Tag Archives: 3.6

3.6 OSPF [v2 and v3]

A designated router (DR) is the router interface elected among all routers on a particular multi-access network segment, generally assumed to be broadcast multi-access. The basic neighbor discovery process (Hello), flooding (224.0.0.6), DR election (priority, RID). Special techniques, often vendor-dependent, may be needed to support the DR function on non-broadcast multi-access (NBMA) media. It is usually wise to configure the individual virtual circuits of a NBMA subnet as individual point-to-point lines.

DRs exist for the purpose of reducing network traffic by providing a source for routing updates. The DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in a multi-access network segment will form a slave/ master relationship with the DR. They will form adjacencies with the DR and BDR only. Every time a router sends an update, it sends it to the DR and BDR on the multicast address 224.0.0.6. The DR will then send the update out to all other routers in the area, to the multicast address 224.0.0.5. This way all the routers do not have to constantly update each other, and can rather get all their updates from a single source. The use of multicasting further reduces the network load . DRs and BDRs are always setup/ elected on OSPF broadcast networks. DR’s can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay. DRs or BDRs are not elected on point-to-point links (such as a point -to-point WAN connection) because the two routers on either sides of the link must become fully adjacent and the bandwidth between them cannot be further optimized. DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3335-3344).  . Kindle Edition.

https://www.ietf.org/rfc/rfc2328.txt

3.6 OSPF [v2 and v3]

here is a mildly interesting situation… as i often use network 0.0.0.0 255.255.255.255 area x, it can prove fatal with summarization…

rene_ospf

router 3’s loopbacks have been configured  using redistributed connected instead of as advertised networks, one of the constraints of the lab… it is also nssa…

Thor(config-router)#do sh run | b router
router ospf 1
log-adjacency-changes
area 3 nssa
summary-address 172.16.0.0 255.255.254.0
redistribute connected subnets
network 3.3.3.0 0.0.0.255 area 0
network 192.168.13.0 0.0.0.255 area 3

if the network statement was the blanket network, the summary address as well as the loopbacks themselves would show up in the routing table of r1, instead of as a single summary below:

Wodan#sh ip route | b Gat
Gateway of last resort is not set

C    192.168.12.0/24 is directly connected, Serial0/0
1.0.0.0/24 is subnetted, 1 subnets
C       1.1.1.0 is directly connected, Loopback0
C    192.168.13.0/24 is directly connected, Serial0/1
2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/65] via 192.168.12.2, 00:09:20, Serial0/0
172.16.0.0/23 is subnetted, 1 subnets
O N2    172.16.0.0 [110/20] via 192.168.13.3, 00:09:20, Serial0/1

ie:

Thor(config-router)#no router ospf 1
Thor(config)#
*Mar  1 01:43:22.151: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0/0 from FULL to DOWN, Neighbor Down: Interface down or detachedrouter ospf 1
Thor(config-router)#netw 0.0.0.0 255.255.255.255 area 3
Thor(config-router)#summary-add 172.16.0.0 255.255.254.0
Thor(config-router)#area 3 nssa
Thor(config-router)#
*Mar  1 01:44:11.899: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0/0 from LOADING to FULL, Loading Done
Thor(config-router)#redistr connect sub

Wodan#sh ip route | b Ga
Gateway of last resort is not set

C    192.168.12.0/24 is directly connected, Serial0/0
1.0.0.0/24 is subnetted, 1 subnets
C       1.1.1.0 is directly connected, Loopback0
C    192.168.13.0/24 is directly connected, Serial0/1
2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/65] via 192.168.12.2, 00:01:39, Serial0/0
3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/65] via 192.168.13.3, 00:01:39, Serial0/1
172.16.0.0/32 is subnetted, 2 subnets
O       172.16.1.3 [110/65] via 192.168.13.3, 00:01:39, Serial0/1
O       172.16.0.3 [110/65] via 192.168.13.3, 00:01:39, Serial0/1

 

3.3.c Compare routing protocol types

3.3.c [ii] Link state

Djikstra:

in his own words…

Construct [a] tree of minimum total length between the n nodes. (The tree is a graph with one and only one path between every two nodes.)
In the course of the construction that we present here, the branches are divided into three sets:
I. the branches definitely assigned to the tree under construction (they will be in a subtree);
II. the branches from which the next branch to be added to set I, will be selected;
III. the remaining branches (rejected or not considered).
The nodes are divided into two sets:
A. the nodes connected by the branches of set I,
B. the remaining nodes (one and only one branch of set II will lead to each of these nodes).

We start the construction by choosing an arbitrary node as the only member of set A, and by placing all branches that end in this node in set II. To start with, set I is empty. From then onwards we perform the following two steps repeatedly.
Step 1: The shortest branch of set II is removed from this set and added to set I. As a result, one node is transferred from set B to set A.
Step 2: Consider the branches leading from the node, which has just been transferred to set A, to the nodes that are still in set B. If the branch under construction is longer than the corresponding branch in set II, it is rejected; if it is shorter, it replaces the corresponding branch in set II, and the latter is rejected.
We then return to step 1 and repeat the process until sets II and B are empty. The branches in set I form the tree required.

doyle’s take on it:

1 A router initializes the Tree database by adding itself as the root. This entry shows the
router as its own neighbor, with a cost of 0.
2 All triples in the link state database describing links to the root router’s neighbors are
added to the Candidate database.
3 The cost from the root to each link in the Candidate database is calculated. The link
in the Candidate database with the lowest cost is moved to the Tree database. If two
or more links are an equally low cost from the root, choose one.
4 The Neighbor ID of the link just added to the Tree database is examined. With the
exception of any triple whose Neighbor ID is already in the Tree database, triples in the
link state database describing that router’s neighbors are added to the Candidate database.
5 If entries remain in the Candidate database, return to step 3. If the Candidate database is
empty, terminate the algorithm. At termination, a single Neighbor ID entry in the Tree
database should represent every router, and the shortest path tree is complete.