General adjacency

Hi,

As a non-native speaker of English, maybe this question is due to a language barrier, but here goes:

What is an adjacency with regards to different routing protocols?

- EIGRP (just neighbors)
- OSPF (point to point links have adjacencies with each router on the link), on LAN's only adjacencies are established with DR/BDR.
- ISIS (adjacencies are established with everyone on the lan segment, either as a level 1 or level 2 adjacency).

So, in other words, is an adjacency when two/more routers exchange routing information?

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singh8281

kpjungle wrote:

Hi,

As a non-native speaker of English, maybe this question is due to a language barrier, but here goes:

What is an adjacency with regards to different routing protocols?

- EIGRP (just neighbors)
- OSPF (point to point links have adjacencies with each router on the link), on LAN's only adjacencies are established with DR/BDR.
- ISIS (adjacencies are established with everyone on the lan segment, either as a level 1 or level 2 adjacency).

So, in other words, is an adjacency when two/more routers exchange routing information?

OSPF routers on a broadcast multiaccess networks will establish 2 way state with all neighbors as well as full adjacency with DR and BDR only.

tech-airman

kpjungle wrote:

Hi,

As a non-native speaker of English, maybe this question is due to a language barrier, but here goes:

What is an adjacency with regards to different routing protocols?

- EIGRP (just neighbors)
- OSPF (point to point links have adjacencies with each router on the link), on LAN's only adjacencies are established with DR/BDR.
- ISIS (adjacencies are established with everyone on the lan segment, either as a level 1 or level 2 adjacency).

So, in other words, is an adjacency when two/more routers exchange routing information?

kpjungle,

Adjacency means different things depending on the routing protocol. Here's the differences:

EIGRP
[list=1:2507734a6a]
The local EIGRP router sends out an EIGRP Hello packet.
A neighbor EIGRP router hears the EIGRP Hello packet, then adds the local EIGRP router into it's IP-EIGRP Neighbor Table.
The neighbor EIGRP router responds to the EIGRP Hello packet by sending an EIGRP Hello packet back to the local EIGRP router.
The local EIGRP router receives the EIGRP Hello packet, then adds the neighbor EIGRP router into it's IP-EIGRP Neighbor Table.
Since the local EIGRP router is in the IP-EIGRP Neighbor table of the neighbor EIGRP router and the neighbor EIGRP router is in the IP-EIGRP Neighbor table of the local EIGRP router, the two routers are now adjacent to each other.

[*]OSPF

The local OSPF router sends out an OSPF Hello packet.
The Designated Router for the OSPF area hears the OSPF Hello packet, adds the local OSPF router into it's OSPF Neighbor Table.
The Designated Router sends an OSPF Hello packet back to the local OSPF router.
The local OSPF router receives the OSPF Hello packet from the Designated Router, adds the Designated Router into it's OSPF Neighbor Table.
The local OSPF router and the Designated Router are now adjacent.

[*]Integrated IS-IS

The local ISIS router sends out an ISIS Hello packet.
The neighbor ISIS router hears the ISIS Hello packet, then responds with an ISIS Hello packet of it's own back to the local ISIS router.
[list=1:2507734a6a]
A local Level 1 ISIS router will form adjacencies with:
[list=1:2507734a6a]
A neighbor Level 1 ISIS router
A neighbor Level 1/2 ISIS router

[*]A local Level 1/2 ISIS router will form adjacencies with:

A neighbor Level 1 ISIS router
A neighbor Level 1/2 ISIS router
A neighbor Level 2 ISIS router

[*]A local Level 2 ISIS router will form adjacencies with:

A neighbor Level 1/2 ISIS router
A neighbor Level 2 ISIS router

[/list:o:2507734a6a]
[*]The local ISIS router and the neighbor ISIS router are now adjacent.
[/list:o:2507734a6a]
[/list:o:2507734a6a]

So in summary (a little BSCI humor attempt), think of router adjacencies like the routers became friends by saying "Hello" to each other.

I hope this helps.

kpjungle

Yep it helps.

Guess what im confused about is that according to the material, ISIS maintains adjacencies with all routers (IS's) on a lan, where OSPF only do so with the DR/BDR. Does that mean that ISIS routers sync their LSDB with all the routers? I know they only advertise that they have a connection to the pseudo-node, but do they sync with all the other routers, or just the DIS?

tech-airman

kpjungle wrote:

Yep it helps.

Guess what im confused about is that according to the material, ISIS maintains adjacencies with all routers (IS's) on a lan, where OSPF only do so with the DR/BDR. Does that mean that ISIS routers sync their LSDB with all the routers? I know they only advertise that they have a connection to the pseudo-node, but do they sync with all the other routers, or just the DIS?

kpjungle,

From what I understand, ISIS is rather strict with the Level 1 or Level 2 heirarchy. That means all Level 1 ISIS routers for an ISIS area are adjacent to each other as well as the DIS for the area. All level 2 ISIS routers for the ISIS inter-area backbone would form an adjacency with each other as well as the Level 2 DIS. So that means a Level 1 ISIS router does NOT form an adjacency with a Level 2 ISIS router. It is the Level 1/2 ISIS router that interconnects the area to the backbone, kinda like the ABR for OSPF.

So in summary, for ISIS, each router in an area (level 1) are fully meshed with each other as well as the DIS and so their LSDBs are synchronized. All Level 2 ISIS routers are fully meshed with each other as well as the DIS and so also their LSDbs are synchronized. ISIS doesn't care if the DIS changes within an area or in the backbone. Keep in mind that the ISIS adjacencies are formed as long as it's within the boundary of the given area or backbone segment.

For OSPF however, there's that whole DR and BDR election process that occurs. After the DR is elected, all OSPF routers in the network then form adjacencies with the DR. So with OSPF, the adjacencies are between the OSPF router and the DR and NOT a full mesh with all of the OPSF routers for an area.

I hope this helps.

scheistermeister

tech-airman wrote:

So in summary, for ISIS, each router in an area (level 1) are fully meshed with each other as well as the DIS and so their LSDBs are synchronized. All Level 2 ISIS routers are fully meshed with each other as well as the DIS and so also their LSDbs are synchronized. ISIS doesn't care if the DIS changes within an area or in the backbone. Keep in mind that the ISIS adjacencies are formed as long as it's within the boundary of the given area or backbone segment.

So what would be the point of the DIS if all routers of the same level and area formed adjacencies with each other and did not care if the DIS changed? There has to be something or else it wouldn't be there.

scheistermeister

Found some helpful info...

http://www.ciscopress.com/articles/article.asp?p=26850&seqNum=5

The responsibilities of LAN Level 1 and Level 2 DISs include the following:

*

Generating pseudonode link-state packets to report links to all systems on the broadcast subnetwork
*

Carrying out flooding over the LAN for the corresponding routing level

The newly elected or resigning DIS is also responsible for purging the old pseudonode LSP from the network. A DIS might resign when preempted or when disconnected from the link either by an interface shutdown or the disabling of the IS-IS process. Because of its critical role, detection of DIS failure is expedited using a shorter hello interval, which is 3.3 seconds rather than the 10 seconds used for ordinary nodes.

Forming LAN Adjacencies

When a LAN interface is enabled for IS-IS routing, the router immediately sends out IIH packets with a locally defined LAN ID, consisting of its own SysID and a unique local circuit ID. It also begins to listen to ESHs, ISHs, and IIHs to discover any connected adjacencies. It subsequently runs the DIS election process, depending on its configuration, to determine whether it is eligible to be a Level 1 or Level 2 DIS on the LAN.

The manner in which a router processes received IIHs depends on its configuration (IS type and circuit type). As in the case of point-to-point links, all IIHs received are checked for configuration conformity and authentication. The ID Length and Maximum Area Addresses fields in the received IIHs must match local values, and authentication passwords must be confirmed before the adjacency is further processed. Examples of additional information contained in hello packets are the neighbor's SysID, holding timer (holdtime), Level 1 or Level 2 priority, and configured area addresses.

A Level 1 adjacency is formed when the area addresses match unless configured otherwise. A Level 2 adjacency is formed alongside the Level 1 unless the router is configured to be Level 1-only. If no matching areas exist between the configuration of the local router and the area addresses information in the received hello, only a Level 2 adjacency is formed. If the transmitting router is configured for Level 2-only, the receiving router must be capable of forming a Level 2 adjacency; otherwise, no adjacency forms.

When a router receives a hello packet, it checks for an existing adjacency with the transmitter. If an adjacency is known, it resets the holdtime to the value in the hello received. If the neighbor is not known, the receiving router creates one, indicating the type of adjacency (Level 1 or Level 2) and sets its state to initializing until subsequent received hello packets confirm two-way communication. Routers include the MAC addresses of all neighbors on the LAN that they have received hellos from, allowing for a simple mechanism to confirm two-way communication. Two-way communication is confirmed when subsequent hellos received contain the receiving router's MAC address (SNPA) in an IS Neighbors TLV field. Otherwise, communication between the nodes is deemed one-way, and the adjacency stays at the initialized state. An adjacency must be in and up state for a router to send or process received LSPs.
Pseudonodes

As discussed in the preceding section, all IS-IS routers connected over a common LAN multicast hellos to well-known addresses, thereby forming adjacencies with each other.

After adjacency is determined, link-state information is exchanged (also referred to as LSP flooding). LSP flooding is the essence of dynamic routing information exchange between IS-IS routers. The two key requirements for LSP flooding are as follows:

*

Accuracy of information and timeliness of the updates
*

Minimum bandwidth usage and low processing overhead

Accuracy and timeliness imply spontaneous and frequent updates. This contradicts the need to conserve network resources, as stipulated by the requirement for minimum bandwidth usage and low processing overhead. This section focuses on the adjacency formation process and network resource management on multiaccess media.

To minimize the complexity of managing multiple adjacencies on multiaccess media, such as LANs, while enforcing efficient LSP flooding to minimize bandwidth consumption, IS-IS models multiaccess links as nodes, referred to as pseudonodes (see Figure 3-6). As the name implies, this is a virtual node, whose role is played by an elected DIS for the LAN. Separate DISs are elected for Level 1 and Level 2 routing. In the election process, only routers with adjacencies in an up state are considered. Election of the DIS is based on the highest interface priority, with the highest SNPA address (MAC address) breaking ties. The default interface priority on Cisco routers is 64.

Despite the critical role of the DIS in LSP flooding, no backup DIS is elected for either Level 1 or Level 2. Fortunately, this doesn't turn out to be a contentious problem because of the frequency of periodic database synchronization that occurs on broadcast links. If the current DIS fails, another router is immediately elected to play the role. As mentioned previously, the DIS transmits hello packets three times faster than the interval for other routers on the LAN. The default hello interval for the DIS is 3.3 seconds rather than the 10 seconds specified for other nodes. This allows for quick detection of DIS failure and immediate replacement.

Figure 3-6 LAN Pseudonode.

As previously expressed, periodic database synchronization on broadcast links allows preemption of the existing DIS without significant disruption of IS-IS operation on such media. This implies that an elected router is not guaranteed to remain the DIS if a new router with a higher priority shows up on the LAN. Any eligible router at the time of connecting to the LAN immediately takes over the DIS role, assuming the pseudonode functionality. No mechanism is specified for making a router ineligible to be the DIS. However, this is achievable, to some extent, by configuring a router's LAN interface with the lowest priority value relative to the priorities of other nodes on the LAN.

The IS-IS specification (ISO 10589) defines three types of designated intermediate systems, as follows:

* LAN Level 1 DIS
* LAN Level 2 DIS
* Partition-designated Level 2 IS

Election of partition-designated Level 2 ISs is specified in ISO 10589 to provide a means for repairing partitioned Level 1 areas in an IS-IS domain. An IS-IS virtual link is established over the Level 2 backbone between partition-designated Level 2 routers, which are elected from among the Level 2 routers in the partitions. Intra-area traffic is then forwarded between the partitions over the virtual link. IS-IS partition repair is not supported on Cisco routers and, therefore, is not discussed further in this book.

kpjungle

scheistermeister wrote:

Found some helpful info...

http://www.ciscopress.com/articles/article.asp?p=26850&seqNum=5

The responsibilities of LAN Level 1 and Level 2 DISs include the following:

*

Generating pseudonode link-state packets to report links to all systems on the broadcast subnetwork
*

Carrying out flooding over the LAN for the corresponding routing level

The newly elected or resigning DIS is also responsible for purging the old pseudonode LSP from the network. A DIS might resign when preempted or when disconnected from the link either by an interface shutdown or the disabling of the IS-IS process. Because of its critical role, detection of DIS failure is expedited using a shorter hello interval, which is 3.3 seconds rather than the 10 seconds used for ordinary nodes.

Forming LAN Adjacencies

When a LAN interface is enabled for IS-IS routing, the router immediately sends out IIH packets with a locally defined LAN ID, consisting of its own SysID and a unique local circuit ID. It also begins to listen to ESHs, ISHs, and IIHs to discover any connected adjacencies. It subsequently runs the DIS election process, depending on its configuration, to determine whether it is eligible to be a Level 1 or Level 2 DIS on the LAN.

The manner in which a router processes received IIHs depends on its configuration (IS type and circuit type). As in the case of point-to-point links, all IIHs received are checked for configuration conformity and authentication. The ID Length and Maximum Area Addresses fields in the received IIHs must match local values, and authentication passwords must be confirmed before the adjacency is further processed. Examples of additional information contained in hello packets are the neighbor's SysID, holding timer (holdtime), Level 1 or Level 2 priority, and configured area addresses.

A Level 1 adjacency is formed when the area addresses match unless configured otherwise. A Level 2 adjacency is formed alongside the Level 1 unless the router is configured to be Level 1-only. If no matching areas exist between the configuration of the local router and the area addresses information in the received hello, only a Level 2 adjacency is formed. If the transmitting router is configured for Level 2-only, the receiving router must be capable of forming a Level 2 adjacency; otherwise, no adjacency forms.

When a router receives a hello packet, it checks for an existing adjacency with the transmitter. If an adjacency is known, it resets the holdtime to the value in the hello received. If the neighbor is not known, the receiving router creates one, indicating the type of adjacency (Level 1 or Level 2) and sets its state to initializing until subsequent received hello packets confirm two-way communication. Routers include the MAC addresses of all neighbors on the LAN that they have received hellos from, allowing for a simple mechanism to confirm two-way communication. Two-way communication is confirmed when subsequent hellos received contain the receiving router's MAC address (SNPA) in an IS Neighbors TLV field. Otherwise, communication between the nodes is deemed one-way, and the adjacency stays at the initialized state. An adjacency must be in and up state for a router to send or process received LSPs.
Pseudonodes

As discussed in the preceding section, all IS-IS routers connected over a common LAN multicast hellos to well-known addresses, thereby forming adjacencies with each other.

After adjacency is determined, link-state information is exchanged (also referred to as LSP flooding). LSP flooding is the essence of dynamic routing information exchange between IS-IS routers. The two key requirements for LSP flooding are as follows:

*

Accuracy of information and timeliness of the updates
*

Minimum bandwidth usage and low processing overhead

Accuracy and timeliness imply spontaneous and frequent updates. This contradicts the need to conserve network resources, as stipulated by the requirement for minimum bandwidth usage and low processing overhead. This section focuses on the adjacency formation process and network resource management on multiaccess media.

To minimize the complexity of managing multiple adjacencies on multiaccess media, such as LANs, while enforcing efficient LSP flooding to minimize bandwidth consumption, IS-IS models multiaccess links as nodes, referred to as pseudonodes (see Figure 3-6). As the name implies, this is a virtual node, whose role is played by an elected DIS for the LAN. Separate DISs are elected for Level 1 and Level 2 routing. In the election process, only routers with adjacencies in an up state are considered. Election of the DIS is based on the highest interface priority, with the highest SNPA address (MAC address) breaking ties. The default interface priority on Cisco routers is 64.

Despite the critical role of the DIS in LSP flooding, no backup DIS is elected for either Level 1 or Level 2. Fortunately, this doesn't turn out to be a contentious problem because of the frequency of periodic database synchronization that occurs on broadcast links. If the current DIS fails, another router is immediately elected to play the role. As mentioned previously, the DIS transmits hello packets three times faster than the interval for other routers on the LAN. The default hello interval for the DIS is 3.3 seconds rather than the 10 seconds specified for other nodes. This allows for quick detection of DIS failure and immediate replacement.

Figure 3-6 LAN Pseudonode.

As previously expressed, periodic database synchronization on broadcast links allows preemption of the existing DIS without significant disruption of IS-IS operation on such media. This implies that an elected router is not guaranteed to remain the DIS if a new router with a higher priority shows up on the LAN. Any eligible router at the time of connecting to the LAN immediately takes over the DIS role, assuming the pseudonode functionality. No mechanism is specified for making a router ineligible to be the DIS. However, this is achievable, to some extent, by configuring a router's LAN interface with the lowest priority value relative to the priorities of other nodes on the LAN.

The IS-IS specification (ISO 10589) defines three types of designated intermediate systems, as follows:

* LAN Level 1 DIS
* LAN Level 2 DIS
* Partition-designated Level 2 IS

Election of partition-designated Level 2 ISs is specified in ISO 10589 to provide a means for repairing partitioned Level 1 areas in an IS-IS domain. An IS-IS virtual link is established over the Level 2 backbone between partition-designated Level 2 routers, which are elected from among the Level 2 routers in the partitions. Intra-area traffic is then forwarded between the partitions over the virtual link. IS-IS partition repair is not supported on Cisco routers and, therefore, is not discussed further in this book.

Thanks, will read it through alot more times

I just find it weird that i still see non-DIS routers send LSP's directly to other non-DIS routers. I dont see the DIS flood anything else than CSNP's every 10 seconds and then hello's every 3.3 seconds.

The only solid information I have why a DIS exist, is a section that states that in order to run the SPF, you must have a node representing the multiaccess network.

tech-airman

scheistermeister wrote:

tech-airman wrote:

So in summary, for ISIS, each router in an area (level 1) are fully meshed with each other as well as the DIS and so their LSDBs are synchronized. All Level 2 ISIS routers are fully meshed with each other as well as the DIS and so also their LSDbs are synchronized. ISIS doesn't care if the DIS changes within an area or in the backbone. Keep in mind that the ISIS adjacencies are formed as long as it's within the boundary of the given area or backbone segment.

So what would be the point of the DIS if all routers of the same level and area formed adjacencies with each other and did not care if the DIS changed? There has to be something or else it wouldn't be there.

scheistermeister,

As you may or may not know, OSPF and ISIS both use the SPF algorithm. The SPF algorithm requires a "starting point." For OSPF, that's the DR. However, for ISIS, there is no specific router in the area or backbone that is elected as the DR, so the SPF algorithm doesn't have a "starting point" to start calculating the Shortest Paths from. So ISIS has to create a "starting point" for the purposes of the SPF algorithm, that's where the DIS comes in. Unlike OSPF where you can basically rig the DR election process for OSPF by configuring the priority values so that a specific router is most likely to become elected as the DR or even to configure the priority value to 0 making the OSPF router a DROTHER, ISIS doesn't care which router becomes the DIS for the area or backbone segment. There is an orderly designation and redesignation process of one of the ISIS routers to take on the role of DIS and an orderly resignation process of the DIS so that there will be a DIS at all times.

The point of the DIS for ISIS is to serve as a "starting point" for the SPF calculations. The point of the DR in OSPF is to serve as a "starting point" for the SPF calculations. The difference between ISIS and OSPF is that if an ISIS router wants to talk to another ISIS router, it talks to the other router directly since they're adjacent to each other. However, with OSPF, if a non-DR/BDR OSPF router wants to talk to another non-DR/BDR OSPF router, the first router MUST FIRST talk to the DR, then the DR will relay the message to the other non-DR/BDR OSPF router because non-DR/BDR OSPF routers only form adjacencies with the DR and no one else.