Categories
CCNP (Pre 2020) Cisco Routing

OSPF Route Summarisation

Scalability of the routing protocol is an important factor for large networks such as service providers. By splitting up an OSPF network into multiple OSPF areas, it can reduce the size of the LSDB for that area. The number of routers and networks remain the same but Type 1 and Type 2 LSAs are exchanged for summarised Type 3 LSAs.

Type 3 LSAs give an opportunity for further summarisation with larger network prefixes; allowing SPF calculations to run faster and provide easier processing for the CPU and memory. It can do this easily simply by hiding the multiple smaller prefixes behind a larger prefix that is advertised to the rest of the OSPF network.

Interarea Summarisation

Interarea summarisation reduces the number of type 3 LSAs that the area border router (ABR) advertises into an area when it receives type 1 LSAs.

When the Type 1 LSA within the summarisation range reaches the area border router from the source area, the area border router creates a Type 3 LSA for the summarised network range with the more specific type 3 LSAs being suppressed.

The default metric for a summarised LSA is the lowest metric associated with it. For example if there were three more specific prefixes in the summarised prefix consisting of 15, 73, 109, 15 would be the chosen metric used in the summary route.

Configuring Interarea Summarisation

Under the OSPF process configuration mode, enter the command area <area number> range <network> <subnet-mask>

There are optional keywords to the command, advertise or not-advertise. Advertise is implicitly included so does not need to be specified explicitly. Adding a metric on the end with the keyword metric <value> allows for a specific metric to be set.

Categories
CCNP (Pre 2020) CCNP Enterprise Core (350-401) Routing

Static Routing

A static route configured by an administrator can give absolute control on determining where packets are routed. In comparison to dynamic routing protocols it can be a large administrative overheard for the network engineer when modifying or updating the routes on multiple routers.

A static route configured on a router does not require any bandwidth, as the routes are configured on the local router and are not transmitted elsewhere.

Due to the static route only being local to the router, any links going down does not result in any peers being updated with new routes.

Static routes are only useful when:

  • Dynamic routes cannot be used due to it not being supported, limited memory, or CPU power.
  • Routes that are learned through a dynamic routing protocol need to be overridden.
Categories
CCNP (Pre 2020) Cisco Routing

Route Metrics

Metric calculations can vary between different routing protocols. The general common concept across them is that internally learned routes are preferred than externally (such as through distribution) and the lowest metric results in a preferred route chosen.

Categories
CCNP (Pre 2020) CCNP Enterprise Core (350-401) Cisco Datalink Layer

UDLD – Unidirectional Link Detection

Unidirectional Link Detection allows for monitoring of physical media that separates its send and receive media, like fibre optic cables.

UDLD operates by transmitting a packet to the neighbour device that contains a system and port ID of the originating interface. The receiving interface gets this information and mirrors it back including its own system and port ID. This process continues to ensure that both send and receive parts of the link have communication between them.

Unidirectional Link Detection has two operating modes, normal and aggressive.

  • Aggressive mode sends eight additional packets in one second intervals when a UDLD packet is not acknowledged by the neighbouring switch. If none of those packets are acknowledged the port is changed into a error disabled state.
  • Normal mode does not send additional packets or disable the interface when a packet is not acknowledged. It changes the link status to undetermined and the port continues to be utilised regardless of what fault state it is in.

Unidirectional Link Detection can be enabled globally with the command udld enable with the optional additional suffix keyword of aggressive to enable aggressive mode. Enabling UDLD globally will switch on the UDLD mechanisms on all SFP ports.

Individual ports can have UDLD capabilities disabled in interface configuration mode with the command udld port disable

In order for UDLD to function correctly, both ends of the switch must have UDLD enabled to communicate packets correctly. The status of UDLD can be checked with the command show udld neighbors

Categories
CCNP (Pre 2020) TSHOOT

TSHOOT Topology

The new TSHOOT exam requires you to isolate issues on a defined network topology. The purpose of the exam is to evaluate troubleshooting skills, not how quickly you can grasp a network topology you have never seen. To this end, it is Cisco’s intent that the exam network topology be available for review before the candidate attempt the exam.

https://learningnetwork.cisco.com/thread/10965

As practice I mirrored the topology in the linked thread to an easy to read topology below.

A - VLAN 10 - 10.2.1.0/24DSW1 - 10.2.1.1DSW2 - 10.2.1.2
B - VLAN 20 - 10.2.2.0/24
DSW1 - 10.2.2.2
DSW2 - 10.2.2.1
C - DSW1 <-> R4 - 10.1.4.4/30
DSW1 - 10.1.4.6
R4 - 10.1.4.5
D - DSW2 <-> R4 - 10.1.4.8/30
DSW2 - 10.1.4.10 
R4 - 10.1.4.9
E - R4 <-> R3 - 10.1.1.8/30
R4 - 10.1.1.10
R3 - 10.1.1.9
F - R3 <-> R2 - 10.1.1.4/30
R3 - 10.1.1.6
R2 - 10.1.1.5
G - R2 <-> R1 - 10.1.1.0/30
R2 - 10.1.1.2
R1 - 10.1.1.1
H - R1 <-> WAN - 209.65.200.224/30
R1 - 209.65.200.225
ISP Router - 209.65.200.226
X - Link between DSW1 and DSW2 (not shown) - 10.2.4.12/30
DSW1 - 10.2.4.13
DSW2 - 10.2.4.14