. without it, you'd get backward-compatibility problems, and nobody wants that 😭
when an area data structure gets created, DefaultExclusionCapability is disabled by default 🌈
if DefaultExclusionCapability is disabled:
- The MT-bit MUST be cleared in Hello packets and SHOULD be cleared in Database Description packets.
- If a link participates in a non-default topology, it's automatically included in the default topology too, so MT and non-MT routers can keep talking to each other. this happens using the TOS 0 metric field, same as in [OSPF] ✨
if DefaultExclusionCapability is enabled:
- The MT-bit MUST be set in Hello packets and SHOULD be set in Database Description packets.
- The router will only accept a Hello packet if the MT-bit is set (see Section 4.3).
when DefaultExclusionCapability is set to enabled, the router is officially said to be operating in DefaultExclusionCapability mode 👑
4.3. Adjacency Formation with Link Exclusion Capability 🦋
so for a smooth glow-up from a non-MT area to an MT area, an MT router with DefaultExclusionCapability disabled will still form adjacencies with non-MT routers, and it'll include all links as part of the default topology 🫶
a link can stop participating in the default topology once DefaultExclusionCapability is enabled. in that state, the router will only form an adjacency with routers that set the MT-bit in their Hello packets. this way, every router has DefaultExclusionCapability enabled before the default topology can get disabled on a link. no one gets left behind 💜
receiving OSPF Hello packets as defined in Section 10.5 of [OSPF] gets modified like this:
- If the DefaultExclusionCapability in the Area Data structure is set to enabled, Hello packets are discarded if the received packet does not have the MT-bit set in the Header Options.
receiving OSPF Database Description packets as defined in Section 10.6 of [OSPF] stays unchanged. packet options get validated in Hello packets, but the only option check for Database Description packets is making sure the options don't change during the database exchange process 🙈
4.4. OSPF Control Packets Transmission over Excluded Links 🧚
if DefaultExclusionCapability is enabled, the default topology can be disabled on an interface. turning off the default topology on an interface does NOT stop connected routes for that interface from getting installed in the default topology. it only changes what a router advertises in its Router-LSA ✨
and this is the best part: OSPF control packets can still be sent and received over an interface even when the default topology is disabled on that interface 💖
4.5. OSPF LSA Advertisement and SPF Computation for Excluded Links 🔥
when DefaultExclusionCapability is enabled and the link does NOT participate in the default topology, the MT-ID#0 metric doesn't get advertised. the link's TOS 0 metric gets ignored during the default topology SPF computation 💀
when DefaultExclusionCapability is enabled and a link DOES participate in the default topology, the MT-ID#0 metric is used to advertise the metric for the default topology. the link's TOS 0 metric is still ignored during the default topology SPF computation 🌸
no matter what DefaultExclusionCapability is set to, the TOS 0 metric is used for Summary-LSAs and Type-5 and Type-7 AS-external-LSAs:
- If the prefix or router does not exist in the default topology, the TOS 0 metric is set to infinity (0xFFFFFF).
- If the prefix or router exists in the default topology, the TOS 0 metric is used to advertise the metric in the default topology.
and during the summary and external prefix calculation for the default topology, the TOS 0 metric is used for Summary-LSAs and Type-5 and Type-7 AS-external-LSAs too 🎀
5. Interoperability between MT-Capable and Non-MT-Capable Routers 🫶
okay this section is about the chaos that happens when MT-capable and non-MT routers share a network 😭 the default metric field is mandatory in all LSAs (even when the metric value is 0). even if a link or prefix doesn't exist in the default topology, a non-MT router will treat that zero in the metric field as a valid metric and count the link or prefix as part of the default topology. yikes 💀
to prevent that mess, an MT-capable router will include all links as part of the default topology. if links need to be removed from the default topology, an MT-capable router must be configured in DefaultExclusionCapability mode. in this mode, routers make sure every other router in the area is also in DefaultExclusionCapability mode before they consider the MT-ID#0 metric in the SPF calculation. only then is it safe to ignore the TOS 0 metric field in Router-LSAs during the default topology SPF computation ✨
one more thing 💅 for any prefix or router to become reachable in a certain topology, there has to be a contiguous path inside that topology between the calculating router and the destination prefix or router. no gaps allowed!
5.1. Demand Circuit Compatibility Considerations 🌈
changing an area's DefaultExclusionCapability needs extra processing for area neighbors that are suppressing Hello packets, as specified in "Extending OSPF to Support Demand Circuits" [DEMAND]. when the DefaultExclusionCapability for an area changes, Hello suppression MUST be disabled for these neighbors for a period of RouterDeadInterval seconds. that means Hello packets get sent with the DC-bit clear, as specified in Section 3.2.1 of [DEMAND], during that time 🙈
after RouterDeadInterval seconds, one of two things happens: either the adjacency gets taken down because of rejected Hello packets with a conflicting MT-bit, or Hello suppression gets renegotiated 💖
6. Migration from Non-MT-Area to MT-Area 🦋
introducing MT-OSPF into a network can be done gradually. that way, MT routers and non-MT routers can both participate in the default topology, while MT routers handle the other topologies 🧚
if there's a requirement to exclude some links from the default topology in an area, ALL routers in the area MUST be in DefaultExclusionCapability mode. okay, so here's the migration plan for going from a non-MT network to an MT network 📝
picture a network with a backbone area and a bunch of non-backbone areas running in standard OSPF mode. we'd like to migrate to an MT network, either partially or all the way 🔥
- As required, part of an area is upgraded to be MT capable. The MT routers will interact with non-MT routers in the default topology and participate in other topologies as required.
- If a new non-backbone area is created for MT routers, it may be configured in DefaultExclusionCapability mode since there is no interaction required with non-MT routers. In this mode, the default topology can be excluded on links as required.
- If there are several non-backbone areas where MT is being used, it is desirable that the backbone area first be upgraded to be MT capable so that inter-area routing is ensured for MT destinations in different areas.
- Gradually, the whole network can be made MT capable.
one thing to keep in mind 💜 inter-area routing for the MT area still depends on the backbone area. so if different areas configured for a given topology need to talk to each other, the backbone area needs to be configured for that topology too.
7. MT Network Management Considerations 👑
when multiple OSPF topologies exist within a domain, some routers can be set up to participate in only a subset of the MTs in the network. this section talks about some of the options for letting operations teams or network management stations reach those routers 🌸
7.1. Create Dedicated Management Topology to Include All the Nodes 💅
this approach sets up a dedicated management topology, also known as an 'in-band' management topology. this 'mgmt' topology includes all the routers that need to be managed. the computed routes in that topology get installed into the 'mgmt' Routing Information Base (RIB). if the 'mgmt' topology uses a set of non-overlapping address space with the default topology, those 'mgmt' routes can also optionally be installed into the default RIB 🎀
there are a few perks to having duplicate 'mgmt' routes in both RIBs. the network management utilities on the system don't have to be modified to use any RIB other than the default RIB, and the 'mgmt' topology can share the same link with the default topology if it's designed that way ✨
7.2. Extend the Default Topology to All the Nodes ✨
okay so even if some nodes in the IP forwarding aren't using the default topology, we might still want to extend it to those nodes for network management 🛠️💖 the trick is that operators SHOULD set a high cost on the links in that extended part of the default topology 🧚 that way, IP data traffic won't get forwarded through those nodes when the network topology changes. no detours through the bestie who isn't on the main route 💅
8. Security Considerations 🔥
this document doesn't raise any security issues that aren't already covered in [OSPF]. so it's all the same drama as the base spec, and no new plot twists here 🫶
9. IANA Considerations 👑
the T-bit that [TOS-OSPF] defined for a router's TOS capability is now redefined as the MT-bit in this document. IANA has assigned the MT-bit as defined in Section 4.1 🦋
similarly, the TOS field for Router-LSAs, Summary-LSAs, and Type-5 and Type-7 AS-external-LSAs (as defined in [OSPF]) is redefined as MT-ID in Section 3.7 🌈
IANA also created a brand new registry called "OSPF Multi-Topology ID Values" 🎀 its assignments and registration policies are listed in Section 3.7 of this document, so go check it out 🍓
10. References 📚
10.1. Normative References
[DEMAND] Moy, J., "Extending OSPF to Support Demand Circuits",
RFC 1793, April 1995.
[NSSA] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA)
Option", RFC 3101, January 2003.
[OSPF] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[RFC-KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3692] Narten, T., "Assigning Experimental and Testing
Numbers Considered Useful", RFC 3692, January 2004.
[TOS-OSPF] Moy, J., "OSPF Version 2", RFC 1583, March 1994.10.2. Informative References
[M-ISIS] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS:
Multi Topology (MT) Routing in IS-IS", Work
in Progress, October 2005.
[STUB] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement",
RFC 3137, June 2001.Appendix A. Acknowledgments 💜
the authors would like to thank Scott Sturgess, Alvaro Retana, David Kushi, Yakov Rekhter, Tony Przygienda, and Naiming Shen for their comments on the document 🫶 special thanks to Acee Lindem for editing, and to Tom Henderson for an extensive review during the OSPF Working Group last call 👑✨
Appendix B. OSPF Data Formats 🧚
LSA content defined in [OSPF] is modified to introduce the MT-ID 💅
B.1. Router-LSAs 🌸
Router-LSAs are the Type 1 LSAs. each router in an area originates a router-LSA. the LSA describes the state and cost of the router's links (i.e., interfaces) to the area. all of the router's links to the area must be described in a single router-LSA. for details on how to construct router-LSAs, see Section 12.4.1 of [OSPF] 🦋
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|*|*|*|N|W|V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # MT-ID | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | 0 | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | 0 | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Figure 2: Router-LSA FormatB.2. Network-LSAs 🍓
Network-LSAs are the Type 2 LSAs. a network-LSA is originated for each broadcast and Non-Broadcast Multi-Access (NBMA) network in the area that supports two or more routers. the network-LSA is originated by the network's Designated Router. the LSA describes all routers attached to the network, including the Designated Router itself. the LSA's Link State ID field lists the IP interface address of the Designated Router 🌈
the distance from the network to all attached routers is zero. this is why metric fields don't need to be specified in the network-LSA. for details on how to construct network-LSAs, see Section 12.4.2 of [OSPF] ✨
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Figure 3: Network-LSA Formatnote that the network-LSA doesn't contain any MT-ID fields, because the cost of the network to the attached routers is 0 and the DR is shared by all topologies 🦋
B.3. Summary-LSAs 💖
Summary-LSAs are the Type 3 and 4 LSAs. these LSAs are originated by area border routers. Summary-LSAs describe inter-area destinations. for details on how to construct summary-LSAs, see Section 12.4.3 of [OSPF] 🧚
Type 3 summary-LSAs are used when the destination is an IP network. in this case the LSA's Link State ID field is an IP network number (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix E of [OSPF] for details). when the destination is an AS boundary router, a Type 4 summary-LSA is used, and the Link State ID field is the AS boundary router's OSPF Router ID. (to see why it's necessary to advertise the location of each ASBR, consult Section 16.4 of [OSPF].) other than the difference in the Link State ID field, the format of Type 3 and 4 summary-LSAs is identical 🎀
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Summary-LSA FormatB.4. AS-external-LSAs ✨
AS-external-LSAs are the Type 5 LSAs 🔥. They get originated by AS boundary routers, and their whole job is describing destinations that live outside the AS 🌍. If you want the nitty-gritty on how they're built, check out Section 12.4.3 of [OSPF] 📖.
Okay, so AS-external-LSAs usually describe one specific external destination 🎯. For these, the Link State ID field holds an IP network number 🧚. If it's needed, the Link State ID can also have one or more of the network's "host" bits set 🌸. Appendix E of [OSPF] has all the details on that 💜.
Now here's the cute part 🎀. AS-external-LSAs also describe a default route 🛣️. Default routes are what you fall back on when there's no specific route to the destination 🫶. When describing a default route, the Link State ID is always set to DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0 💅.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Figure 5: AS-External-LSA Format 🖼️
B.5. Type-7 AS-external-LSAs 🌈
Type-7 AS-external-LSAs are originated by AS boundary routers that sit local to an NSSA (Not-So-Stubby Area) 🦋, and they describe destinations external to the AS. The changes to Type-7 AS-external-LSAs are identical to the ones for AS-external-LSAs (Appendix A.4.5 of [OSPF]) 💜. If you want the details on how they're constructed, see Section 2.4 of [NSSA] 📚.
Authors' Addresses 🎀
Peter Psenak Cisco Systems Mlynske Nivy 43 821 09 Bratislava Slovakia
EMail: ppsenak@cisco.com
Sina Mirtorabi Force10 Networks 1440 McCarthy Blvd Milpitas, CA 95035 USA
EMail: sina@force10networks.com
Abhay Roy Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA
EMail: akr@cisco.com
Liem Nguyen Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA
EMail: lhnguyen@cisco.com
Padma Pillay-Esnault Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA
EMail: ppe@cisco.com
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📦 where this page comes from
- ietf: rfc4915 2007-06
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms