ISIS part 2

ISIS router has 2 network types which are broadcasts (LAN and Multipoint WAN) and point-to-point (everything else). If the router configuration is point-to-point, the adjacency will be formed and the routers will send a complete sequence number package, which contain the routers link-state database and can therefore synchronize with the other. If the routers are in a broadcast network, the network will elect a DIS (Designated Intermediate System) to send the latest configuration to the other routers. The DIS is elected to the router with the highest ISIS interface priority (default = 64). If the interface priorities are the same, the highest SNPA (Sub network Point of Attachment) will determine the DIS. The SNPA is the highest MAC address if it is a LAN configuration, and the highest DLCI number in a Frame Relay configuration. A DIS does not sync with all other routers. Therefore, a pseudonode (a virtual router) is created by the DIS. In this way, all routers forms an adjacency with the pseudonode, including the DIS. The DIS can detect problems quickly because it sends hello packets every 3.3 seconds, whereas CSNP sends hello packets every 10 seconds.

ISIS has 4 types of PDU which are Hello, LSP, CSNP and PSNP packets.

Hello packets consists of 3 different packets:

1. - ESH (End System Hello) which is sent by an ES (host) to an IS (router)

2. - ISH (Intermediate System Hello) which is sent by IS and listened by ES.

3. - IIH (IS-IS Hello) which is send from an IS to another IS to form and adjeceny.

The router is able to send multiple hello packets at the same time (ISH and IIH).

LSP (Link State Packets) is used to build a link-state database sent from adjacent routers, which are necessary to build a routing table together with the SPF algorithm. There are 2 types of LSP which are the L1 LSP and L2 LSP.

CSNP (Complete Sequence Number PDU) contains a complete list of LSP, sent to adjacent routers to update and sync the adjacent router. This list is sent by the DIS every 10 seconds.

PSNP (Partial Sequence Number PDU) is used to request LSP and to acknowledge if an LSP has been received.

-- 1 March 2011 --

EIGRP Administrative Distance, EIGRP Route Summarization, EIGRP Stub

The default administrative distance - AD of EIGRP internal route is 90 which is marked by the "D" symbol. The AD of an external route or considered as an external route using route distribution is 170 which is marked by the "D EX" symbol. There is also an eigrp summary route with the AD of 5. All of this can be shown with the #show ip route eigrp command.

To change the AD of EIGRP use the command:
(config)#router eigrp
(config-router)#distance eigrp

Auto summarization is a default feature at EIGRP. The purpose of this feature is used to automatically summarize the routes that are advertised through classfull network boundaries (EIGRP and RIPv2). It is therefore almost always necessary to turn this feature off in order make the router available to advertise classless networks routes by using the no auto-summary command. It is important to configure this route on the router sending the route. Remember that when applying the summarization, the adjecency goes down for a moment before it goes up again.

There are several advantages when applying a route summarization. A crucial advantage is that the route tables are getting smaller, because several routes are summarized. Another advantage is that CPU processing will be lighter, as the routes in the table are smaller.

To configure auto summarization use the following command:
(config)#int
(config-if)#ip summary-address eigrp <>

There are several ways to summarize routes where each route needs to have a specific bandwidth:

1. If there are multiple routes with the same CIRs, then add the amount of the CIRs together and use that as the minimum bandwidth needed. To set this use the command:
(config-if)#bandwidth

2. If there are multiple routes with different CIRs, then configure the route using a point to point (sub-)interface mode and assign each VC with the required CIR bandwidth value. To set this use the command on each sub-interface:
(config)#int serial point-to-point
(config-subif)#ip address
(config-subif)#bandwidth

3. There is also a possibility to configure a multipoint sub-interfaces. This can be done by adding all the CIRs from the routers. To set this use the command on the main router:
(config)#int serial multipoint
(config-subif)#bandwidth

The EIGRP can also be configured as a stub. However the stub has to be set only in the hub router. The spoke router does not need to be configured for stub (although possible) because the spoke router does not need to keep the routing table. It simply forwards all the routers to the hub router. This feature can also combat the SIA (stuck in active) problem because in stub networks, spoke routers will not be queried for routes when the hub does not have a feasible successor. To configure stub use the command:
(config)#router eigrp 100
(config-router)#eigrp stub

-- 10 February 2011 --

Static Route, Floating Static Route and Default Static Route

Static route is used when one wants to define the routing process manually.

The static route command is as below:

(config)#ip route

On the exit interface of the router part, the interface can be defined as the type of interface (e.g. serial0, fa0/2) or it can be defined as the ip address of the outbound interface of the router. The difference is that when we set it with the type of interface, the route is configured to be directly connected (AD=0), while the option of setting the IP address of the outbound interface will be considered as static route (AD=1)

The example below shows the 2 different configuration, but has actually the same meaning:

(config)#ip route 10.10.10.10 255.255.255.255 172.168.10.10 --> static route AD=1
(config)#ip route 10.10.10.10 255.255.255.255 serial0 --> direct connected AD=0

A floating static route is a route that is used to back up a dynamic protocol. So, the floating static route will take over the routing when the dynamic protocol such as RIP or OSPF goes down for some reason on an interface. The key in setting a floating static route is the same as in configuring a static route, but adding the administrative distance of it. The AD must be higher than the current value of the dynamic protocol. If we use RIP as the protocol we must set the floating route higher than 120. A value lower than 120 will have the opposite effect; the static route will be applied in routing rather than the RIP.

The floating static route can be done with the command:

(config)#ip route

The default static route is used as a gateway of last resort. If the router does not have a routing table of an incoming packet, then the router will send the packet to according to the default static route. If this route is not set, unknown incoming packets will be dropped.


The command for default static route can be shown below:

(config)#ip route 0.0.0.0 0.0.0.0 /

The symbol for default static route will be S* --> AD=0 (directly connected)/1(static)

Sometimes when updating routing policies the routes are not updated directly. A useful way to refresh the changes is to clear the ip route, and let the router discover the table again by applying the command:

RX#clear ip route*

Routing Table Operation

Routers use their routing tables to determine the outgoing route of the incoming packets. The routing table operation goes as follow in the following order:

1. The route with the longest prefix (or the shortest subnet mask) will be prioritized first, despite of its routing protocol. So if there is a RIP route of 172.10.10.0/28 and an OSPF route of 172.10.10.0/29, it will choose the OSPF route when sending outgoing packets because 172.10.10.0/29 (6 hosts) has a longer prefix and thus more specific hosts than the 172.10.10.0/28 (14 hosts) route.

2. If the route has the same prefix length, it will look for its administrative distance (AD) of the connection to the neighboring routes. Administrative distances are distances that are measured based on the reliability of the connection and routing protocol of one router to the other. Routes with a lower AD are considered more reliable and will be chosen over the other routes with a higher AD number.

For example, directly connected routes are considered very reliable and has therefore an AD=0. RIP routes are considered simple and not so reliable and has therefore an AD=120. OSPF are considered more reliable than RIP and is given an AD=110.

3. If (1) and (2) are the same for multiple routes, the router will look for its metrics according to its table. The metric is the cost of a route to go from the source address to the destination address. Metrics are different from each other's routing protocol.

RIP metrics uses hop counts where the path with the lowest hops of routers is considered as the shortest path. OSPF metric uses cost which is the inversely proportional bandwidth of the current connection and gets prioritized. Lower cost means a faster interface with higher speed and gets prioritized. IGRP uses a composite metric, based on a the composite of bandwidth, delay, load, reliability and max. MTU. By default, IGRP chooses its route based on bandwidth and delay only. Lowest composite metric means better connection and gets prioritized.

4. If (1) to (3) has the same values, then equal-cost load sharing will be applied, where the load will be equally shared between the multiple connections.

In any of the 4 cases above, only the best route is shown on the show ip route command. The alternative route is there when the best route is down, but the alternative route configuration will not be shown in the show ip route command.

To show the IP route type the command:
RX#show ip route

It will show the routing configuration. A B[C/D] via E (optional), F, G
where:
A = type of connection (direct, RIP, IGRP etc)
B = the destination group IP address
C = administrative distance
D = metric
E = next hop inbound interface (not the final destination)
F = length of connection
G = interface connected to

for example:

R 172.10.0.0 [120/5] via 10.20.30.40, 0:02:34, serial0

means that the connection uses RIP routing protocol to the destination 172.10.0.0 with the administrative distance of RIP=120 and the metric of 5 hops. 10.20.30.40 is the inline interface of the next hop and the router is already connected for 2 minutes and 34 seconds. The connection is connected to the serial0 of the router.

-- 6 December 2010 --